.. SPDX-License-Identifier: GPL-2.0

====================
The /proc Filesystem
====================

=====================  =======================================  ================
/proc/sys              Terrehon Bowden <terrehon@pacbell.net>,  October 7 1999
                       Bodo Bauer <bb@ricochet.net>
2.4.x update	       Jorge Nerin <comandante@zaralinux.com>   November 14 2000
move /proc/sys	       Shen Feng <shen@cn.fujitsu.com>	        April 1 2009
fixes/update part 1.1  Stefani Seibold <stefani@seibold.net>    June 9 2009
=====================  =======================================  ================


.. Table of Contents

  0     Preface
  0.1	Introduction/Credits
  0.2	Legal Stuff

  1	Collecting System Information
  1.1	Process-Specific Subdirectories
  1.2	Kernel data
  1.3	IDE devices in /proc/ide
  1.4	Networking info in /proc/net
  1.5	SCSI info
  1.6	Parallel port info in /proc/parport
  1.7	TTY info in /proc/tty
  1.8	Miscellaneous kernel statistics in /proc/stat
  1.9	Ext4 file system parameters

  2	Modifying System Parameters

  3	Per-Process Parameters
  3.1	/proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj - Adjust the oom-killer
								score
  3.2	/proc/<pid>/oom_score - Display current oom-killer score
  3.3	/proc/<pid>/io - Display the IO accounting fields
  3.4	/proc/<pid>/coredump_filter - Core dump filtering settings
  3.5	/proc/<pid>/mountinfo - Information about mounts
  3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
  3.7   /proc/<pid>/task/<tid>/children - Information about task children
  3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file
  3.9   /proc/<pid>/map_files - Information about memory mapped files
  3.10  /proc/<pid>/timerslack_ns - Task timerslack value
  3.11	/proc/<pid>/patch_state - Livepatch patch operation state
  3.12	/proc/<pid>/arch_status - Task architecture specific information
  3.13  /proc/<pid>/fd - List of symlinks to open files
  3.14  /proc/<pid/ksm_stat - Information about the process's ksm status.

  4	Configuring procfs
  4.1	Mount options

  5	Filesystem behavior

Preface
=======

0.1 Introduction/Credits
------------------------

We'd like  to  thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
other people for help compiling this documentation. We'd also like to extend a
special thank  you to Andi Kleen for documentation, which we relied on heavily
to create  this  document,  as well as the additional information he provided.
Thanks to  everybody  else  who contributed source or docs to the Linux kernel
and helped create a great piece of software... :)

The   latest   version    of   this   document   is    available   online   at
https://www.kernel.org/doc/html/latest/filesystems/proc.html

0.2 Legal Stuff
---------------

We don't  guarantee  the  correctness  of this document, and if you come to us
complaining about  how  you  screwed  up  your  system  because  of  incorrect
documentation, we won't feel responsible...

Chapter 1: Collecting System Information
========================================

In This Chapter
---------------
* Investigating  the  properties  of  the  pseudo  file  system  /proc and its
  ability to provide information on the running Linux system
* Examining /proc's structure
* Uncovering  various  information  about the kernel and the processes running
  on the system

------------------------------------------------------------------------------

The proc  file  system acts as an interface to internal data structures in the
kernel. It  can  be  used to obtain information about the system and to change
certain kernel parameters at runtime (sysctl).

First, we'll  take  a  look  at the read-only parts of /proc. In Chapter 2, we
show you how you can use /proc/sys to change settings.

1.1 Process-Specific Subdirectories
-----------------------------------

The directory  /proc  contains  (among other things) one subdirectory for each
process running on the system, which is named after the process ID (PID).

The link  'self'  points to  the process reading the file system. Each process
subdirectory has the entries listed in Table 1-1.

A process can read its own information from /proc/PID/* with no extra
permissions. When reading /proc/PID/* information for other processes, reading
process is required to have either CAP_SYS_PTRACE capability with
PTRACE_MODE_READ access permissions, or, alternatively, CAP_PERFMON
capability. This applies to all read-only information like `maps`, `environ`,
`pagemap`, etc. The only exception is `mem` file due to its read-write nature,
which requires CAP_SYS_PTRACE capabilities with more elevated
PTRACE_MODE_ATTACH permissions; CAP_PERFMON capability does not grant access
to /proc/PID/mem for other processes.

Note that an open file descriptor to /proc/<pid> or to any of its
contained files or subdirectories does not prevent <pid> being reused
for some other process in the event that <pid> exits. Operations on
open /proc/<pid> file descriptors corresponding to dead processes
never act on any new process that the kernel may, through chance, have
also assigned the process ID <pid>. Instead, operations on these FDs
usually fail with ESRCH.

.. table:: Table 1-1: Process specific entries in /proc

 =============  ===============================================================
 File		Content
 =============  ===============================================================
 clear_refs	Clears page referenced bits shown in smaps output
 cmdline	Command line arguments
 cpu		Current and last cpu in which it was executed	(2.4)(smp)
 cwd		Link to the current working directory
 environ	Values of environment variables
 exe		Link to the executable of this process
 fd		Directory, which contains all file descriptors
 maps		Memory maps to executables and library files	(2.4)
 mem		Memory held by this process
 root		Link to the root directory of this process
 stat		Process status
 statm		Process memory status information
 status		Process status in human readable form
 wchan		Present with CONFIG_KALLSYMS=y: it shows the kernel function
		symbol the task is blocked in - or "0" if not blocked.
 pagemap	Page table
 stack		Report full stack trace, enable via CONFIG_STACKTRACE
 smaps		An extension based on maps, showing the memory consumption of
		each mapping and flags associated with it
 smaps_rollup	Accumulated smaps stats for all mappings of the process.  This
		can be derived from smaps, but is faster and more convenient
 numa_maps	An extension based on maps, showing the memory locality and
		binding policy as well as mem usage (in pages) of each mapping.
 =============  ===============================================================

For example, to get the status information of a process, all you have to do is
read the file /proc/PID/status::

  >cat /proc/self/status
  Name:   cat
  State:  R (running)
  Tgid:   5452
  Pid:    5452
  PPid:   743
  TracerPid:      0						(2.4)
  Uid:    501     501     501     501
  Gid:    100     100     100     100
  FDSize: 256
  Groups: 100 14 16
  Kthread:    0
  VmPeak:     5004 kB
  VmSize:     5004 kB
  VmLck:         0 kB
  VmHWM:       476 kB
  VmRSS:       476 kB
  RssAnon:             352 kB
  RssFile:             120 kB
  RssShmem:              4 kB
  VmData:      156 kB
  VmStk:        88 kB
  VmExe:        68 kB
  VmLib:      1412 kB
  VmPTE:        20 kb
  VmSwap:        0 kB
  HugetlbPages:          0 kB
  CoreDumping:    0
  THP_enabled:	  1
  Threads:        1
  SigQ:   0/28578
  SigPnd: 0000000000000000
  ShdPnd: 0000000000000000
  SigBlk: 0000000000000000
  SigIgn: 0000000000000000
  SigCgt: 0000000000000000
  CapInh: 00000000fffffeff
  CapPrm: 0000000000000000
  CapEff: 0000000000000000
  CapBnd: ffffffffffffffff
  CapAmb: 0000000000000000
  NoNewPrivs:     0
  Seccomp:        0
  Speculation_Store_Bypass:       thread vulnerable
  SpeculationIndirectBranch:      conditional enabled
  voluntary_ctxt_switches:        0
  nonvoluntary_ctxt_switches:     1

This shows you nearly the same information you would get if you viewed it with
the ps  command.  In  fact,  ps  uses  the  proc  file  system  to  obtain its
information.  But you get a more detailed  view of the  process by reading the
file /proc/PID/status. It fields are described in table 1-2.

The  statm  file  contains  more  detailed  information about the process
memory usage. Its seven fields are explained in Table 1-3.  The stat file
contains detailed information about the process itself.  Its fields are
explained in Table 1-4.

(for SMP CONFIG users)

For making accounting scalable, RSS related information are handled in an
asynchronous manner and the value may not be very precise. To see a precise
snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
It's slow but very precise.

.. table:: Table 1-2: Contents of the status fields (as of 4.19)

 ==========================  ===================================================
 Field                       Content
 ==========================  ===================================================
 Name                        filename of the executable
 Umask                       file mode creation mask
 State                       state (R is running, S is sleeping, D is sleeping
                             in an uninterruptible wait, Z is zombie,
			     T is traced or stopped)
 Tgid                        thread group ID
 Ngid                        NUMA group ID (0 if none)
 Pid                         process id
 PPid                        process id of the parent process
 TracerPid                   PID of process tracing this process (0 if not, or
                             the tracer is outside of the current pid namespace)
 Uid                         Real, effective, saved set, and  file system UIDs
 Gid                         Real, effective, saved set, and  file system GIDs
 FDSize                      number of file descriptor slots currently allocated
 Groups                      supplementary group list
 NStgid                      descendant namespace thread group ID hierarchy
 NSpid                       descendant namespace process ID hierarchy
 NSpgid                      descendant namespace process group ID hierarchy
 NSsid                       descendant namespace session ID hierarchy
 Kthread                     kernel thread flag, 1 is yes, 0 is no
 VmPeak                      peak virtual memory size
 VmSize                      total program size
 VmLck                       locked memory size
 VmPin                       pinned memory size
 VmHWM                       peak resident set size ("high water mark")
 VmRSS                       size of memory portions. It contains the three
                             following parts
                             (VmRSS = RssAnon + RssFile + RssShmem)
 RssAnon                     size of resident anonymous memory
 RssFile                     size of resident file mappings
 RssShmem                    size of resident shmem memory (includes SysV shm,
                             mapping of tmpfs and shared anonymous mappings)
 VmData                      size of private data segments
 VmStk                       size of stack segments
 VmExe                       size of text segment
 VmLib                       size of shared library code
 VmPTE                       size of page table entries
 VmSwap                      amount of swap used by anonymous private data
                             (shmem swap usage is not included)
 HugetlbPages                size of hugetlb memory portions
 CoreDumping                 process's memory is currently being dumped
                             (killing the process may lead to a corrupted core)
 THP_enabled                 process is allowed to use THP (returns 0 when
                             PR_SET_THP_DISABLE is set on the process to disable
                             THP completely, not just partially)
 Threads                     number of threads
 SigQ                        number of signals queued/max. number for queue
 SigPnd                      bitmap of pending signals for the thread
 ShdPnd                      bitmap of shared pending signals for the process
 SigBlk                      bitmap of blocked signals
 SigIgn                      bitmap of ignored signals
 SigCgt                      bitmap of caught signals
 CapInh                      bitmap of inheritable capabilities
 CapPrm                      bitmap of permitted capabilities
 CapEff                      bitmap of effective capabilities
 CapBnd                      bitmap of capabilities bounding set
 CapAmb                      bitmap of ambient capabilities
 NoNewPrivs                  no_new_privs, like prctl(PR_GET_NO_NEW_PRIV, ...)
 Seccomp                     seccomp mode, like prctl(PR_GET_SECCOMP, ...)
 Speculation_Store_Bypass    speculative store bypass mitigation status
 SpeculationIndirectBranch   indirect branch speculation mode
 Cpus_allowed                mask of CPUs on which this process may run
 Cpus_allowed_list           Same as previous, but in "list format"
 Mems_allowed                mask of memory nodes allowed to this process
 Mems_allowed_list           Same as previous, but in "list format"
 voluntary_ctxt_switches     number of voluntary context switches
 nonvoluntary_ctxt_switches  number of non voluntary context switches
 ==========================  ===================================================


.. table:: Table 1-3: Contents of the statm fields (as of 2.6.8-rc3)

 ======== ===============================	==============================
 Field    Content
 ======== ===============================	==============================
 size     total program size (pages)		(same as VmSize in status)
 resident size of memory portions (pages)	(same as VmRSS in status)
 shared   number of pages that are shared	(i.e. backed by a file, same
						as RssFile+RssShmem in status)
 trs      number of pages that are 'code'	(not including libs; broken,
						includes data segment)
 lrs      number of pages of library		(always 0 on 2.6)
 drs      number of pages of data/stack		(including libs; broken,
						includes library text)
 dt       number of dirty pages			(always 0 on 2.6)
 ======== ===============================	==============================


.. table:: Table 1-4: Contents of the stat fields (as of 2.6.30-rc7)

  ============= ===============================================================
  Field         Content
  ============= ===============================================================
  pid           process id
  tcomm         filename of the executable
  state         state (R is running, S is sleeping, D is sleeping in an
                uninterruptible wait, Z is zombie, T is traced or stopped)
  ppid          process id of the parent process
  pgrp          pgrp of the process
  sid           session id
  tty_nr        tty the process uses
  tty_pgrp      pgrp of the tty
  flags         task flags
  min_flt       number of minor faults
  cmin_flt      number of minor faults with child's
  maj_flt       number of major faults
  cmaj_flt      number of major faults with child's
  utime         user mode jiffies
  stime         kernel mode jiffies
  cutime        user mode jiffies with child's
  cstime        kernel mode jiffies with child's
  priority      priority level
  nice          nice level
  num_threads   number of threads
  it_real_value	(obsolete, always 0)
  start_time    time the process started after system boot
  vsize         virtual memory size
  rss           resident set memory size
  rsslim        current limit in bytes on the rss
  start_code    address above which program text can run
  end_code      address below which program text can run
  start_stack   address of the start of the main process stack
  esp           current value of ESP
  eip           current value of EIP
  pending       bitmap of pending signals
  blocked       bitmap of blocked signals
  sigign        bitmap of ignored signals
  sigcatch      bitmap of caught signals
  0		(place holder, used to be the wchan address,
		use /proc/PID/wchan instead)
  0             (place holder)
  0             (place holder)
  exit_signal   signal to send to parent thread on exit
  task_cpu      which CPU the task is scheduled on
  rt_priority   realtime priority
  policy        scheduling policy (man sched_setscheduler)
  blkio_ticks   time spent waiting for block IO
  gtime         guest time of the task in jiffies
  cgtime        guest time of the task children in jiffies
  start_data    address above which program data+bss is placed
  end_data      address below which program data+bss is placed
  start_brk     address above which program heap can be expanded with brk()
  arg_start     address above which program command line is placed
  arg_end       address below which program command line is placed
  env_start     address above which program environment is placed
  env_end       address below which program environment is placed
  exit_code     the thread's exit_code in the form reported by the waitpid
		system call
  ============= ===============================================================

The /proc/PID/maps file contains the currently mapped memory regions and
their access permissions.

The format is::

    address           perms offset  dev   inode      pathname

    08048000-08049000 r-xp 00000000 03:00 8312       /opt/test
    08049000-0804a000 rw-p 00001000 03:00 8312       /opt/test
    0804a000-0806b000 rw-p 00000000 00:00 0          [heap]
    a7cb1000-a7cb2000 ---p 00000000 00:00 0
    a7cb2000-a7eb2000 rw-p 00000000 00:00 0
    a7eb2000-a7eb3000 ---p 00000000 00:00 0
    a7eb3000-a7ed5000 rw-p 00000000 00:00 0
    a7ed5000-a8008000 r-xp 00000000 03:00 4222       /lib/libc.so.6
    a8008000-a800a000 r--p 00133000 03:00 4222       /lib/libc.so.6
    a800a000-a800b000 rw-p 00135000 03:00 4222       /lib/libc.so.6
    a800b000-a800e000 rw-p 00000000 00:00 0
    a800e000-a8022000 r-xp 00000000 03:00 14462      /lib/libpthread.so.0
    a8022000-a8023000 r--p 00013000 03:00 14462      /lib/libpthread.so.0
    a8023000-a8024000 rw-p 00014000 03:00 14462      /lib/libpthread.so.0
    a8024000-a8027000 rw-p 00000000 00:00 0
    a8027000-a8043000 r-xp 00000000 03:00 8317       /lib/ld-linux.so.2
    a8043000-a8044000 r--p 0001b000 03:00 8317       /lib/ld-linux.so.2
    a8044000-a8045000 rw-p 0001c000 03:00 8317       /lib/ld-linux.so.2
    aff35000-aff4a000 rw-p 00000000 00:00 0          [stack]
    ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

where "address" is the address space in the process that it occupies, "perms"
is a set of permissions::

 r = read
 w = write
 x = execute
 s = shared
 p = private (copy on write)

"offset" is the offset into the mapping, "dev" is the device (major:minor), and
"inode" is the inode  on that device.  0 indicates that  no inode is associated
with the memory region, as the case would be with BSS (uninitialized data).
The "pathname" shows the name associated file for this mapping.  If the mapping
is not associated with a file:

 ===================        ===========================================
 [heap]                     the heap of the program
 [stack]                    the stack of the main process
 [vdso]                     the "virtual dynamic shared object",
                            the kernel system call handler
 [anon:<name>]              a private anonymous mapping that has been
                            named by userspace
 [anon_shmem:<name>]        an anonymous shared memory mapping that has
                            been named by userspace
 ===================        ===========================================

 or if empty, the mapping is anonymous.

Starting with 6.11 kernel, /proc/PID/maps provides an alternative
ioctl()-based API that gives ability to flexibly and efficiently query and
filter individual VMAs. This interface is binary and is meant for more
efficient and easy programmatic use. `struct procmap_query`, defined in
linux/fs.h UAPI header, serves as an input/output argument to the
`PROCMAP_QUERY` ioctl() command. See comments in linus/fs.h UAPI header for
details on query semantics, supported flags, data returned, and general API
usage information.

The /proc/PID/smaps is an extension based on maps, showing the memory
consumption for each of the process's mappings. For each mapping (aka Virtual
Memory Area, or VMA) there is a series of lines such as the following::

    08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash

    Size:               1084 kB
    KernelPageSize:        4 kB
    MMUPageSize:           4 kB
    Rss:                 892 kB
    Pss:                 374 kB
    Pss_Dirty:             0 kB
    Shared_Clean:        892 kB
    Shared_Dirty:          0 kB
    Private_Clean:         0 kB
    Private_Dirty:         0 kB
    Referenced:          892 kB
    Anonymous:             0 kB
    KSM:                   0 kB
    LazyFree:              0 kB
    AnonHugePages:         0 kB
    ShmemPmdMapped:        0 kB
    Shared_Hugetlb:        0 kB
    Private_Hugetlb:       0 kB
    Swap:                  0 kB
    SwapPss:               0 kB
    KernelPageSize:        4 kB
    MMUPageSize:           4 kB
    Locked:                0 kB
    THPeligible:           0
    VmFlags: rd ex mr mw me dw

The first of these lines shows the same information as is displayed for
the mapping in /proc/PID/maps.  Following lines show the size of the
mapping (size); the size of each page allocated when backing a VMA
(KernelPageSize), which is usually the same as the size in the page table
entries; the page size used by the MMU when backing a VMA (in most cases,
the same as KernelPageSize); the amount of the mapping that is currently
resident in RAM (RSS); the process's proportional share of this mapping
(PSS); and the number of clean and dirty shared and private pages in the
mapping.

The "proportional set size" (PSS) of a process is the count of pages it has
in memory, where each page is divided by the number of processes sharing it.
So if a process has 1000 pages all to itself, and 1000 shared with one other
process, its PSS will be 1500.  "Pss_Dirty" is the portion of PSS which
consists of dirty pages.  ("Pss_Clean" is not included, but it can be
calculated by subtracting "Pss_Dirty" from "Pss".)

Traditionally, a page is accounted as "private" if it is mapped exactly once,
and a page is accounted as "shared" when mapped multiple times, even when
mapped in the same process multiple times. Note that this accounting is
independent of MAP_SHARED.

In some kernel configurations, the semantics of pages part of a larger
allocation (e.g., THP) can differ: a page is accounted as "private" if all
pages part of the corresponding large allocation are *certainly* mapped in the
same process, even if the page is mapped multiple times in that process. A
page is accounted as "shared" if any page page of the larger allocation
is *maybe* mapped in a different process. In some cases, a large allocation
might be treated as "maybe mapped by multiple processes" even though this
is no longer the case.

Some kernel configurations do not track the precise number of times a page part
of a larger allocation is mapped. In this case, when calculating the PSS, the
average number of mappings per page in this larger allocation might be used
as an approximation for the number of mappings of a page. The PSS calculation
will be imprecise in this case.

"Referenced" indicates the amount of memory currently marked as referenced or
accessed.

"Anonymous" shows the amount of memory that does not belong to any file.  Even
a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
and a page is modified, the file page is replaced by a private anonymous copy.

"KSM" reports how many of the pages are KSM pages. Note that KSM-placed zeropages
are not included, only actual KSM pages.

"LazyFree" shows the amount of memory which is marked by madvise(MADV_FREE).
The memory isn't freed immediately with madvise(). It's freed in memory
pressure if the memory is clean. Please note that the printed value might
be lower than the real value due to optimizations used in the current
implementation. If this is not desirable please file a bug report.

"AnonHugePages" shows the amount of memory backed by transparent hugepage.

"ShmemPmdMapped" shows the amount of shared (shmem/tmpfs) memory backed by
huge pages.

"Shared_Hugetlb" and "Private_Hugetlb" show the amounts of memory backed by
hugetlbfs page which is *not* counted in "RSS" or "PSS" field for historical
reasons. And these are not included in {Shared,Private}_{Clean,Dirty} field.

"Swap" shows how much would-be-anonymous memory is also used, but out on swap.

For shmem mappings, "Swap" includes also the size of the mapped (and not
replaced by copy-on-write) part of the underlying shmem object out on swap.
"SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
does not take into account swapped out page of underlying shmem objects.
"Locked" indicates whether the mapping is locked in memory or not.

"THPeligible" indicates whether the mapping is eligible for allocating
naturally aligned THP pages of any currently enabled size. 1 if true, 0
otherwise.

"VmFlags" field deserves a separate description. This member represents the
kernel flags associated with the particular virtual memory area in two letter
encoded manner. The codes are the following:

    ==    =============================================================
    rd    readable
    wr    writeable
    ex    executable
    sh    shared
    mr    may read
    mw    may write
    me    may execute
    ms    may share
    gd    stack segment growns down
    pf    pure PFN range
    lo    pages are locked in memory
    io    memory mapped I/O area
    sr    sequential read advise provided
    rr    random read advise provided
    dc    do not copy area on fork
    de    do not expand area on remapping
    ac    area is accountable
    nr    swap space is not reserved for the area
    ht    area uses huge tlb pages
    sf    synchronous page fault
    ar    architecture specific flag
    wf    wipe on fork
    dd    do not include area into core dump
    sd    soft dirty flag
    mm    mixed map area
    hg    huge page advise flag
    nh    no huge page advise flag
    mg    mergeable advise flag
    bt    arm64 BTI guarded page
    mt    arm64 MTE allocation tags are enabled
    um    userfaultfd missing tracking
    uw    userfaultfd wr-protect tracking
    ui    userfaultfd minor fault
    ss    shadow/guarded control stack page
    sl    sealed
    lf    lock on fault pages
    dp    always lazily freeable mapping
    gu    maybe contains guard regions (if not set, definitely doesn't)
    ==    =============================================================

Note that there is no guarantee that every flag and associated mnemonic will
be present in all further kernel releases. Things get changed, the flags may
be vanished or the reverse -- new added. Interpretation of their meaning
might change in future as well. So each consumer of these flags has to
follow each specific kernel version for the exact semantic.

This file is only present if the CONFIG_MMU kernel configuration option is
enabled.

Note: reading /proc/PID/maps or /proc/PID/smaps is inherently racy (consistent
output can be achieved only in the single read call).

This typically manifests when doing partial reads of these files while the
memory map is being modified.  Despite the races, we do provide the following
guarantees:

1) The mapped addresses never go backwards, which implies no two
   regions will ever overlap.
2) If there is something at a given vaddr during the entirety of the
   life of the smaps/maps walk, there will be some output for it.

The /proc/PID/smaps_rollup file includes the same fields as /proc/PID/smaps,
but their values are the sums of the corresponding values for all mappings of
the process.  Additionally, it contains these fields:

- Pss_Anon
- Pss_File
- Pss_Shmem

They represent the proportional shares of anonymous, file, and shmem pages, as
described for smaps above.  These fields are omitted in smaps since each
mapping identifies the type (anon, file, or shmem) of all pages it contains.
Thus all information in smaps_rollup can be derived from smaps, but at a
significantly higher cost.

The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
bits on both physical and virtual pages associated with a process, and the
soft-dirty bit on pte (see Documentation/admin-guide/mm/soft-dirty.rst
for details).
To clear the bits for all the pages associated with the process::

    > echo 1 > /proc/PID/clear_refs

To clear the bits for the anonymous pages associated with the process::

    > echo 2 > /proc/PID/clear_refs

To clear the bits for the file mapped pages associated with the process::

    > echo 3 > /proc/PID/clear_refs

To clear the soft-dirty bit::

    > echo 4 > /proc/PID/clear_refs

To reset the peak resident set size ("high water mark") to the process's
current value::

    > echo 5 > /proc/PID/clear_refs

Any other value written to /proc/PID/clear_refs will have no effect.

The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags
using /proc/kpageflags and number of times a page is mapped using
/proc/kpagecount. For detailed explanation, see
Documentation/admin-guide/mm/pagemap.rst.

The /proc/pid/numa_maps is an extension based on maps, showing the memory
locality and binding policy, as well as the memory usage (in pages) of
each mapping. The output follows a general format where mapping details get
summarized separated by blank spaces, one mapping per each file line::

    address   policy    mapping details

    00400000 default file=/usr/local/bin/app mapped=1 active=0 N3=1 kernelpagesize_kB=4
    00600000 default file=/usr/local/bin/app anon=1 dirty=1 N3=1 kernelpagesize_kB=4
    3206000000 default file=/lib64/ld-2.12.so mapped=26 mapmax=6 N0=24 N3=2 kernelpagesize_kB=4
    320621f000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
    3206220000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
    3206221000 default anon=1 dirty=1 N3=1 kernelpagesize_kB=4
    3206800000 default file=/lib64/libc-2.12.so mapped=59 mapmax=21 active=55 N0=41 N3=18 kernelpagesize_kB=4
    320698b000 default file=/lib64/libc-2.12.so
    3206b8a000 default file=/lib64/libc-2.12.so anon=2 dirty=2 N3=2 kernelpagesize_kB=4
    3206b8e000 default file=/lib64/libc-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
    3206b8f000 default anon=3 dirty=3 active=1 N3=3 kernelpagesize_kB=4
    7f4dc10a2000 default anon=3 dirty=3 N3=3 kernelpagesize_kB=4
    7f4dc10b4000 default anon=2 dirty=2 active=1 N3=2 kernelpagesize_kB=4
    7f4dc1200000 default file=/anon_hugepage\040(deleted) huge anon=1 dirty=1 N3=1 kernelpagesize_kB=2048
    7fff335f0000 default stack anon=3 dirty=3 N3=3 kernelpagesize_kB=4
    7fff3369d000 default mapped=1 mapmax=35 active=0 N3=1 kernelpagesize_kB=4

Where:

"address" is the starting address for the mapping;

"policy" reports the NUMA memory policy set for the mapping (see Documentation/admin-guide/mm/numa_memory_policy.rst);

"mapping details" summarizes mapping data such as mapping type, page usage counters,
node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
size, in KB, that is backing the mapping up.

Note that some kernel configurations do not track the precise number of times
a page part of a larger allocation (e.g., THP) is mapped. In these
configurations, "mapmax" might corresponds to the average number of mappings
per page in such a larger allocation instead.

1.2 Kernel data
---------------

Similar to  the  process entries, the kernel data files give information about
the running kernel. The files used to obtain this information are contained in
/proc and  are  listed  in Table 1-5. Not all of these will be present in your
system. It  depends  on the kernel configuration and the loaded modules, which
files are there, and which are missing.

.. table:: Table 1-5: Kernel info in /proc

 ============ ===============================================================
 File         Content
 ============ ===============================================================
 allocinfo    Memory allocations profiling information
 apm          Advanced power management info
 bootconfig   Kernel command line obtained from boot config,
 	      and, if there were kernel parameters from the
	      boot loader, a "# Parameters from bootloader:"
	      line followed by a line containing those
	      parameters prefixed by "# ".			(5.5)
 buddyinfo    Kernel memory allocator information (see text)	(2.5)
 bus          Directory containing bus specific information
 cmdline      Kernel command line, both from bootloader and embedded
              in the kernel image
 cpuinfo      Info about the CPU
 devices      Available devices (block and character)
 dma          Used DMS channels
 filesystems  Supported filesystems
 driver       Various drivers grouped here, currently rtc	(2.4)
 execdomains  Execdomains, related to security			(2.4)
 fb 	      Frame Buffer devices				(2.4)
 fs 	      File system parameters, currently nfs/exports	(2.4)
 ide          Directory containing info about the IDE subsystem
 interrupts   Interrupt usage
 iomem 	      Memory map					(2.4)
 ioports      I/O port usage
 irq 	      Masks for irq to cpu affinity			(2.4)(smp?)
 isapnp       ISA PnP (Plug&Play) Info				(2.4)
 kcore        Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
 kmsg         Kernel messages
 ksyms        Kernel symbol table
 loadavg      Load average of last 1, 5 & 15 minutes;
                number of processes currently runnable (running or on ready queue);
                total number of processes in system;
                last pid created.
                All fields are separated by one space except "number of
                processes currently runnable" and "total number of processes
                in system", which are separated by a slash ('/'). Example:
                0.61 0.61 0.55 3/828 22084
 locks        Kernel locks
 meminfo      Memory info
 misc         Miscellaneous
 modules      List of loaded modules
 mounts       Mounted filesystems
 net          Networking info (see text)
 pagetypeinfo Additional page allocator information (see text)  (2.5)
 partitions   Table of partitions known to the system
 pci 	      Deprecated info of PCI bus (new way -> /proc/bus/pci/,
              decoupled by lspci				(2.4)
 rtc          Real time clock
 scsi         SCSI info (see text)
 slabinfo     Slab pool info
 softirqs     softirq usage
 stat         Overall statistics
 swaps        Swap space utilization
 sys          See chapter 2
 sysvipc      Info of SysVIPC Resources (msg, sem, shm)		(2.4)
 tty 	      Info of tty drivers
 uptime       Wall clock since boot, combined idle time of all cpus
 version      Kernel version
 video 	      bttv info of video resources			(2.4)
 vmallocinfo  Show vmalloced areas
 ============ ===============================================================

You can,  for  example,  check  which interrupts are currently in use and what
they are used for by looking in the file /proc/interrupts::

  > cat /proc/interrupts
             CPU0
    0:    8728810          XT-PIC  timer
    1:        895          XT-PIC  keyboard
    2:          0          XT-PIC  cascade
    3:     531695          XT-PIC  aha152x
    4:    2014133          XT-PIC  serial
    5:      44401          XT-PIC  pcnet_cs
    8:          2          XT-PIC  rtc
   11:          8          XT-PIC  i82365
   12:     182918          XT-PIC  PS/2 Mouse
   13:          1          XT-PIC  fpu
   14:    1232265          XT-PIC  ide0
   15:          7          XT-PIC  ide1
  NMI:          0

In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
output of a SMP machine)::

  > cat /proc/interrupts

             CPU0       CPU1
    0:    1243498    1214548    IO-APIC-edge  timer
    1:       8949       8958    IO-APIC-edge  keyboard
    2:          0          0          XT-PIC  cascade
    5:      11286      10161    IO-APIC-edge  soundblaster
    8:          1          0    IO-APIC-edge  rtc
    9:      27422      27407    IO-APIC-edge  3c503
   12:     113645     113873    IO-APIC-edge  PS/2 Mouse
   13:          0          0          XT-PIC  fpu
   14:      22491      24012    IO-APIC-edge  ide0
   15:       2183       2415    IO-APIC-edge  ide1
   17:      30564      30414   IO-APIC-level  eth0
   18:        177        164   IO-APIC-level  bttv
  NMI:    2457961    2457959
  LOC:    2457882    2457881
  ERR:       2155

NMI is incremented in this case because every timer interrupt generates a NMI
(Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.

LOC is the local interrupt counter of the internal APIC of every CPU.

ERR is incremented in the case of errors in the IO-APIC bus (the bus that
connects the CPUs in a SMP system. This means that an error has been detected,
the IO-APIC automatically retry the transmission, so it should not be a big
problem, but you should read the SMP-FAQ.

In 2.6.2* /proc/interrupts was expanded again.  This time the goal was for
/proc/interrupts to display every IRQ vector in use by the system, not
just those considered 'most important'.  The new vectors are:

THR
  interrupt raised when a machine check threshold counter
  (typically counting ECC corrected errors of memory or cache) exceeds
  a configurable threshold.  Only available on some systems.

TRM
  a thermal event interrupt occurs when a temperature threshold
  has been exceeded for the CPU.  This interrupt may also be generated
  when the temperature drops back to normal.

SPU
  a spurious interrupt is some interrupt that was raised then lowered
  by some IO device before it could be fully processed by the APIC.  Hence
  the APIC sees the interrupt but does not know what device it came from.
  For this case the APIC will generate the interrupt with a IRQ vector
  of 0xff. This might also be generated by chipset bugs.

RES, CAL, TLB
  rescheduling, call and TLB flush interrupts are
  sent from one CPU to another per the needs of the OS.  Typically,
  their statistics are used by kernel developers and interested users to
  determine the occurrence of interrupts of the given type.

The above IRQ vectors are displayed only when relevant.  For example,
the threshold vector does not exist on x86_64 platforms.  Others are
suppressed when the system is a uniprocessor.  As of this writing, only
i386 and x86_64 platforms support the new IRQ vector displays.

Of some interest is the introduction of the /proc/irq directory to 2.4.
It could be used to set IRQ to CPU affinity. This means that you can "hook" an
IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
irq subdir is one subdir for each IRQ, and two files; default_smp_affinity and
prof_cpu_mask.

For example::

  > ls /proc/irq/
  0  10  12  14  16  18  2  4  6  8  prof_cpu_mask
  1  11  13  15  17  19  3  5  7  9  default_smp_affinity
  > ls /proc/irq/0/
  smp_affinity

smp_affinity is a bitmask, in which you can specify which CPUs can handle the
IRQ. You can set it by doing::

  > echo 1 > /proc/irq/10/smp_affinity

This means that only the first CPU will handle the IRQ, but you can also echo
5 which means that only the first and third CPU can handle the IRQ.

The contents of each smp_affinity file is the same by default::

  > cat /proc/irq/0/smp_affinity
  ffffffff

There is an alternate interface, smp_affinity_list which allows specifying
a CPU range instead of a bitmask::

  > cat /proc/irq/0/smp_affinity_list
  1024-1031

The default_smp_affinity mask applies to all non-active IRQs, which are the
IRQs which have not yet been allocated/activated, and hence which lack a
/proc/irq/[0-9]* directory.

The node file on an SMP system shows the node to which the device using the IRQ
reports itself as being attached. This hardware locality information does not
include information about any possible driver locality preference.

prof_cpu_mask specifies which CPUs are to be profiled by the system wide
profiler. Default value is ffffffff (all CPUs if there are only 32 of them).

The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
between all the CPUs which are allowed to handle it. As usual the kernel has
more info than you and does a better job than you, so the defaults are the
best choice for almost everyone.  [Note this applies only to those IO-APIC's
that support "Round Robin" interrupt distribution.]

There are  three  more  important subdirectories in /proc: net, scsi, and sys.
The general  rule  is  that  the  contents,  or  even  the  existence of these
directories, depend  on your kernel configuration. If SCSI is not enabled, the
directory scsi  may  not  exist. The same is true with the net, which is there
only when networking support is present in the running kernel.

The slabinfo  file  gives  information  about  memory usage at the slab level.
Linux uses  slab  pools for memory management above page level in version 2.2.
Commonly used  objects  have  their  own  slab  pool (such as network buffers,
directory cache, and so on).

::

    > cat /proc/buddyinfo

    Node 0, zone      DMA      0      4      5      4      4      3 ...
    Node 0, zone   Normal      1      0      0      1    101      8 ...
    Node 0, zone  HighMem      2      0      0      1      1      0 ...

External fragmentation is a problem under some workloads, and buddyinfo is a
useful tool for helping diagnose these problems.  Buddyinfo will give you a
clue as to how big an area you can safely allocate, or why a previous
allocation failed.

Each column represents the number of pages of a certain order which are
available.  In this case, there are 0 chunks of 2^0*PAGE_SIZE available in
ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE
available in ZONE_NORMAL, etc...

More information relevant to external fragmentation can be found in
pagetypeinfo::

    > cat /proc/pagetypeinfo
    Page block order: 9
    Pages per block:  512

    Free pages count per migrate type at order       0      1      2      3      4      5      6      7      8      9     10
    Node    0, zone      DMA, type    Unmovable      0      0      0      1      1      1      1      1      1      1      0
    Node    0, zone      DMA, type  Reclaimable      0      0      0      0      0      0      0      0      0      0      0
    Node    0, zone      DMA, type      Movable      1      1      2      1      2      1      1      0      1      0      2
    Node    0, zone      DMA, type      Reserve      0      0      0      0      0      0      0      0      0      1      0
    Node    0, zone      DMA, type      Isolate      0      0      0      0      0      0      0      0      0      0      0
    Node    0, zone    DMA32, type    Unmovable    103     54     77      1      1      1     11      8      7      1      9
    Node    0, zone    DMA32, type  Reclaimable      0      0      2      1      0      0      0      0      1      0      0
    Node    0, zone    DMA32, type      Movable    169    152    113     91     77     54     39     13      6      1    452
    Node    0, zone    DMA32, type      Reserve      1      2      2      2      2      0      1      1      1      1      0
    Node    0, zone    DMA32, type      Isolate      0      0      0      0      0      0      0      0      0      0      0

    Number of blocks type     Unmovable  Reclaimable      Movable      Reserve      Isolate
    Node 0, zone      DMA            2            0            5            1            0
    Node 0, zone    DMA32           41            6          967            2            0

Fragmentation avoidance in the kernel works by grouping pages of different
migrate types into the same contiguous regions of memory called page blocks.
A page block is typically the size of the default hugepage size, e.g. 2MB on
X86-64. By keeping pages grouped based on their ability to move, the kernel
can reclaim pages within a page block to satisfy a high-order allocation.

The pagetypinfo begins with information on the size of a page block. It
then gives the same type of information as buddyinfo except broken down
by migrate-type and finishes with details on how many page blocks of each
type exist.

If min_free_kbytes has been tuned correctly (recommendations made by hugeadm
from libhugetlbfs https://github.com/libhugetlbfs/libhugetlbfs/), one can
make an estimate of the likely number of huge pages that can be allocated
at a given point in time. All the "Movable" blocks should be allocatable
unless memory has been mlock()'d. Some of the Reclaimable blocks should
also be allocatable although a lot of filesystem metadata may have to be
reclaimed to achieve this.


allocinfo
~~~~~~~~~

Provides information about memory allocations at all locations in the code
base. Each allocation in the code is identified by its source file, line
number, module (if originates from a loadable module) and the function calling
the allocation. The number of bytes allocated and number of calls at each
location are reported. The first line indicates the version of the file, the
second line is the header listing fields in the file.
If file version is 2.0 or higher then each line may contain additional
<key>:<value> pairs representing extra information about the call site.
For example if the counters are not accurate, the line will be appended with
"accurate:no" pair.

Supported markers in v2:
accurate:no

              Absolute values of the counters in this line are not accurate
              because of the failure to allocate memory to track some of the
              allocations made at this location.  Deltas in these counters are
              accurate, therefore counters can be used to track allocation size
              and count changes.

Example output.

::

    > tail -n +3 /proc/allocinfo | sort -rn
   127664128    31168 mm/page_ext.c:270 func:alloc_page_ext
    56373248     4737 mm/slub.c:2259 func:alloc_slab_page
    14880768     3633 mm/readahead.c:247 func:page_cache_ra_unbounded
    14417920     3520 mm/mm_init.c:2530 func:alloc_large_system_hash
    13377536      234 block/blk-mq.c:3421 func:blk_mq_alloc_rqs
    11718656     2861 mm/filemap.c:1919 func:__filemap_get_folio
     9192960     2800 kernel/fork.c:307 func:alloc_thread_stack_node
     4206592        4 net/netfilter/nf_conntrack_core.c:2567 func:nf_ct_alloc_hashtable
     4136960     1010 drivers/staging/ctagmod/ctagmod.c:20 [ctagmod] func:ctagmod_start
     3940352      962 mm/memory.c:4214 func:alloc_anon_folio
     2894464    22613 fs/kernfs/dir.c:615 func:__kernfs_new_node
     ...


meminfo
~~~~~~~

Provides information about distribution and utilization of memory.  This
varies by architecture and compile options.  Some of the counters reported
here overlap.  The memory reported by the non overlapping counters may not
add up to the overall memory usage and the difference for some workloads
can be substantial.  In many cases there are other means to find out
additional memory using subsystem specific interfaces, for instance
/proc/net/sockstat for TCP memory allocations.

Example output. You may not have all of these fields.

::

    > cat /proc/meminfo

    MemTotal:       32858820 kB
    MemFree:        21001236 kB
    MemAvailable:   27214312 kB
    Buffers:          581092 kB
    Cached:          5587612 kB
    SwapCached:            0 kB
    Active:          3237152 kB
    Inactive:        7586256 kB
    Active(anon):      94064 kB
    Inactive(anon):  4570616 kB
    Active(file):    3143088 kB
    Inactive(file):  3015640 kB
    Unevictable:           0 kB
    Mlocked:               0 kB
    SwapTotal:             0 kB
    SwapFree:              0 kB
    Zswap:              1904 kB
    Zswapped:           7792 kB
    Dirty:                12 kB
    Writeback:             0 kB
    AnonPages:       4654780 kB
    Mapped:           266244 kB
    Shmem:              9976 kB
    KReclaimable:     517708 kB
    Slab:             660044 kB
    SReclaimable:     517708 kB
    SUnreclaim:       142336 kB
    KernelStack:       11168 kB
    PageTables:        20540 kB
    SecPageTables:         0 kB
    NFS_Unstable:          0 kB
    Bounce:                0 kB
    WritebackTmp:          0 kB
    CommitLimit:    16429408 kB
    Committed_AS:    7715148 kB
    VmallocTotal:   34359738367 kB
    VmallocUsed:       40444 kB
    VmallocChunk:          0 kB
    Percpu:            29312 kB
    EarlyMemtestBad:       0 kB
    HardwareCorrupted:     0 kB
    AnonHugePages:   4149248 kB
    ShmemHugePages:        0 kB
    ShmemPmdMapped:        0 kB
    FileHugePages:         0 kB
    FilePmdMapped:         0 kB
    CmaTotal:              0 kB
    CmaFree:               0 kB
    Unaccepted:            0 kB
    Balloon:               0 kB
    HugePages_Total:       0
    HugePages_Free:        0
    HugePages_Rsvd:        0
    HugePages_Surp:        0
    Hugepagesize:       2048 kB
    Hugetlb:               0 kB
    DirectMap4k:      401152 kB
    DirectMap2M:    10008576 kB
    DirectMap1G:    24117248 kB

MemTotal
              Total usable RAM (i.e. physical RAM minus a few reserved
              bits and the kernel binary code)
MemFree
              Total free RAM. On highmem systems, the sum of LowFree+HighFree
MemAvailable
              An estimate of how much memory is available for starting new
              applications, without swapping. Calculated from MemFree,
              SReclaimable, the size of the file LRU lists, and the low
              watermarks in each zone.
              The estimate takes into account that the system needs some
              page cache to function well, and that not all reclaimable
              slab will be reclaimable, due to items being in use. The
              impact of those factors will vary from system to system.
Buffers
              Relatively temporary storage for raw disk blocks
              shouldn't get tremendously large (20MB or so)
Cached
              In-memory cache for files read from the disk (the
              pagecache) as well as tmpfs & shmem.
              Doesn't include SwapCached.
SwapCached
              Memory that once was swapped out, is swapped back in but
              still also is in the swapfile (if memory is needed it
              doesn't need to be swapped out AGAIN because it is already
              in the swapfile. This saves I/O)
Active
              Memory that has been used more recently and usually not
              reclaimed unless absolutely necessary.
Inactive
              Memory which has been less recently used.  It is more
              eligible to be reclaimed for other purposes
Unevictable
              Memory allocated for userspace which cannot be reclaimed, such
              as mlocked pages, ramfs backing pages, secret memfd pages etc.
Mlocked
              Memory locked with mlock().
HighTotal, HighFree
              Highmem is all memory above ~860MB of physical memory.
              Highmem areas are for use by userspace programs, or
              for the pagecache.  The kernel must use tricks to access
              this memory, making it slower to access than lowmem.
LowTotal, LowFree
              Lowmem is memory which can be used for everything that
              highmem can be used for, but it is also available for the
              kernel's use for its own data structures.  Among many
              other things, it is where everything from the Slab is
              allocated.  Bad things happen when you're out of lowmem.
SwapTotal
              total amount of swap space available
SwapFree
              Memory which has been evicted from RAM, and is temporarily
              on the disk
Zswap
              Memory consumed by the zswap backend (compressed size)
Zswapped
              Amount of anonymous memory stored in zswap (original size)
Dirty
              Memory which is waiting to get written back to the disk
Writeback
              Memory which is actively being written back to the disk
AnonPages
              Non-file backed pages mapped into userspace page tables. Note that
              some kernel configurations might consider all pages part of a
              larger allocation (e.g., THP) as "mapped", as soon as a single
              page is mapped.
Mapped
              files which have been mmapped, such as libraries. Note that some
              kernel configurations might consider all pages part of a larger
              allocation (e.g., THP) as "mapped", as soon as a single page is
              mapped.
Shmem
              Total memory used by shared memory (shmem) and tmpfs
KReclaimable
              Kernel allocations that the kernel will attempt to reclaim
              under memory pressure. Includes SReclaimable (below), and other
              direct allocations with a shrinker.
Slab
              in-kernel data structures cache
SReclaimable
              Part of Slab, that might be reclaimed, such as caches
SUnreclaim
              Part of Slab, that cannot be reclaimed on memory pressure
KernelStack
              Memory consumed by the kernel stacks of all tasks
PageTables
              Memory consumed by userspace page tables
SecPageTables
              Memory consumed by secondary page tables, this currently includes
              KVM mmu and IOMMU allocations on x86 and arm64.
NFS_Unstable
              Always zero. Previously counted pages which had been written to
              the server, but has not been committed to stable storage.
Bounce
              Always zero. Previously memory used for block device
              "bounce buffers".
WritebackTmp
              Always zero. Previously memory used by FUSE for temporary
              writeback buffers.
CommitLimit
              Based on the overcommit ratio ('vm.overcommit_ratio'),
              this is the total amount of  memory currently available to
              be allocated on the system. This limit is only adhered to
              if strict overcommit accounting is enabled (mode 2 in
              'vm.overcommit_memory').

              The CommitLimit is calculated with the following formula::

                CommitLimit = ([total RAM pages] - [total huge TLB pages]) *
                               overcommit_ratio / 100 + [total swap pages]

              For example, on a system with 1G of physical RAM and 7G
              of swap with a `vm.overcommit_ratio` of 30 it would
              yield a CommitLimit of 7.3G.

              For more details, see the memory overcommit documentation
              in mm/overcommit-accounting.
Committed_AS
              The amount of memory presently allocated on the system.
              The committed memory is a sum of all of the memory which
              has been allocated by processes, even if it has not been
              "used" by them as of yet. A process which malloc()'s 1G
              of memory, but only touches 300M of it will show up as
              using 1G. This 1G is memory which has been "committed" to
              by the VM and can be used at any time by the allocating
              application. With strict overcommit enabled on the system
              (mode 2 in 'vm.overcommit_memory'), allocations which would
              exceed the CommitLimit (detailed above) will not be permitted.
              This is useful if one needs to guarantee that processes will
              not fail due to lack of memory once that memory has been
              successfully allocated.
VmallocTotal
              total size of vmalloc virtual address space
VmallocUsed
              amount of vmalloc area which is used
VmallocChunk
              largest contiguous block of vmalloc area which is free
Percpu
              Memory allocated to the percpu allocator used to back percpu
              allocations. This stat excludes the cost of metadata.
EarlyMemtestBad
              The amount of RAM/memory in kB, that was identified as corrupted
              by early memtest. If memtest was not run, this field will not
              be displayed at all. Size is never rounded down to 0 kB.
              That means if 0 kB is reported, you can safely assume
              there was at least one pass of memtest and none of the passes
              found a single faulty byte of RAM.
HardwareCorrupted
              The amount of RAM/memory in KB, the kernel identifies as
              corrupted.
AnonHugePages
              Non-file backed huge pages mapped into userspace page tables
ShmemHugePages
              Memory used by shared memory (shmem) and tmpfs allocated
              with huge pages
ShmemPmdMapped
              Shared memory mapped into userspace with huge pages
FileHugePages
              Memory used for filesystem data (page cache) allocated
              with huge pages
FilePmdMapped
              Page cache mapped into userspace with huge pages
CmaTotal
              Memory reserved for the Contiguous Memory Allocator (CMA)
CmaFree
              Free remaining memory in the CMA reserves
Unaccepted
              Memory that has not been accepted by the guest
Balloon
              Memory returned to Host by VM Balloon Drivers
HugePages_Total, HugePages_Free, HugePages_Rsvd, HugePages_Surp, Hugepagesize, Hugetlb
              See Documentation/admin-guide/mm/hugetlbpage.rst.
DirectMap4k, DirectMap2M, DirectMap1G
              Breakdown of page table sizes used in the kernel's
              identity mapping of RAM

vmallocinfo
~~~~~~~~~~~

Provides information about vmalloced/vmaped areas. One line per area,
containing the virtual address range of the area, size in bytes,
caller information of the creator, and optional information depending
on the kind of area:

 ==========  ===================================================
 pages=nr    number of pages
 phys=addr   if a physical address was specified
 ioremap     I/O mapping (ioremap() and friends)
 vmalloc     vmalloc() area
 vmap        vmap()ed pages
 user        VM_USERMAP area
 vpages      buffer for pages pointers was vmalloced (huge area)
 N<node>=nr  (Only on NUMA kernels)
             Number of pages allocated on memory node <node>
 ==========  ===================================================

::

    > cat /proc/vmallocinfo
    0xffffc20000000000-0xffffc20000201000 2101248 alloc_large_system_hash+0x204 ...
    /0x2c0 pages=512 vmalloc N0=128 N1=128 N2=128 N3=128
    0xffffc20000201000-0xffffc20000302000 1052672 alloc_large_system_hash+0x204 ...
    /0x2c0 pages=256 vmalloc N0=64 N1=64 N2=64 N3=64
    0xffffc20000302000-0xffffc20000304000    8192 acpi_tb_verify_table+0x21/0x4f...
    phys=7fee8000 ioremap
    0xffffc20000304000-0xffffc20000307000   12288 acpi_tb_verify_table+0x21/0x4f...
    phys=7fee7000 ioremap
    0xffffc2000031d000-0xffffc2000031f000    8192 init_vdso_vars+0x112/0x210
    0xffffc2000031f000-0xffffc2000032b000   49152 cramfs_uncompress_init+0x2e ...
    /0x80 pages=11 vmalloc N0=3 N1=3 N2=2 N3=3
    0xffffc2000033a000-0xffffc2000033d000   12288 sys_swapon+0x640/0xac0      ...
    pages=2 vmalloc N1=2
    0xffffc20000347000-0xffffc2000034c000   20480 xt_alloc_table_info+0xfe ...
    /0x130 [x_tables] pages=4 vmalloc N0=4
    0xffffffffa0000000-0xffffffffa000f000   61440 sys_init_module+0xc27/0x1d00 ...
    pages=14 vmalloc N2=14
    0xffffffffa000f000-0xffffffffa0014000   20480 sys_init_module+0xc27/0x1d00 ...
    pages=4 vmalloc N1=4
    0xffffffffa0014000-0xffffffffa0017000   12288 sys_init_module+0xc27/0x1d00 ...
    pages=2 vmalloc N1=2
    0xffffffffa0017000-0xffffffffa0022000   45056 sys_init_module+0xc27/0x1d00 ...
    pages=10 vmalloc N0=10


softirqs
~~~~~~~~

Provides counts of softirq handlers serviced since boot time, for each CPU.

::

    > cat /proc/softirqs
		  CPU0       CPU1       CPU2       CPU3
	HI:          0          0          0          0
    TIMER:       27166      27120      27097      27034
    NET_TX:          0          0          0         17
    NET_RX:         42          0          0         39
    BLOCK:           0          0        107       1121
    TASKLET:         0          0          0        290
    SCHED:       27035      26983      26971      26746
    HRTIMER:         0          0          0          0
	RCU:      1678       1769       2178       2250

1.3 Networking info in /proc/net
--------------------------------

The subdirectory  /proc/net  follows  the  usual  pattern. Table 1-8 shows the
additional values  you  get  for  IP  version 6 if you configure the kernel to
support this. Table 1-9 lists the files and their meaning.


.. table:: Table 1-8: IPv6 info in /proc/net

 ========== =====================================================
 File       Content
 ========== =====================================================
 udp6       UDP sockets (IPv6)
 tcp6       TCP sockets (IPv6)
 raw6       Raw device statistics (IPv6)
 igmp6      IP multicast addresses, which this host joined (IPv6)
 if_inet6   List of IPv6 interface addresses
 ipv6_route Kernel routing table for IPv6
 rt6_stats  Global IPv6 routing tables statistics
 sockstat6  Socket statistics (IPv6)
 snmp6      Snmp data (IPv6)
 ========== =====================================================

.. table:: Table 1-9: Network info in /proc/net

 ============= ================================================================
 File          Content
 ============= ================================================================
 arp           Kernel  ARP table
 dev           network devices with statistics
 dev_mcast     the Layer2 multicast groups a device is listening too
               (interface index, label, number of references, number of bound
               addresses).
 dev_stat      network device status
 ip_fwchains   Firewall chain linkage
 ip_fwnames    Firewall chain names
 ip_masq       Directory containing the masquerading tables
 ip_masquerade Major masquerading table
 netstat       Network statistics
 raw           raw device statistics
 route         Kernel routing table
 rpc           Directory containing rpc info
 rt_cache      Routing cache
 snmp          SNMP data
 sockstat      Socket statistics
 softnet_stat  Per-CPU incoming packets queues statistics of online CPUs
 tcp           TCP  sockets
 udp           UDP sockets
 unix          UNIX domain sockets
 wireless      Wireless interface data (Wavelan etc)
 igmp          IP multicast addresses, which this host joined
 psched        Global packet scheduler parameters.
 netlink       List of PF_NETLINK sockets
 ip_mr_vifs    List of multicast virtual interfaces
 ip_mr_cache   List of multicast routing cache
 ============= ================================================================

You can  use  this  information  to see which network devices are available in
your system and how much traffic was routed over those devices::

  > cat /proc/net/dev
  Inter-|Receive                                                   |[...
   face |bytes    packets errs drop fifo frame compressed multicast|[...
      lo:  908188   5596     0    0    0     0          0         0 [...
    ppp0:15475140  20721   410    0    0   410          0         0 [...
    eth0:  614530   7085     0    0    0     0          0         1 [...

  ...] Transmit
  ...] bytes    packets errs drop fifo colls carrier compressed
  ...]  908188     5596    0    0    0     0       0          0
  ...] 1375103    17405    0    0    0     0       0          0
  ...] 1703981     5535    0    0    0     3       0          0

In addition, each Channel Bond interface has its own directory.  For
example, the bond0 device will have a directory called /proc/net/bond0/.
It will contain information that is specific to that bond, such as the
current slaves of the bond, the link status of the slaves, and how
many times the slaves link has failed.

1.4 SCSI info
-------------

If you have a SCSI or ATA host adapter in your system, you'll find a
subdirectory named after the driver for this adapter in /proc/scsi.
You'll also see a list of all recognized SCSI devices in /proc/scsi::

  >cat /proc/scsi/scsi
  Attached devices:
  Host: scsi0 Channel: 00 Id: 00 Lun: 00
    Vendor: IBM      Model: DGHS09U          Rev: 03E0
    Type:   Direct-Access                    ANSI SCSI revision: 03
  Host: scsi0 Channel: 00 Id: 06 Lun: 00
    Vendor: PIONEER  Model: CD-ROM DR-U06S   Rev: 1.04
    Type:   CD-ROM                           ANSI SCSI revision: 02


The directory  named  after  the driver has one file for each adapter found in
the system.  These  files  contain information about the controller, including
the used  IRQ  and  the  IO  address range. The amount of information shown is
dependent on  the adapter you use. The example shows the output for an Adaptec
AHA-2940 SCSI adapter::

  > cat /proc/scsi/aic7xxx/0

  Adaptec AIC7xxx driver version: 5.1.19/3.2.4
  Compile Options:
    TCQ Enabled By Default : Disabled
    AIC7XXX_PROC_STATS     : Disabled
    AIC7XXX_RESET_DELAY    : 5
  Adapter Configuration:
             SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
                             Ultra Wide Controller
      PCI MMAPed I/O Base: 0xeb001000
   Adapter SEEPROM Config: SEEPROM found and used.
        Adaptec SCSI BIOS: Enabled
                      IRQ: 10
                     SCBs: Active 0, Max Active 2,
                           Allocated 15, HW 16, Page 255
               Interrupts: 160328
        BIOS Control Word: 0x18b6
     Adapter Control Word: 0x005b
     Extended Translation: Enabled
  Disconnect Enable Flags: 0xffff
       Ultra Enable Flags: 0x0001
   Tag Queue Enable Flags: 0x0000
  Ordered Queue Tag Flags: 0x0000
  Default Tag Queue Depth: 8
      Tagged Queue By Device array for aic7xxx host instance 0:
        {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
      Actual queue depth per device for aic7xxx host instance 0:
        {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
  Statistics:
  (scsi0:0:0:0)
    Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
    Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
    Total transfers 160151 (74577 reads and 85574 writes)
  (scsi0:0:6:0)
    Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
    Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
    Total transfers 0 (0 reads and 0 writes)


1.5 Parallel port info in /proc/parport
---------------------------------------

The directory  /proc/parport  contains information about the parallel ports of
your system.  It  has  one  subdirectory  for  each port, named after the port
number (0,1,2,...).

These directories contain the four files shown in Table 1-10.


.. table:: Table 1-10: Files in /proc/parport

 ========= ====================================================================
 File      Content
 ========= ====================================================================
 autoprobe Any IEEE-1284 device ID information that has been acquired.
 devices   list of the device drivers using that port. A + will appear by the
           name of the device currently using the port (it might not appear
           against any).
 hardware  Parallel port's base address, IRQ line and DMA channel.
 irq       IRQ that parport is using for that port. This is in a separate
           file to allow you to alter it by writing a new value in (IRQ
           number or none).
 ========= ====================================================================

1.6 TTY info in /proc/tty
-------------------------

Information about  the  available  and actually used tty's can be found in the
directory /proc/tty. You'll find  entries  for drivers and line disciplines in
this directory, as shown in Table 1-11.


.. table:: Table 1-11: Files in /proc/tty

 ============= ==============================================
 File          Content
 ============= ==============================================
 drivers       list of drivers and their usage
 ldiscs        registered line disciplines
 driver/serial usage statistic and status of single tty lines
 ============= ==============================================

To see  which  tty's  are  currently in use, you can simply look into the file
/proc/tty/drivers::

  > cat /proc/tty/drivers
  pty_slave            /dev/pts      136   0-255 pty:slave
  pty_master           /dev/ptm      128   0-255 pty:master
  pty_slave            /dev/ttyp       3   0-255 pty:slave
  pty_master           /dev/pty        2   0-255 pty:master
  serial               /dev/cua        5   64-67 serial:callout
  serial               /dev/ttyS       4   64-67 serial
  /dev/tty0            /dev/tty0       4       0 system:vtmaster
  /dev/ptmx            /dev/ptmx       5       2 system
  /dev/console         /dev/console    5       1 system:console
  /dev/tty             /dev/tty        5       0 system:/dev/tty
  unknown              /dev/tty        4    1-63 console


1.7 Miscellaneous kernel statistics in /proc/stat
-------------------------------------------------

Various pieces   of  information about  kernel activity  are  available in the
/proc/stat file.  All  of  the numbers reported  in  this file are  aggregates
since the system first booted.  For a quick look, simply cat the file::

  > cat /proc/stat
  cpu  237902850 368826709 106375398 1873517540 1135548 0 14507935 0 0 0
  cpu0 60045249 91891769 26331539 468411416 495718 0 5739640 0 0 0
  cpu1 59746288 91759249 26609887 468860630 312281 0 4384817 0 0 0
  cpu2 59489247 92985423 26904446 467808813 171668 0 2268998 0 0 0
  cpu3 58622065 92190267 26529524 468436680 155879 0 2114478 0 0 0
  intr 8688370575 8 3373 0 0 0 0 0 0 1 40791 0 0 353317 0 0 0 0 224789828 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 190974333 41958554 123983334 43 0 224593 0 0 0 <more 0's deleted>
  ctxt 22848221062
  btime 1605316999
  processes 746787147
  procs_running 2
  procs_blocked 0
  softirq 12121874454 100099120 3938138295 127375644 2795979 187870761 0 173808342 3072582055 52608 224184354

The very first  "cpu" line aggregates the  numbers in all  of the other "cpuN"
lines.  These numbers identify the amount of time the CPU has spent performing
different kinds of work.  Time units are in USER_HZ (typically hundredths of a
second).  The meanings of the columns are as follows, from left to right:

- user: normal processes executing in user mode
- nice: niced processes executing in user mode
- system: processes executing in kernel mode
- idle: twiddling thumbs
- iowait: In a word, iowait stands for waiting for I/O to complete. But there
  are several problems:

  1. CPU will not wait for I/O to complete, iowait is the time that a task is
     waiting for I/O to complete. When CPU goes into idle state for
     outstanding task I/O, another task will be scheduled on this CPU.
  2. In a multi-core CPU, the task waiting for I/O to complete is not running
     on any CPU, so the iowait of each CPU is difficult to calculate.
  3. The value of iowait field in /proc/stat will decrease in certain
     conditions.

  So, the iowait is not reliable by reading from /proc/stat.
- irq: servicing interrupts
- softirq: servicing softirqs
- steal: involuntary wait
- guest: running a normal guest
- guest_nice: running a niced guest

The "intr" line gives counts of interrupts  serviced since boot time, for each
of the  possible system interrupts.   The first  column  is the  total of  all
interrupts serviced  including  unnumbered  architecture specific  interrupts;
each  subsequent column is the  total for that particular numbered interrupt.
Unnumbered interrupts are not shown, only summed into the total.

The "ctxt" line gives the total number of context switches across all CPUs.

The "btime" line gives  the time at which the  system booted, in seconds since
the Unix epoch.

The "processes" line gives the number  of processes and threads created, which
includes (but  is not limited  to) those  created by  calls to the  fork() and
clone() system calls.

The "procs_running" line gives the total number of threads that are
running or ready to run (i.e., the total number of runnable threads).

The   "procs_blocked" line gives  the  number of  processes currently blocked,
waiting for I/O to complete.

The "softirq" line gives counts of softirqs serviced since boot time, for each
of the possible system softirqs. The first column is the total of all
softirqs serviced; each subsequent column is the total for that particular
softirq.


1.8 Ext4 file system parameters
-------------------------------

Information about mounted ext4 file systems can be found in
/proc/fs/ext4.  Each mounted filesystem will have a directory in
/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
/proc/fs/ext4/sda9 or /proc/fs/ext4/dm-0).   The files in each per-device
directory are shown in Table 1-12, below.

.. table:: Table 1-12: Files in /proc/fs/ext4/<devname>

 ==============  ==========================================================
 File            Content
 mb_groups       details of multiblock allocator buddy cache of free blocks
 ==============  ==========================================================

1.9 /proc/consoles
-------------------
Shows registered system console lines.

To see which character device lines are currently used for the system console
/dev/console, you may simply look into the file /proc/consoles::

  > cat /proc/consoles
  tty0                 -WU (ECp)       4:7
  ttyS0                -W- (Ep)        4:64

The columns are:

+--------------------+-------------------------------------------------------+
| device             | name of the device                                    |
+====================+=======================================================+
| operations         | * R = can do read operations                          |
|                    | * W = can do write operations                         |
|                    | * U = can do unblank                                  |
+--------------------+-------------------------------------------------------+
| flags              | * E = it is enabled                                   |
|                    | * C = it is preferred console                         |
|                    | * B = it is primary boot console                      |
|                    | * p = it is used for printk buffer                    |
|                    | * b = it is not a TTY but a Braille device            |
|                    | * a = it is safe to use when cpu is offline           |
+--------------------+-------------------------------------------------------+
| major:minor        | major and minor number of the device separated by a   |
|                    | colon                                                 |
+--------------------+-------------------------------------------------------+

Summary
-------

The /proc file system serves information about the running system. It not only
allows access to process data but also allows you to request the kernel status
by reading files in the hierarchy.

The directory  structure  of /proc reflects the types of information and makes
it easy, if not obvious, where to look for specific data.

Chapter 2: Modifying System Parameters
======================================

In This Chapter
---------------

* Modifying kernel parameters by writing into files found in /proc/sys
* Exploring the files which modify certain parameters
* Review of the /proc/sys file tree

------------------------------------------------------------------------------

A very  interesting part of /proc is the directory /proc/sys. This is not only
a source  of  information,  it also allows you to change parameters within the
kernel. Be  very  careful  when attempting this. You can optimize your system,
but you  can  also  cause  it  to  crash.  Never  alter kernel parameters on a
production system.  Set  up  a  development machine and test to make sure that
everything works  the  way  you want it to. You may have no alternative but to
reboot the machine once an error has been made.

To change  a  value,  simply  echo  the new value into the file.
You need to be root to do this. You  can  create  your  own  boot script
to perform this every time your system boots.

The files  in /proc/sys can be used to fine tune and monitor miscellaneous and
general things  in  the operation of the Linux kernel. Since some of the files
can inadvertently  disrupt  your  system,  it  is  advisable  to  read  both
documentation and  source  before actually making adjustments. In any case, be
very careful  when  writing  to  any  of these files. The entries in /proc may
change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
review the kernel documentation in the directory linux/Documentation.
This chapter  is  heavily  based  on the documentation included in the pre 2.2
kernels, and became part of it in version 2.2.1 of the Linux kernel.

Please see: Documentation/admin-guide/sysctl/ directory for descriptions of
these entries.

Summary
-------

Certain aspects  of  kernel  behavior  can be modified at runtime, without the
need to  recompile  the kernel, or even to reboot the system. The files in the
/proc/sys tree  can  not only be read, but also modified. You can use the echo
command to write value into these files, thereby changing the default settings
of the kernel.


Chapter 3: Per-process Parameters
=================================

3.1 /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj- Adjust the oom-killer score
--------------------------------------------------------------------------------

These files can be used to adjust the badness heuristic used to select which
process gets killed in out of memory (oom) conditions.

The badness heuristic assigns a value to each candidate task ranging from 0
(never kill) to 1000 (always kill) to determine which process is targeted.  The
units are roughly a proportion along that range of allowed memory the process
may allocate from based on an estimation of its current memory and swap use.
For example, if a task is using all allowed memory, its badness score will be
1000.  If it is using half of its allowed memory, its score will be 500.

The amount of "allowed" memory depends on the context in which the oom killer
was called.  If it is due to the memory assigned to the allocating task's cpuset
being exhausted, the allowed memory represents the set of mems assigned to that
cpuset.  If it is due to a mempolicy's node(s) being exhausted, the allowed
memory represents the set of mempolicy nodes.  If it is due to a memory
limit (or swap limit) being reached, the allowed memory is that configured
limit.  Finally, if it is due to the entire system being out of memory, the
allowed memory represents all allocatable resources.

The value of /proc/<pid>/oom_score_adj is added to the badness score before it
is used to determine which task to kill.  Acceptable values range from -1000
(OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX).  This allows userspace to
polarize the preference for oom killing either by always preferring a certain
task or completely disabling it.  The lowest possible value, -1000, is
equivalent to disabling oom killing entirely for that task since it will always
report a badness score of 0.

Consequently, it is very simple for userspace to define the amount of memory to
consider for each task.  Setting a /proc/<pid>/oom_score_adj value of +500, for
example, is roughly equivalent to allowing the remainder of tasks sharing the
same system, cpuset, mempolicy, or memory controller resources to use at least
50% more memory.  A value of -500, on the other hand, would be roughly
equivalent to discounting 50% of the task's allowed memory from being considered
as scoring against the task.

For backwards compatibility with previous kernels, /proc/<pid>/oom_adj may also
be used to tune the badness score.  Its acceptable values range from -16
(OOM_ADJUST_MIN) to +15 (OOM_ADJUST_MAX) and a special value of -17
(OOM_DISABLE) to disable oom killing entirely for that task.  Its value is
scaled linearly with /proc/<pid>/oom_score_adj.

The value of /proc/<pid>/oom_score_adj may be reduced no lower than the last
value set by a CAP_SYS_RESOURCE process. To reduce the value any lower
requires CAP_SYS_RESOURCE.


3.2 /proc/<pid>/oom_score - Display current oom-killer score
-------------------------------------------------------------

This file can be used to check the current score used by the oom-killer for
any given <pid>. Use it together with /proc/<pid>/oom_score_adj to tune which
process should be killed in an out-of-memory situation.

Please note that the exported value includes oom_score_adj so it is
effectively in range [0,2000].


3.3  /proc/<pid>/io - Display the IO accounting fields
-------------------------------------------------------

This file contains IO statistics for each running process.

Example
~~~~~~~

::

    test:/tmp # dd if=/dev/zero of=/tmp/test.dat &
    [1] 3828

    test:/tmp # cat /proc/3828/io
    rchar: 323934931
    wchar: 323929600
    syscr: 632687
    syscw: 632675
    read_bytes: 0
    write_bytes: 323932160
    cancelled_write_bytes: 0


Description
~~~~~~~~~~~

rchar
^^^^^

I/O counter: chars read
The number of bytes which this task has caused to be read from storage. This
is simply the sum of bytes which this process passed to read() and pread().
It includes things like tty IO and it is unaffected by whether or not actual
physical disk IO was required (the read might have been satisfied from
pagecache).


wchar
^^^^^

I/O counter: chars written
The number of bytes which this task has caused, or shall cause to be written
to disk. Similar caveats apply here as with rchar.


syscr
^^^^^

I/O counter: read syscalls
Attempt to count the number of read I/O operations, i.e. syscalls like read()
and pread().


syscw
^^^^^

I/O counter: write syscalls
Attempt to count the number of write I/O operations, i.e. syscalls like
write() and pwrite().


read_bytes
^^^^^^^^^^

I/O counter: bytes read
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer. Done at the submit_bio() level, so it is
accurate for block-backed filesystems. <please add status regarding NFS and
CIFS at a later time>


write_bytes
^^^^^^^^^^^

I/O counter: bytes written
Attempt to count the number of bytes which this process caused to be sent to
the storage layer. This is done at page-dirtying time.


cancelled_write_bytes
^^^^^^^^^^^^^^^^^^^^^

The big inaccuracy here is truncate. If a process writes 1MB to a file and
then deletes the file, it will in fact perform no writeout. But it will have
been accounted as having caused 1MB of write.
In other words: The number of bytes which this process caused to not happen,
by truncating pagecache. A task can cause "negative" IO too. If this task
truncates some dirty pagecache, some IO which another task has been accounted
for (in its write_bytes) will not be happening. We _could_ just subtract that
from the truncating task's write_bytes, but there is information loss in doing
that.


.. Note::

   At its current implementation state, this is a bit racy on 32-bit machines:
   if process A reads process B's /proc/pid/io while process B is updating one
   of those 64-bit counters, process A could see an intermediate result.


More information about this can be found within the taskstats documentation in
Documentation/accounting.

3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
---------------------------------------------------------------
When a process is dumped, all anonymous memory is written to a core file as
long as the size of the core file isn't limited. But sometimes we don't want
to dump some memory segments, for example, huge shared memory or DAX.
Conversely, sometimes we want to save file-backed memory segments into a core
file, not only the individual files.

/proc/<pid>/coredump_filter allows you to customize which memory segments
will be dumped when the <pid> process is dumped. coredump_filter is a bitmask
of memory types. If a bit of the bitmask is set, memory segments of the
corresponding memory type are dumped, otherwise they are not dumped.

The following 9 memory types are supported:

  - (bit 0) anonymous private memory
  - (bit 1) anonymous shared memory
  - (bit 2) file-backed private memory
  - (bit 3) file-backed shared memory
  - (bit 4) ELF header pages in file-backed private memory areas (it is
    effective only if the bit 2 is cleared)
  - (bit 5) hugetlb private memory
  - (bit 6) hugetlb shared memory
  - (bit 7) DAX private memory
  - (bit 8) DAX shared memory

  Note that MMIO pages such as frame buffer are never dumped and vDSO pages
  are always dumped regardless of the bitmask status.

  Note that bits 0-4 don't affect hugetlb or DAX memory. hugetlb memory is
  only affected by bit 5-6, and DAX is only affected by bits 7-8.

The default value of coredump_filter is 0x33; this means all anonymous memory
segments, ELF header pages and hugetlb private memory are dumped.

If you don't want to dump all shared memory segments attached to pid 1234,
write 0x31 to the process's proc file::

  $ echo 0x31 > /proc/1234/coredump_filter

When a new process is created, the process inherits the bitmask status from its
parent. It is useful to set up coredump_filter before the program runs.
For example::

  $ echo 0x7 > /proc/self/coredump_filter
  $ ./some_program

3.5	/proc/<pid>/mountinfo - Information about mounts
--------------------------------------------------------

This file contains lines of the form::

    36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
    (1)(2)(3)   (4)   (5)      (6)     (n…m) (m+1)(m+2) (m+3)         (m+4)

    (1)   mount ID:        unique identifier of the mount (may be reused after umount)
    (2)   parent ID:       ID of parent (or of self for the top of the mount tree)
    (3)   major:minor:     value of st_dev for files on filesystem
    (4)   root:            root of the mount within the filesystem
    (5)   mount point:     mount point relative to the process's root
    (6)   mount options:   per mount options
    (n…m) optional fields: zero or more fields of the form "tag[:value]"
    (m+1) separator:       marks the end of the optional fields
    (m+2) filesystem type: name of filesystem of the form "type[.subtype]"
    (m+3) mount source:    filesystem specific information or "none"
    (m+4) super options:   per super block options

Parsers should ignore all unrecognised optional fields.  Currently the
possible optional fields are:

================  ==============================================================
shared:X          mount is shared in peer group X
master:X          mount is slave to peer group X
propagate_from:X  mount is slave and receives propagation from peer group X [#]_
unbindable        mount is unbindable
================  ==============================================================

.. [#] X is the closest dominant peer group under the process's root.  If
       X is the immediate master of the mount, or if there's no dominant peer
       group under the same root, then only the "master:X" field is present
       and not the "propagate_from:X" field.

For more information on mount propagation see:

  Documentation/filesystems/sharedsubtree.rst


3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
--------------------------------------------------------
These files provide a method to access a task's comm value. It also allows for
a task to set its own or one of its thread siblings comm value. The comm value
is limited in size compared to the cmdline value, so writing anything longer
then the kernel's TASK_COMM_LEN (currently 16 chars, including the NUL
terminator) will result in a truncated comm value.


3.7	/proc/<pid>/task/<tid>/children - Information about task children
-------------------------------------------------------------------------
This file provides a fast way to retrieve first level children pids
of a task pointed by <pid>/<tid> pair. The format is a space separated
stream of pids.

Note the "first level" here -- if a child has its own children they will
not be listed here; one needs to read /proc/<children-pid>/task/<tid>/children
to obtain the descendants.

Since this interface is intended to be fast and cheap it doesn't
guarantee to provide precise results and some children might be
skipped, especially if they've exited right after we printed their
pids, so one needs to either stop or freeze processes being inspected
if precise results are needed.


3.8	/proc/<pid>/fdinfo/<fd> - Information about opened file
---------------------------------------------------------------
This file provides information associated with an opened file. The regular
files have at least four fields -- 'pos', 'flags', 'mnt_id' and 'ino'.
The 'pos' represents the current offset of the opened file in decimal
form [see lseek(2) for details], 'flags' denotes the octal O_xxx mask the
file has been created with [see open(2) for details] and 'mnt_id' represents
mount ID of the file system containing the opened file [see 3.5
/proc/<pid>/mountinfo for details]. 'ino' represents the inode number of
the file.

A typical output is::

	pos:	0
	flags:	0100002
	mnt_id:	19
	ino:	63107

All locks associated with a file descriptor are shown in its fdinfo too::

    lock:       1: FLOCK  ADVISORY  WRITE 359 00:13:11691 0 EOF

The files such as eventfd, fsnotify, signalfd, epoll among the regular pos/flags
pair provide additional information particular to the objects they represent.

Eventfd files
~~~~~~~~~~~~~

::

	pos:	0
	flags:	04002
	mnt_id:	9
	ino:	63107
	eventfd-count:	5a

where 'eventfd-count' is hex value of a counter.

Signalfd files
~~~~~~~~~~~~~~

::

	pos:	0
	flags:	04002
	mnt_id:	9
	ino:	63107
	sigmask:	0000000000000200

where 'sigmask' is hex value of the signal mask associated
with a file.

Epoll files
~~~~~~~~~~~

::

	pos:	0
	flags:	02
	mnt_id:	9
	ino:	63107
	tfd:        5 events:       1d data: ffffffffffffffff pos:0 ino:61af sdev:7

where 'tfd' is a target file descriptor number in decimal form,
'events' is events mask being watched and the 'data' is data
associated with a target [see epoll(7) for more details].

The 'pos' is current offset of the target file in decimal form
[see lseek(2)], 'ino' and 'sdev' are inode and device numbers
where target file resides, all in hex format.

Fsnotify files
~~~~~~~~~~~~~~
For inotify files the format is the following::

	pos:	0
	flags:	02000000
	mnt_id:	9
	ino:	63107
	inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d

where 'wd' is a watch descriptor in decimal form, i.e. a target file
descriptor number, 'ino' and 'sdev' are inode and device where the
target file resides and the 'mask' is the mask of events, all in hex
form [see inotify(7) for more details].

If the kernel was built with exportfs support, the path to the target
file is encoded as a file handle.  The file handle is provided by three
fields 'fhandle-bytes', 'fhandle-type' and 'f_handle', all in hex
format.

If the kernel is built without exportfs support the file handle won't be
printed out.

If there is no inotify mark attached yet the 'inotify' line will be omitted.

For fanotify files the format is::

	pos:	0
	flags:	02
	mnt_id:	9
	ino:	63107
	fanotify flags:10 event-flags:0
	fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
	fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4

where fanotify 'flags' and 'event-flags' are values used in fanotify_init
call, 'mnt_id' is the mount point identifier, 'mflags' is the value of
flags associated with mark which are tracked separately from events
mask. 'ino' and 'sdev' are target inode and device, 'mask' is the events
mask and 'ignored_mask' is the mask of events which are to be ignored.
All are in hex format. Incorporation of 'mflags', 'mask' and 'ignored_mask'
provide information about flags and mask used in fanotify_mark
call [see fsnotify manpage for details].

While the first three lines are mandatory and always printed, the rest is
optional and may be omitted if no marks created yet.

Timerfd files
~~~~~~~~~~~~~

::

	pos:	0
	flags:	02
	mnt_id:	9
	ino:	63107
	clockid: 0
	ticks: 0
	settime flags: 01
	it_value: (0, 49406829)
	it_interval: (1, 0)

where 'clockid' is the clock type and 'ticks' is the number of the timer expirations
that have occurred [see timerfd_create(2) for details]. 'settime flags' are
flags in octal form been used to setup the timer [see timerfd_settime(2) for
details]. 'it_value' is remaining time until the timer expiration.
'it_interval' is the interval for the timer. Note the timer might be set up
with TIMER_ABSTIME option which will be shown in 'settime flags', but 'it_value'
still exhibits timer's remaining time.

DMA Buffer files
~~~~~~~~~~~~~~~~

::

	pos:	0
	flags:	04002
	mnt_id:	9
	ino:	63107
	size:   32768
	count:  2
	exp_name:  system-heap

where 'size' is the size of the DMA buffer in bytes. 'count' is the file count of
the DMA buffer file. 'exp_name' is the name of the DMA buffer exporter.

VFIO Device files
~~~~~~~~~~~~~~~~~

::

	pos:    0
	flags:  02000002
	mnt_id: 17
	ino:    5122
	vfio-device-syspath: /sys/devices/pci0000:e0/0000:e0:01.1/0000:e1:00.0/0000:e2:05.0/0000:e8:00.0

where 'vfio-device-syspath' is the sysfs path corresponding to the VFIO device
file.

3.9	/proc/<pid>/map_files - Information about memory mapped files
---------------------------------------------------------------------
This directory contains symbolic links which represent memory mapped files
the process is maintaining.  Example output::

     | lr-------- 1 root root 64 Jan 27 11:24 333c600000-333c620000 -> /usr/lib64/ld-2.18.so
     | lr-------- 1 root root 64 Jan 27 11:24 333c81f000-333c820000 -> /usr/lib64/ld-2.18.so
     | lr-------- 1 root root 64 Jan 27 11:24 333c820000-333c821000 -> /usr/lib64/ld-2.18.so
     | ...
     | lr-------- 1 root root 64 Jan 27 11:24 35d0421000-35d0422000 -> /usr/lib64/libselinux.so.1
     | lr-------- 1 root root 64 Jan 27 11:24 400000-41a000 -> /usr/bin/ls

The name of a link represents the virtual memory bounds of a mapping, i.e.
vm_area_struct::vm_start-vm_area_struct::vm_end.

The main purpose of the map_files is to retrieve a set of memory mapped
files in a fast way instead of parsing /proc/<pid>/maps or
/proc/<pid>/smaps, both of which contain many more records.  At the same
time one can open(2) mappings from the listings of two processes and
comparing their inode numbers to figure out which anonymous memory areas
are actually shared.

3.10	/proc/<pid>/timerslack_ns - Task timerslack value
---------------------------------------------------------
This file provides the value of the task's timerslack value in nanoseconds.
This value specifies an amount of time that normal timers may be deferred
in order to coalesce timers and avoid unnecessary wakeups.

This allows a task's interactivity vs power consumption tradeoff to be
adjusted.

Writing 0 to the file will set the task's timerslack to the default value.

Valid values are from 0 - ULLONG_MAX

An application setting the value must have PTRACE_MODE_ATTACH_FSCREDS level
permissions on the task specified to change its timerslack_ns value.

3.11	/proc/<pid>/patch_state - Livepatch patch operation state
-----------------------------------------------------------------
When CONFIG_LIVEPATCH is enabled, this file displays the value of the
patch state for the task.

A value of '-1' indicates that no patch is in transition.

A value of '0' indicates that a patch is in transition and the task is
unpatched.  If the patch is being enabled, then the task hasn't been
patched yet.  If the patch is being disabled, then the task has already
been unpatched.

A value of '1' indicates that a patch is in transition and the task is
patched.  If the patch is being enabled, then the task has already been
patched.  If the patch is being disabled, then the task hasn't been
unpatched yet.

3.12 /proc/<pid>/arch_status - task architecture specific status
-------------------------------------------------------------------
When CONFIG_PROC_PID_ARCH_STATUS is enabled, this file displays the
architecture specific status of the task.

Example
~~~~~~~

::

 $ cat /proc/6753/arch_status
 AVX512_elapsed_ms:      8

Description
~~~~~~~~~~~

x86 specific entries
~~~~~~~~~~~~~~~~~~~~~

AVX512_elapsed_ms
^^^^^^^^^^^^^^^^^^

  If AVX512 is supported on the machine, this entry shows the milliseconds
  elapsed since the last time AVX512 usage was recorded. The recording
  happens on a best effort basis when a task is scheduled out. This means
  that the value depends on two factors:

    1) The time which the task spent on the CPU without being scheduled
       out. With CPU isolation and a single runnable task this can take
       several seconds.

    2) The time since the task was scheduled out last. Depending on the
       reason for being scheduled out (time slice exhausted, syscall ...)
       this can be arbitrary long time.

  As a consequence the value cannot be considered precise and authoritative
  information. The application which uses this information has to be aware
  of the overall scenario on the system in order to determine whether a
  task is a real AVX512 user or not. Precise information can be obtained
  with performance counters.

  A special value of '-1' indicates that no AVX512 usage was recorded, thus
  the task is unlikely an AVX512 user, but depends on the workload and the
  scheduling scenario, it also could be a false negative mentioned above.

3.13 /proc/<pid>/fd - List of symlinks to open files
-------------------------------------------------------
This directory contains symbolic links which represent open files
the process is maintaining.  Example output::

  lr-x------ 1 root root 64 Sep 20 17:53 0 -> /dev/null
  l-wx------ 1 root root 64 Sep 20 17:53 1 -> /dev/null
  lrwx------ 1 root root 64 Sep 20 17:53 10 -> 'socket:[12539]'
  lrwx------ 1 root root 64 Sep 20 17:53 11 -> 'socket:[12540]'
  lrwx------ 1 root root 64 Sep 20 17:53 12 -> 'socket:[12542]'

The number of open files for the process is stored in 'size' member
of stat() output for /proc/<pid>/fd for fast access.
-------------------------------------------------------

3.14 /proc/<pid/ksm_stat - Information about the process's ksm status
---------------------------------------------------------------------
When CONFIG_KSM is enabled, each process has this file which displays
the information of ksm merging status.

Example
~~~~~~~

::

    / # cat /proc/self/ksm_stat
    ksm_rmap_items 0
    ksm_zero_pages 0
    ksm_merging_pages 0
    ksm_process_profit 0
    ksm_merge_any: no
    ksm_mergeable: no

Description
~~~~~~~~~~~

ksm_rmap_items
^^^^^^^^^^^^^^

The number of ksm_rmap_item structures in use.  The structure
ksm_rmap_item stores the reverse mapping information for virtual
addresses.  KSM will generate a ksm_rmap_item for each ksm-scanned page of
the process.

ksm_zero_pages
^^^^^^^^^^^^^^

When /sys/kernel/mm/ksm/use_zero_pages is enabled, it represent how many
empty pages are merged with kernel zero pages by KSM.

ksm_merging_pages
^^^^^^^^^^^^^^^^^

It represents how many pages of this process are involved in KSM merging
(not including ksm_zero_pages). It is the same with what
/proc/<pid>/ksm_merging_pages shows.

ksm_process_profit
^^^^^^^^^^^^^^^^^^

The profit that KSM brings (Saved bytes). KSM can save memory by merging
identical pages, but also can consume additional memory, because it needs
to generate a number of rmap_items to save each scanned page's brief rmap
information. Some of these pages may be merged, but some may not be abled
to be merged after being checked several times, which are unprofitable
memory consumed.

ksm_merge_any
^^^^^^^^^^^^^

It specifies whether the process's 'mm is added by prctl() into the
candidate list of KSM or not, and if KSM scanning is fully enabled at
process level.

ksm_mergeable
^^^^^^^^^^^^^

It specifies whether any VMAs of the process''s mms are currently
applicable to KSM.

More information about KSM can be found in
Documentation/admin-guide/mm/ksm.rst.


Chapter 4: Configuring procfs
=============================

4.1	Mount options
---------------------

The following mount options are supported:

	=========	========================================================
	hidepid=	Set /proc/<pid>/ access mode.
	gid=		Set the group authorized to learn processes information.
	subset=		Show only the specified subset of procfs.
	pidns=		Specify a the namespace used by this procfs.
	=========	========================================================

hidepid=off or hidepid=0 means classic mode - everybody may access all
/proc/<pid>/ directories (default).

hidepid=noaccess or hidepid=1 means users may not access any /proc/<pid>/
directories but their own.  Sensitive files like cmdline, sched*, status are now
protected against other users.  This makes it impossible to learn whether any
user runs specific program (given the program doesn't reveal itself by its
behaviour).  As an additional bonus, as /proc/<pid>/cmdline is unaccessible for
other users, poorly written programs passing sensitive information via program
arguments are now protected against local eavesdroppers.

hidepid=invisible or hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be
fully invisible to other users.  It doesn't mean that it hides a fact whether a
process with a specific pid value exists (it can be learned by other means, e.g.
by "kill -0 $PID"), but it hides process's uid and gid, which may be learned by
stat()'ing /proc/<pid>/ otherwise.  It greatly complicates an intruder's task of
gathering information about running processes, whether some daemon runs with
elevated privileges, whether other user runs some sensitive program, whether
other users run any program at all, etc.

hidepid=ptraceable or hidepid=4 means that procfs should only contain
/proc/<pid>/ directories that the caller can ptrace.

gid= defines a group authorized to learn processes information otherwise
prohibited by hidepid=.  If you use some daemon like identd which needs to learn
information about processes information, just add identd to this group.

subset=pid hides all top level files and directories in the procfs that
are not related to tasks.

pidns= specifies a pid namespace (either as a string path to something like
`/proc/$pid/ns/pid`, or a file descriptor when using `FSCONFIG_SET_FD`) that
will be used by the procfs instance when translating pids. By default, procfs
will use the calling process's active pid namespace. Note that the pid
namespace of an existing procfs instance cannot be modified (attempting to do
so will give an `-EBUSY` error).

Chapter 5: Filesystem behavior
==============================

Originally, before the advent of pid namespace, procfs was a global file
system. It means that there was only one procfs instance in the system.

When pid namespace was added, a separate procfs instance was mounted in
each pid namespace. So, procfs mount options are global among all
mountpoints within the same namespace::

	# grep ^proc /proc/mounts
	proc /proc proc rw,relatime,hidepid=2 0 0

	# strace -e mount mount -o hidepid=1 -t proc proc /tmp/proc
	mount("proc", "/tmp/proc", "proc", 0, "hidepid=1") = 0
	+++ exited with 0 +++

	# grep ^proc /proc/mounts
	proc /proc proc rw,relatime,hidepid=2 0 0
	proc /tmp/proc proc rw,relatime,hidepid=2 0 0

and only after remounting procfs mount options will change at all
mountpoints::

	# mount -o remount,hidepid=1 -t proc proc /tmp/proc

	# grep ^proc /proc/mounts
	proc /proc proc rw,relatime,hidepid=1 0 0
	proc /tmp/proc proc rw,relatime,hidepid=1 0 0

This behavior is different from the behavior of other filesystems.

The new procfs behavior is more like other filesystems. Each procfs mount
creates a new procfs instance. Mount options affect own procfs instance.
It means that it became possible to have several procfs instances
displaying tasks with different filtering options in one pid namespace::

	# mount -o hidepid=invisible -t proc proc /proc
	# mount -o hidepid=noaccess -t proc proc /tmp/proc
	# grep ^proc /proc/mounts
	proc /proc proc rw,relatime,hidepid=invisible 0 0
	proc /tmp/proc proc rw,relatime,hidepid=noaccess 0 0