Linux Programmer's Manual
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PROC(5) Linux Programmer's Manual PROC(5)
NAME top
proc - process information pseudo-filesystem
DESCRIPTION top
The proc filesystem is a pseudo-filesystem which provides an
interface to kernel data structures. It is commonly mounted at
/proc. Typically, it is mounted automatically by the system, but it
can also be mounted manually using a command such as:
mount -t proc proc /proc
Most of the files in the proc filesystem are read-only, but some
files are writable, allowing kernel variables to be changed.
Mount options
The proc filesystem supports the following mount options:
hidepid=n (since Linux 3.3)
This option controls who can access the information in
/proc/[pid] directories. The argument, n, is one of the fol‐
lowing values:
0 Everybody may access all /proc/[pid] directories. This is
the traditional behavior, and the default if this mount
option is not specified.
1 Users may not access files and subdirectories inside any
/proc/[pid] directories but their own (the /proc/[pid]
directories themselves remain visible). Sensitive files
such as /proc/[pid]/cmdline and /proc/[pid]/status are now
protected against other users. This makes it impossible
to learn whether any user is running a specific program
(so long as the program doesn't otherwise reveal itself by
its behavior).
2 As for mode 1, but in addition the /proc/[pid] directories
belonging to other users become invisible. This means
that /proc/[pid] entries can no longer be used to discover
the PIDs on the system. This doesn't hide the fact that a
process with a specific PID value exists (it can be
learned by other means, for example, by "kill -0 $PID"),
but it hides a process's UID and GID, which could other‐
wise be learned by employing stat(2) on a /proc/[pid]
directory. This greatly complicates an attacker's task of
gathering information about running processes (e.g., dis‐
covering whether some daemon is running with elevated
privileges, whether another user is running some sensitive
program, whether other users are running any program at
all, and so on).
gid=gid (since Linux 3.3)
Specifies the ID of a group whose members are authorized to
learn process information otherwise prohibited by hidepid
(i.e., users in this group behave as though /proc was mounted
with hidepid=0). This group should be used instead of
approaches such as putting nonroot users into the sudoers(5)
file.
Files and directories
The following list describes many of the files and directories under
the /proc hierarchy.
/proc/[pid]
There is a numerical subdirectory for each running process;
the subdirectory is named by the process ID.
Each /proc/[pid] subdirectory contains the pseudo-files and
directories described below. These files are normally owned
by the effective user and effective group ID of the process.
However, as a security measure, the ownership is made
root:root if the process's "dumpable" attribute is set to a
value other than 1. This attribute may change for the follow‐
ing reasons:
* The attribute was explicitly set via the prctl(2)
PR_SET_DUMPABLE operation.
* The attribute was reset to the value in the file
/proc/sys/fs/suid_dumpable (described below), for the rea‐
sons described in prctl(2).
Resetting the "dumpable" attribute to 1 reverts the ownership
of the /proc/[pid]/* files to the process's real UID and real
GID.
/proc/[pid]/attr
The files in this directory provide an API for security mod‐
ules. The contents of this directory are files that can be
read and written in order to set security-related attributes.
This directory was added to support SELinux, but the intention
was that the API be general enough to support other security
modules. For the purpose of explanation, examples of how
SELinux uses these files are provided below.
This directory is present only if the kernel was configured
with CONFIG_SECURITY.
/proc/[pid]/attr/current (since Linux 2.6.0)
The contents of this file represent the current security
attributes of the process.
In SELinux, this file is used to get the security context of a
process. Prior to Linux 2.6.11, this file could not be used
to set the security context (a write was always denied), since
SELinux limited process security transitions to execve(2) (see
the description of /proc/[pid]/attr/exec, below). Since Linux
2.6.11, SELinux lifted this restriction and began supporting
"set" operations via writes to this node if authorized by pol‐
icy, although use of this operation is only suitable for
applications that are trusted to maintain any desired separa‐
tion between the old and new security contexts. Prior to
Linux 2.6.28, SELinux did not allow threads within a multi-
threaded process to set their security context via this node
as it would yield an inconsistency among the security contexts
of the threads sharing the same memory space. Since Linux
2.6.28, SELinux lifted this restriction and began supporting
"set" operations for threads within a multithreaded process if
the new security context is bounded by the old security con‐
text, where the bounded relation is defined in policy and
guarantees that the new security context has a subset of the
permissions of the old security context. Other security mod‐
ules may choose to support "set" operations via writes to this
node.
/proc/[pid]/attr/exec (since Linux 2.6.0)
This file represents the attributes to assign to the process
upon a subsequent execve(2).
In SELinux, this is needed to support role/domain transitions,
and execve(2) is the preferred point to make such transitions
because it offers better control over the initialization of
the process in the new security label and the inheritance of
state. In SELinux, this attribute is reset on execve(2) so
that the new program reverts to the default behavior for any
execve(2) calls that it may make. In SELinux, a process can
set only its own /proc/[pid]/attr/exec attribute.
/proc/[pid]/attr/fscreate (since Linux 2.6.0)
This file represents the attributes to assign to files created
by subsequent calls to open(2), mkdir(2), symlink(2), and
mknod(2)
SELinux employs this file to support creation of a file (using
the aforementioned system calls) in a secure state, so that
there is no risk of inappropriate access being obtained
between the time of creation and the time that attributes are
set. In SELinux, this attribute is reset on execve(2), so
that the new program reverts to the default behavior for any
file creation calls it may make, but the attribute will per‐
sist across multiple file creation calls within a program
unless it is explicitly reset. In SELinux, a process can set
only its own /proc/[pid]/attr/fscreate attribute.
/proc/[pid]/attr/keycreate (since Linux 2.6.18)
If a process writes a security context into this file, all
subsequently created keys (add_key(2)) will be labeled with
this context. For further information, see the kernel source
file Documentation/security/keys/core.rst (or file Documenta‐
tion/security/keys.txt on Linux between 3.0 and 4.13, or Docu‐
mentation/keys.txt before Linux 3.0).
/proc/[pid]/attr/prev (since Linux 2.6.0)
This file contains the security context of the process before
the last execve(2); that is, the previous value of
/proc/[pid]/attr/current.
/proc/[pid]/attr/socketcreate (since Linux 2.6.18)
If a process writes a security context into this file, all
subsequently created sockets will be labeled with this con‐
text.
/proc/[pid]/autogroup (since Linux 2.6.38)
See sched(7).
/proc/[pid]/auxv (since 2.6.0-test7)
This contains the contents of the ELF interpreter information
passed to the process at exec time. The format is one
unsigned long ID plus one unsigned long value for each entry.
The last entry contains two zeros. See also getauxval(3).
Permission to access this file is governed by a ptrace access
mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
/proc/[pid]/cgroup (since Linux 2.6.24)
See cgroups(7).
/proc/[pid]/clear_refs (since Linux 2.6.22)
This is a write-only file, writable only by owner of the
process.
The following values may be written to the file:
1 (since Linux 2.6.22)
Reset the PG_Referenced and ACCESSED/YOUNG bits for all
the pages associated with the process. (Before kernel
2.6.32, writing any nonzero value to this file had this
effect.)
2 (since Linux 2.6.32)
Reset the PG_Referenced and ACCESSED/YOUNG bits for all
anonymous pages associated with the process.
3 (since Linux 2.6.32)
Reset the PG_Referenced and ACCESSED/YOUNG bits for all
file-mapped pages associated with the process.
Clearing the PG_Referenced and ACCESSED/YOUNG bits provides a
method to measure approximately how much memory a process is
using. One first inspects the values in the "Referenced"
fields for the VMAs shown in /proc/[pid]/smaps to get an idea
of the memory footprint of the process. One then clears the
PG_Referenced and ACCESSED/YOUNG bits and, after some measured
time interval, once again inspects the values in the "Refer‐
enced" fields to get an idea of the change in memory footprint
of the process during the measured interval. If one is inter‐
ested only in inspecting the selected mapping types, then the
value 2 or 3 can be used instead of 1.
Further values can be written to affect different properties:
4 (since Linux 3.11)
Clear the soft-dirty bit for all the pages associated
with the process. This is used (in conjunction with
/proc/[pid]/pagemap) by the check-point restore system
to discover which pages of a process have been dirtied
since the file /proc/[pid]/clear_refs was written to.
5 (since Linux 4.0)
Reset the peak resident set size ("high water mark") to
the process's current resident set size value.
Writing any value to /proc/[pid]/clear_refs other than those
listed above has no effect.
The /proc/[pid]/clear_refs file is present only if the CON‐
FIG_PROC_PAGE_MONITOR kernel configuration option is enabled.
/proc/[pid]/cmdline
This read-only file holds the complete command line for the
process, unless the process is a zombie. In the latter case,
there is nothing in this file: that is, a read on this file
will return 0 characters. The command-line arguments appear
in this file as a set of strings separated by null bytes
('\0'), with a further null byte after the last string.
/proc/[pid]/comm (since Linux 2.6.33)
This file exposes the process's comm value—that is, the com‐
mand name associated with the process. Different threads in
the same process may have different comm values, accessible
via /proc/[pid]/task/[tid]/comm. A thread may modify its comm
value, or that of any of other thread in the same thread group
(see the discussion of CLONE_THREAD in clone(2)), by writing
to the file /proc/self/task/[tid]/comm. Strings longer than
TASK_COMM_LEN (16) characters are silently truncated.
This file provides a superset of the prctl(2) PR_SET_NAME and
PR_GET_NAME operations, and is employed by
pthread_setname_np(3) when used to rename threads other than
the caller.
/proc/[pid]/coredump_filter (since Linux 2.6.23)
See core(5).
/proc/[pid]/cpuset (since Linux 2.6.12)
See cpuset(7).
/proc/[pid]/cwd
This is a symbolic link to the current working directory of
the process. To find out the current working directory of
process 20, for instance, you can do this:
$ cd /proc/20/cwd; /bin/pwd
Note that the pwd command is often a shell built-in, and might
not work properly. In bash(1), you may use pwd -P.
In a multithreaded process, the contents of this symbolic link
are not available if the main thread has already terminated
(typically by calling pthread_exit(3)).
Permission to dereference or read (readlink(2)) this symbolic
link is governed by a ptrace access mode
PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
/proc/[pid]/environ
This file contains the initial environment that was set when
the currently executing program was started via execve(2).
The entries are separated by null bytes ('\0'), and there may
be a null byte at the end. Thus, to print out the environment
of process 1, you would do:
$ strings /proc/1/environ
If, after an execve(2), the process modifies its environment
(e.g., by calling functions such as putenv(3) or modifying the
environ(7) variable directly), this file will not reflect
those changes.
Furthermore, a process may change the memory location that
this file refers via prctl(2) operations such as
PR_SET_MM_ENV_START.
Permission to access this file is governed by a ptrace access
mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
/proc/[pid]/exe
Under Linux 2.2 and later, this file is a symbolic link con‐
taining the actual pathname of the executed command. This
symbolic link can be dereferenced normally; attempting to open
it will open the executable. You can even type
/proc/[pid]/exe to run another copy of the same executable
that is being run by process [pid]. If the pathname has been
unlinked, the symbolic link will contain the string
'(deleted)' appended to the original pathname. In a multi‐
threaded process, the contents of this symbolic link are not
available if the main thread has already terminated (typically
by calling pthread_exit(3)).
Permission to dereference or read (readlink(2)) this symbolic
link is governed by a ptrace access mode
PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
Under Linux 2.0 and earlier, /proc/[pid]/exe is a pointer to
the binary which was executed, and appears as a symbolic link.
A readlink(2) call on this file under Linux 2.0 returns a
string in the format:
[device]:inode
For example, [0301]:1502 would be inode 1502 on device major
03 (IDE, MFM, etc. drives) minor 01 (first partition on the
first drive).
find(1) with the -inum option can be used to locate the file.
/proc/[pid]/fd/
This is a subdirectory containing one entry for each file
which the process has open, named by its file descriptor, and
which is a symbolic link to the actual file. Thus, 0 is stan‐
dard input, 1 standard output, 2 standard error, and so on.
For file descriptors for pipes and sockets, the entries will
be symbolic links whose content is the file type with the
inode. A readlink(2) call on this file returns a string in
the format:
type:[inode]
For example, socket:[2248868] will be a socket and its inode
is 2248868. For sockets, that inode can be used to find more
information in one of the files under /proc/net/.
For file descriptors that have no corresponding inode (e.g.,
file descriptors produced by bpf(2), epoll_create(2),
eventfd(2), inotify_init(2), perf_event_open(2), signalfd(2),
timerfd_create(2), and userfaultfd(2)), the entry will be a
symbolic link with contents of the form
anon_inode:
In many cases (but not all), the file-type is surrounded by
square brackets.
For example, an epoll file descriptor will have a symbolic
link whose content is the string anon_inode:[eventpoll].
In a multithreaded process, the contents of this directory are
not available if the main thread has already terminated (typi‐
cally by calling pthread_exit(3)).
Programs that take a filename as a command-line argument, but
don't take input from standard input if no argument is sup‐
plied, and programs that write to a file named as a command-
line argument, but don't send their output to standard output
if no argument is supplied, can nevertheless be made to use
standard input or standard output by using /proc/[pid]/fd
files as command-line arguments. For example, assuming that
-i is the flag designating an input file and -o is the flag
designating an output file:
$ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...
and you have a working filter.
/proc/self/fd/N is approximately the same as /dev/fd/N in some
UNIX and UNIX-like systems. Most Linux MAKEDEV scripts sym‐
bolically link /dev/fd to /proc/self/fd, in fact.
Most systems provide symbolic links /dev/stdin, /dev/stdout,
and /dev/stderr, which respectively link to the files 0, 1,
and 2 in /proc/self/fd. Thus the example command above could
be written as:
$ foobar -i /dev/stdin -o /dev/stdout ...
Permission to dereference or read (readlink(2)) the symbolic
links in this directory is governed by a ptrace access mode
PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
Note that for file descriptors referring to inodes (pipes and
sockets, see above), those inodes still have permission bits
and ownership information distinct from those of the
/proc/[pid]/fd entry, and that the owner may differ from the
user and group IDs of the process. An unprivileged process
may lack permissions to open them, as in this example:
$ echo test | sudo -u nobody cat
test
$ echo test | sudo -u nobody cat /proc/self/fd/0
cat: /proc/self/fd/0: Permission denied
File descriptor 0 refers to the pipe created by the shell and
owned by that shell's user, which is not nobody, so cat does
not have permission to create a new file descriptor to read
from that inode, even though it can still read from its exist‐
ing file descriptor 0.
/proc/[pid]/fdinfo/ (since Linux 2.6.22)
This is a subdirectory containing one entry for each file
which the process has open, named by its file descriptor. The
files in this directory are readable only by the owner of the
process. The contents of each file can be read to obtain
information about the corresponding file descriptor. The con‐
tent depends on the type of file referred to by the corre‐
sponding file descriptor.
For regular files and directories, we see something like:
$ cat /proc/12015/fdinfo/4
pos: 1000
flags: 01002002
mnt_id: 21
The fields are as follows:
pos This is a decimal number showing the file offset.
flags This is an octal number that displays the file access
mode and file status flags (see open(2)). If the
close-on-exec file descriptor flag is set, then flags
will also include the value O_CLOEXEC.
Before Linux 3.1, this field incorrectly displayed the
setting of O_CLOEXEC at the time the file was opened,
rather than the current setting of the close-on-exec
flag.
mnt_id This field, present since Linux 3.15, is the ID of the
mount point containing this file. See the description
of /proc/[pid]/mountinfo.
For eventfd file descriptors (see eventfd(2)), we see (since
Linux 3.8) the following fields:
pos: 0
flags: 02
mnt_id: 10
eventfd-count: 40
eventfd-count is the current value of the eventfd counter, in
hexadecimal.
For epoll file descriptors (see epoll(7)), we see (since Linux
3.8) the following fields:
pos: 0
flags: 02
mnt_id: 10
tfd: 9 events: 19 data: 74253d2500000009
tfd: 7 events: 19 data: 74253d2500000007
Each of the lines beginning tfd describes one of the file
descriptors being monitored via the epoll file descriptor (see
epoll_ctl(2) for some details). The tfd field is the number
of the file descriptor. The events field is a hexadecimal
mask of the events being monitored for this file descriptor.
The data field is the data value associated with this file
descriptor.
For signalfd file descriptors (see signalfd(2)), we see (since
Linux 3.8) the following fields:
pos: 0
flags: 02
mnt_id: 10
sigmask: 0000000000000006
sigmask is the hexadecimal mask of signals that are accepted
via this signalfd file descriptor. (In this example, bits 2
and 3 are set, corresponding to the signals SIGINT and
SIGQUIT; see signal(7).)
For inotify file descriptors (see inotify(7)), we see (since
Linux 3.8) the following fields:
pos: 0
flags: 00
mnt_id: 11
inotify wd:2 ino:7ef82a sdev:800001 mask:800afff ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:2af87e00220ffd73
inotify wd:1 ino:192627 sdev:800001 mask:800afff ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:27261900802dfd73
Each of the lines beginning with "inotify" displays informa‐
tion about one file or directory that is being monitored. The
fields in this line are as follows:
wd A watch descriptor number (in decimal).
ino The inode number of the target file (in hexadecimal).
sdev The ID of the device where the target file resides (in
hexadecimal).
mask The mask of events being monitored for the target file
(in hexadecimal).
If the kernel was built with exportfs support, the path to the
target file is exposed as a file handle, via three hexadecimal
fields: fhandle-bytes, fhandle-type, and f_handle.
For fanotify file descriptors (see fanotify(7)), we see (since
Linux 3.8) the following fields:
pos: 0
flags: 02
mnt_id: 11
fanotify flags:0 event-flags:88002
fanotify ino:19264f sdev:800001 mflags:0 mask:1 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:4f261900a82dfd73
The fourth line displays information defined when the fanotify
group was created via fanotify_init(2):
flags The flags argument given to fanotify_init(2) (expressed
in hexadecimal).
event-flags
The event_f_flags argument given to fanotify_init(2)
(expressed in hexadecimal).
Each additional line shown in the file contains information
about one of the marks in the fanotify group. Most of these
fields are as for inotify, except:
mflags The flags associated with the mark (expressed in hexa‐
decimal).
mask The events mask for this mark (expressed in hexadeci‐
mal).
ignored_mask
The mask of events that are ignored for this mark
(expressed in hexadecimal).
For details on these fields, see fanotify_mark(2).
/proc/[pid]/gid_map (since Linux 3.5)
See user_namespaces(7).
/proc/[pid]/io (since kernel 2.6.20)
This file contains I/O statistics for the process, for exam‐
ple:
# cat /proc/3828/io
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0
The fields are as follows:
rchar: characters 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(2) and similar system
calls. It includes things such as terminal I/O and is
unaffected by whether or not actual physical disk I/O
was required (the read might have been satisfied from
pagecache).
wchar: characters 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: read syscalls
Attempt to count the number of read I/O operations—that
is, system calls such as read(2) and pread(2).
syscw: write syscalls
Attempt to count the number of write I/O operations—
that is, system calls such as write(2) and pwrite(2).
read_bytes: bytes read
Attempt to count the number of bytes which this process
really did cause to be fetched from the storage layer.
This is accurate for block-backed filesystems.
write_bytes: bytes written
Attempt to count the number of bytes which this process
caused to be sent to the storage layer.
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: this field represents the number of bytes which
this process caused to not happen, by truncating page‐
cache. A task can cause "negative" I/O too. If this
task truncates some dirty pagecache, some I/O which
another task has been accounted for (in its
write_bytes) will not be happening.
Note: In the current implementation, things are a bit racy on
32-bit systems: if process A reads process B's /proc/[pid]/io
while process B is updating one of these 64-bit counters,
process A could see an intermediate result.
Permission to access this file is governed by a ptrace access
mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
/proc/[pid]/limits (since Linux 2.6.24)
This file displays the soft limit, hard limit, and units of
measurement for each of the process's resource limits (see
getrlimit(2)). Up to and including Linux 2.6.35, this file is
protected to allow reading only by the real UID of the
process. Since Linux 2.6.36, this file is readable by all
users on the system.
/proc/[pid]/map_files/ (since kernel 3.3)
This subdirectory contains entries corresponding to memory-
mapped files (see mmap(2)). Entries are named by memory
region start and end address pair (expressed as hexadecimal
numbers), and are symbolic links to the mapped files them‐
selves. Here is an example, with the output wrapped and
reformatted to fit on an 80-column display:
# ls -l /proc/self/map_files/
lr--------. 1 root root 64 Apr 16 21:31
3252e00000-3252e20000 -> /usr/lib64/ld-2.15.so
...
Although these entries are present for memory regions that
were mapped with the MAP_FILE flag, the way anonymous shared
memory (regions created with the MAP_ANON | MAP_SHARED flags)
is implemented in Linux means that such regions also appear on
this directory. Here is an example where the target file is
the deleted /dev/zero one:
lrw-------. 1 root root 64 Apr 16 21:33
7fc075d2f000-7fc075e6f000 -> /dev/zero (deleted)
This directory appears only if the CONFIG_CHECKPOINT_RESTORE
kernel configuration option is enabled. Privilege
(CAP_SYS_ADMIN) is required to view the contents of this
directory.
/proc/[pid]/maps
A file containing the currently mapped memory regions and
their access permissions. See mmap(2) for some further infor‐
mation about memory mappings.
Permission to access this file is governed by a ptrace access
mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
The format of the file is:
address perms offset dev inode pathname
00400000-00452000 r-xp 00000000 08:02 173521 /usr/bin/dbus-daemon
00651000-00652000 r--p 00051000 08:02 173521 /usr/bin/dbus-daemon
00652000-00655000 rw-p 00052000 08:02 173521 /usr/bin/dbus-daemon
00e03000-00e24000 rw-p 00000000 00:00 0 [heap]
00e24000-011f7000 rw-p 00000000 00:00 0 [heap]
...
35b1800000-35b1820000 r-xp 00000000 08:02 135522 /usr/lib64/ld-2.15.so
35b1a1f000-35b1a20000 r--p 0001f000 08:02 135522 /usr/lib64/ld-2.15.so
35b1a20000-35b1a21000 rw-p 00020000 08:02 135522 /usr/lib64/ld-2.15.so
35b1a21000-35b1a22000 rw-p 00000000 00:00 0
35b1c00000-35b1dac000 r-xp 00000000 08:02 135870 /usr/lib64/libc-2.15.so
35b1dac000-35b1fac000 ---p 001ac000 08:02 135870 /usr/lib64/libc-2.15.so
35b1fac000-35b1fb0000 r--p 001ac000 08:02 135870 /usr/lib64/libc-2.15.so
35b1fb0000-35b1fb2000 rw-p 001b0000 08:02 135870 /usr/lib64/libc-2.15.so
...
f2c6ff8c000-7f2c7078c000 rw-p 00000000 00:00 0 [stack:986]
...
7fffb2c0d000-7fffb2c2e000 rw-p 00000000 00:00 0 [stack]
7fffb2d48000-7fffb2d49000 r-xp 00000000 00:00 0 [vdso]
The address field is the address space in the process that the
mapping occupies. The perms field is a set of permissions:
r = read
w = write
x = execute
s = shared
p = private (copy on write)
The offset field is the offset into the file/whatever; dev is
the device (major:minor); inode is the inode on that device.
0 indicates that no inode is associated with the memory
region, as would be the case with BSS (uninitialized data).
The pathname field will usually be the file that is backing
the mapping. For ELF files, you can easily coordinate with
the offset field by looking at the Offset field in the ELF
program headers (readelf -l).
There are additional helpful pseudo-paths:
[stack]
The initial process's (also known as the main
thread's) stack.
[stack:] (since Linux 3.4)
A thread's stack (where the reached"), try increasing this value:
echo 100000 > /proc/sys/fs/file-max
Privileged processes (CAP_SYS_ADMIN) can override the file-max
limit.
/proc/sys/fs/file-nr
This (read-only) file contains three numbers: the number of
allocated file handles (i.e., the number of files presently
opened); the number of free file handles; and the maximum num‐
ber of file handles (i.e., the same value as
/proc/sys/fs/file-max). If the number of allocated file han‐
dles is close to the maximum, you should consider increasing
the maximum. Before Linux 2.6, the kernel allocated file han‐
dles dynamically, but it didn't free them again. Instead the
free file handles were kept in a list for reallocation; the
"free file handles" value indicates the size of that list. A
large number of free file handles indicates that there was a
past peak in the usage of open file handles. Since Linux 2.6,
the kernel does deallocate freed file handles, and the "free
file handles" value is always zero.
/proc/sys/fs/inode-max (only present until Linux 2.2)
This file contains the maximum number of in-memory inodes.
This value should be 3–4 times larger than the value in file-
max, since stdin, stdout and network sockets also need an
inode to handle them. When you regularly run out of inodes,
you need to increase this value.
Starting with Linux 2.4, there is no longer a static limit on
the number of inodes, and this file is removed.
/proc/sys/fs/inode-nr
This file contains the first two values from inode-state.
/proc/sys/fs/inode-state
This file contains seven numbers: nr_inodes, nr_free_inodes,
preshrink, and four dummy values (always zero).
nr_inodes is the number of inodes the system has allocated.
nr_free_inodes represents the number of free inodes.
preshrink is nonzero when the nr_inodes > inode-max and the
system needs to prune the inode list instead of allocating
more; since Linux 2.4, this field is a dummy value (always
zero).
/proc/sys/fs/inotify (since Linux 2.6.13)
This directory contains files max_queued_events,
max_user_instances, and max_user_watches, that can be used to
limit the amount of kernel memory consumed by the inotify
interface. For further details, see inotify(7).
/proc/sys/fs/lease-break-time
This file specifies the grace period that the kernel grants to
a process holding a file lease (fcntl(2)) after it has sent a
signal to that process notifying it that another process is
waiting to open the file. If the lease holder does not remove
or downgrade the lease within this grace period, the kernel
forcibly breaks the lease.
/proc/sys/fs/leases-enable
This file can be used to enable or disable file leases
(fcntl(2)) on a system-wide basis. If this file contains the
value 0, leases are disabled. A nonzero value enables leases.
/proc/sys/fs/mount-max (since Linux 4.9)
The value in this file specifies the maximum number of mounts
that may exist in a mount namespace. The default value in
this file is 100,000.
/proc/sys/fs/mqueue (since Linux 2.6.6)
This directory contains files msg_max, msgsize_max, and
queues_max, controlling the resources used by POSIX message
queues. See mq_overview(7) for details.
/proc/sys/fs/nr_open (since Linux 2.6.25)
This file imposes ceiling on the value to which the
RLIMIT_NOFILE resource limit can be raised (see getrlimit(2)).
This ceiling is enforced for both unprivileged and privileged
process. The default value in this file is 1048576. (Before
Linux 2.6.25, the ceiling for RLIMIT_NOFILE was hard-coded to
the same value.)
/proc/sys/fs/overflowgid and /proc/sys/fs/overflowuid
These files allow you to change the value of the fixed UID and
GID. The default is 65534. Some filesystems support only
16-bit UIDs and GIDs, although in Linux UIDs and GIDs are 32
bits. When one of these filesystems is mounted with writes
enabled, any UID or GID that would exceed 65535 is translated
to the overflow value before being written to disk.
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
See pipe(7).
/proc/sys/fs/pipe-user-pages-hard (since Linux 4.5)
See pipe(7).
/proc/sys/fs/pipe-user-pages-soft (since Linux 4.5)
See pipe(7).
/proc/sys/fs/protected_hardlinks (since Linux 3.6)
When the value in this file is 0, no restrictions are placed
on the creation of hard links (i.e., this is the historical
behavior before Linux 3.6). When the value in this file is 1,
a hard link can be created to a target file only if one of the
following conditions is true:
* The calling process has the CAP_FOWNER capability in its
user namespace and the file UID has a mapping in the names‐
pace.
* The filesystem UID of the process creating the link matches
the owner (UID) of the target file (as described in
credentials(7), a process's filesystem UID is normally the
same as its effective UID).
* All of the following conditions are true:
· the target is a regular file;
· the target file does not have its set-user-ID mode bit
enabled;
· the target file does not have both its set-group-ID and
group-executable mode bits enabled; and
· the caller has permission to read and write the target
file (either via the file's permissions mask or because
it has suitable capabilities).
The default value in this file is 0. Setting the value to 1
prevents a longstanding class of security issues caused by
hard-link-based time-of-check, time-of-use races, most com‐
monly seen in world-writable directories such as /tmp. The
common method of exploiting this flaw is to cross privilege
boundaries when following a given hard link (i.e., a root
process follows a hard link created by another user). Addi‐
tionally, on systems without separated partitions, this stops
unauthorized users from "pinning" vulnerable set-user-ID and
set-group-ID files against being upgraded by the administra‐
tor, or linking to special files.
/proc/sys/fs/protected_symlinks (since Linux 3.6)
When the value in this file is 0, no restrictions are placed
on following symbolic links (i.e., this is the historical
behavior before Linux 3.6). When the value in this file is 1,
symbolic links are followed only in the following circum‐
stances:
* the filesystem UID of the process following the link
matches the owner (UID) of the symbolic link (as described
in credentials(7), a process's filesystem UID is normally
the same as its effective UID);
* the link is not in a sticky world-writable directory; or
* the symbolic link and its parent directory have the same
owner (UID)
A system call that fails to follow a symbolic link because of
the above restrictions returns the error EACCES in errno.
The default value in this file is 0. Setting the value to 1
avoids a longstanding class of security issues based on time-
of-check, time-of-use races when accessing symbolic links.
/proc/sys/fs/suid_dumpable (since Linux 2.6.13)
The value in this file is assigned to a process's "dumpable"
flag in the circumstances described in prctl(2). In effect,
the value in this file determines whether core dump files are
produced for set-user-ID or otherwise protected/tainted bina‐
ries. The "dumpable" setting also affects the ownership of
files in a process's /proc/[pid] directory, as described
above.
Three different integer values can be specified:
0 (default)
This provides the traditional (pre-Linux 2.6.13) behav‐
ior. A core dump will not be produced for a process
which has changed credentials (by calling seteuid(2),
setgid(2), or similar, or by executing a set-user-ID or
set-group-ID program) or whose binary does not have
read permission enabled.
1 ("debug")
All processes dump core when possible. (Reasons why a
process might nevertheless not dump core are described
in core(5).) The core dump is owned by the filesystem
user ID of the dumping process and no security is
applied. This is intended for system debugging situa‐
tions only: this mode is insecure because it allows
unprivileged users to examine the memory contents of
privileged processes.
2 ("suidsafe")
Any binary which normally would not be dumped (see "0"
above) is dumped readable by root only. This allows
the user to remove the core dump file but not to read
it. For security reasons core dumps in this mode will
not overwrite one another or other files. This mode is
appropriate when administrators are attempting to debug
problems in a normal environment.
Additionally, since Linux 3.6, /proc/sys/ker‐
nel/core_pattern must either be an absolute pathname or
a pipe command, as detailed in core(5). Warnings will
be written to the kernel log if core_pattern does not
follow these rules, and no core dump will be produced.
For details of the effect of a process's "dumpable" setting on
ptrace access mode checking, see ptrace(2).
/proc/sys/fs/super-max
This file controls the maximum number of superblocks, and thus
the maximum number of mounted filesystems the kernel can have.
You need increase only super-max if you need to mount more
filesystems than the current value in super-max allows you to.
/proc/sys/fs/super-nr
This file contains the number of filesystems currently
mounted.
/proc/sys/kernel
This directory contains files controlling a range of kernel
parameters, as described below.
/proc/sys/kernel/acct
This file contains three numbers: highwater, lowwater, and
frequency. If BSD-style process accounting is enabled, these
values control its behavior. If free space on filesystem
where the log lives goes below lowwater percent, accounting
suspends. If free space gets above highwater percent,
accounting resumes. frequency determines how often the kernel
checks the amount of free space (value is in seconds).
Default values are 4, 2 and 30. That is, suspend accounting
if 2% or less space is free; resume it if 4% or more space is
free; consider information about amount of free space valid
for 30 seconds.
/proc/sys/kernel/auto_msgmni (Linux 2.6.27 to 3.18)
From Linux 2.6.27 to 3.18, this file was used to control
recomputing of the value in /proc/sys/kernel/msgmni upon the
addition or removal of memory or upon IPC namespace cre‐
ation/removal. Echoing "1" into this file enabled msgmni
automatic recomputing (and triggered a recomputation of msgmni
based on the current amount of available memory and number of
IPC namespaces). Echoing "0" disabled automatic recomputing.
(Automatic recomputing was also disabled if a value was
explicitly assigned to /proc/sys/kernel/msgmni.) The default
value in auto_msgmni was 1.
Since Linux 3.19, the content of this file has no effect
(because msgmni defaults to near the maximum value possible),
and reads from this file always return the value "0".
/proc/sys/kernel/cap_last_cap (since Linux 3.2)
See capabilities(7).
/proc/sys/kernel/cap-bound (from Linux 2.2 to 2.6.24)
This file holds the value of the kernel capability bounding
set (expressed as a signed decimal number). This set is ANDed
against the capabilities permitted to a process during
execve(2). Starting with Linux 2.6.25, the system-wide capa‐
bility bounding set disappeared, and was replaced by a per-
thread bounding set; see capabilities(7).
/proc/sys/kernel/core_pattern
See core(5).
/proc/sys/kernel/core_pipe_limit
See core(5).
/proc/sys/kernel/core_uses_pid
See core(5).
/proc/sys/kernel/ctrl-alt-del
This file controls the handling of Ctrl-Alt-Del from the key‐
board. When the value in this file is 0, Ctrl-Alt-Del is
trapped and sent to the init(1) program to handle a graceful
restart. When the value is greater than zero, Linux's reac‐
tion to a Vulcan Nerve Pinch (tm) will be an immediate reboot,
without even syncing its dirty buffers. Note: when a program
(like dosemu) has the keyboard in "raw" mode, the ctrl-alt-del
is intercepted by the program before it ever reaches the ker‐
nel tty layer, and it's up to the program to decide what to do
with it.
/proc/sys/kernel/dmesg_restrict (since Linux 2.6.37)
The value in this file determines who can see kernel syslog
contents. A value of 0 in this file imposes no restrictions.
If the value is 1, only privileged users can read the kernel
syslog. (See syslog(2) for more details.) Since Linux 3.4,
only users with the CAP_SYS_ADMIN capability may change the
value in this file.
/proc/sys/kernel/domainname and /proc/sys/kernel/hostname
can be used to set the NIS/YP domainname and the hostname of
your box in exactly the same way as the commands domainname(1)
and hostname(1), that is:
# echo 'darkstar' > /proc/sys/kernel/hostname
# echo 'mydomain' > /proc/sys/kernel/domainname
has the same effect as
# hostname 'darkstar'
# domainname 'mydomain'
Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS (Net‐
work Information Service) or YP (Yellow Pages) domainname.
These two domain names are in general different. For a
detailed discussion see the hostname(1) man page.
/proc/sys/kernel/hotplug
This file contains the path for the hotplug policy agent. The
default value in this file is /sbin/hotplug.
/proc/sys/kernel/htab-reclaim (before Linux 2.4.9.2)
(PowerPC only) If this file is set to a nonzero value, the
PowerPC htab (see kernel file Documentation/pow‐
erpc/ppc_htab.txt) is pruned each time the system hits the
idle loop.
/proc/sys/kernel/keys/*
This directory contains various files that define parameters
and limits for the key-management facility. These files are
described in keyrings(7).
/proc/sys/kernel/kptr_restrict (since Linux 2.6.38)
The value in this file determines whether kernel addresses are
exposed via /proc files and other interfaces. A value of 0 in
this file imposes no restrictions. If the value is 1, kernel
pointers printed using the %pK format specifier will be
replaced with zeros unless the user has the CAP_SYSLOG capa‐
bility. If the value is 2, kernel pointers printed using the
%pK format specifier will be replaced with zeros regardless of
the user's capabilities. The initial default value for this
file was 1, but the default was changed to 0 in Linux 2.6.39.
Since Linux 3.4, only users with the CAP_SYS_ADMIN capability
can change the value in this file.
/proc/sys/kernel/l2cr
(PowerPC only) This file contains a flag that controls the L2
cache of G3 processor boards. If 0, the cache is disabled.
Enabled if nonzero.
/proc/sys/kernel/modprobe
This file contains the path for the kernel module loader. The
default value is /sbin/modprobe. The file is present only if
the kernel is built with the CONFIG_MODULES (CONFIG_KMOD in
Linux 2.6.26 and earlier) option enabled. It is described by
the Linux kernel source file Documentation/kmod.txt (present
only in kernel 2.4 and earlier).
/proc/sys/kernel/modules_disabled (since Linux 2.6.31)
A toggle value indicating if modules are allowed to be loaded
in an otherwise modular kernel. This toggle defaults to off
(0), but can be set true (1). Once true, modules can be nei‐
ther loaded nor unloaded, and the toggle cannot be set back to
false. The file is present only if the kernel is built with
the CONFIG_MODULES option enabled.
/proc/sys/kernel/msgmax (since Linux 2.2)
This file defines a system-wide limit specifying the maximum
number of bytes in a single message written on a System V mes‐
sage queue.
/proc/sys/kernel/msgmni (since Linux 2.4)
This file defines the system-wide limit on the number of mes‐
sage queue identifiers. See also /proc/sys/ker‐
nel/auto_msgmni.
/proc/sys/kernel/msgmnb (since Linux 2.2)
This file defines a system-wide parameter used to initialize
the msg_qbytes setting for subsequently created message
queues. The msg_qbytes setting specifies the maximum number
of bytes that may be written to the message queue.
/proc/sys/kernel/ngroups_max (since Linux 2.6.4)
This is a read-only file that displays the upper limit on the
number of a process's group memberships.
/proc/sys/kernel/ns_last_pid (since Linux 3.3)
See pid_namespaces(7).
/proc/sys/kernel/ostype and /proc/sys/kernel/osrelease
These files give substrings of /proc/version.
/proc/sys/kernel/overflowgid and /proc/sys/kernel/overflowuid
These files duplicate the files /proc/sys/fs/overflowgid and
/proc/sys/fs/overflowuid.
/proc/sys/kernel/panic
This file gives read/write access to the kernel variable
panic_timeout. If this is zero, the kernel will loop on a
panic; if nonzero, it indicates that the kernel should autore‐
boot after this number of seconds. When you use the software
watchdog device driver, the recommended setting is 60.
/proc/sys/kernel/panic_on_oops (since Linux 2.5.68)
This file controls the kernel's behavior when an oops or BUG
is encountered. If this file contains 0, then the system
tries to continue operation. If it contains 1, then the sys‐
tem delays a few seconds (to give klogd time to record the
oops output) and then panics. If the /proc/sys/kernel/panic
file is also nonzero, then the machine will be rebooted.
/proc/sys/kernel/pid_max (since Linux 2.5.34)
This file specifies the value at which PIDs wrap around (i.e.,
the value in this file is one greater than the maximum PID).
PIDs greater than this value are not allocated; thus, the
value in this file also acts as a system-wide limit on the
total number of processes and threads. The default value for
this file, 32768, results in the same range of PIDs as on ear‐
lier kernels. On 32-bit platforms, 32768 is the maximum value
for pid_max. On 64-bit systems, pid_max can be set to any
value up to 2^22 (PID_MAX_LIMIT, approximately 4 million).
/proc/sys/kernel/powersave-nap (PowerPC only)
This file contains a flag. If set, Linux-PPC will use the
"nap" mode of powersaving, otherwise the "doze" mode will be
used.
/proc/sys/kernel/printk
See syslog(2).
/proc/sys/kernel/pty (since Linux 2.6.4)
This directory contains two files relating to the number of
UNIX 98 pseudoterminals (see pts(4)) on the system.
/proc/sys/kernel/pty/max
This file defines the maximum number of pseudoterminals.
/proc/sys/kernel/pty/nr
This read-only file indicates how many pseudoterminals are
currently in use.
/proc/sys/kernel/random
This directory contains various parameters controlling the
operation of the file /dev/random. See random(4) for further
information.
/proc/sys/kernel/random/uuid (since Linux 2.4)
Each read from this read-only file returns a randomly gener‐
ated 128-bit UUID, as a string in the standard UUID format.
/proc/sys/kernel/randomize_va_space (since Linux 2.6.12)
Select the address space layout randomization (ASLR) policy
for the system (on architectures that support ASLR). Three
values are supported for this file:
0 Turn ASLR off. This is the default for architectures that
don't support ASLR, and when the kernel is booted with the
norandmaps parameter.
1 Make the addresses of mmap(2) allocations, the stack, and
the VDSO page randomized. Among other things, this means
that shared libraries will be loaded at randomized
addresses. The text segment of PIE-linked binaries will
also be loaded at a randomized address. This value is the
default if the kernel was configured with CONFIG_COM‐
PAT_BRK.
2 (Since Linux 2.6.25) Also support heap randomization. This
value is the default if the kernel was not configured with
CONFIG_COMPAT_BRK.
/proc/sys/kernel/real-root-dev
This file is documented in the Linux kernel source file Docu‐
mentation/admin-guide/initrd.rst (or Documentation/initrd.txt
before Linux 4.10).
/proc/sys/kernel/reboot-cmd (Sparc only)
This file seems to be a way to give an argument to the SPARC
ROM/Flash boot loader. Maybe to tell it what to do after
rebooting?
/proc/sys/kernel/rtsig-max
(Only in kernels up to and including 2.6.7; see setrlimit(2))
This file can be used to tune the maximum number of POSIX
real-time (queued) signals that can be outstanding in the sys‐
tem.
/proc/sys/kernel/rtsig-nr
(Only in kernels up to and including 2.6.7.) This file shows
the number of POSIX real-time signals currently queued.
/proc/[pid]/sched_autogroup_enabled (since Linux 2.6.38)
See sched(7).
/proc/sys/kernel/sched_child_runs_first (since Linux 2.6.23)
If this file contains the value zero, then, after a fork(2),
the parent is first scheduled on the CPU. If the file con‐
tains a nonzero value, then the child is scheduled first on
the CPU. (Of course, on a multiprocessor system, the parent
and the child might both immediately be scheduled on a CPU.)
/proc/sys/kernel/sched_rr_timeslice_ms (since Linux 3.9)
See sched_rr_get_interval(2).
/proc/sys/kernel/sched_rt_period_us (since Linux 2.6.25)
See sched(7).
/proc/sys/kernel/sched_rt_runtime_us (since Linux 2.6.25)
See sched(7).
/proc/sys/kernel/seccomp (since Linux 4.14)
This directory provides additional seccomp information and
configuration. See seccomp(2) for further details.
/proc/sys/kernel/sem (since Linux 2.4)
This file contains 4 numbers defining limits for System V IPC
semaphores. These fields are, in order:
SEMMSL The maximum semaphores per semaphore set.
SEMMNS A system-wide limit on the number of semaphores in all
semaphore sets.
SEMOPM The maximum number of operations that may be specified
in a semop(2) call.
SEMMNI A system-wide limit on the maximum number of semaphore
identifiers.
/proc/sys/kernel/sg-big-buff
This file shows the size of the generic SCSI device (sg) buf‐
fer. You can't tune it just yet, but you could change it at
compile time by editing include/scsi/sg.h and changing the
value of SG_BIG_BUFF. However, there shouldn't be any reason
to change this value.
/proc/sys/kernel/shm_rmid_forced (since Linux 3.1)
If this file is set to 1, all System V shared memory segments
will be marked for destruction as soon as the number of
attached processes falls to zero; in other words, it is no
longer possible to create shared memory segments that exist
independently of any attached process.
The effect is as though a shmctl(2) IPC_RMID is performed on
all existing segments as well as all segments created in the
future (until this file is reset to 0). Note that existing
segments that are attached to no process will be immediately
destroyed when this file is set to 1. Setting this option
will also destroy segments that were created, but never
attached, upon termination of the process that created the
segment with shmget(2).
Setting this file to 1 provides a way of ensuring that all
System V shared memory segments are counted against the
resource usage and resource limits (see the description of
RLIMIT_AS in getrlimit(2)) of at least one process.
Because setting this file to 1 produces behavior that is non‐
standard and could also break existing applications, the
default value in this file is 0. Set this file to 1 only if
you have a good understanding of the semantics of the applica‐
tions using System V shared memory on your system.
/proc/sys/kernel/shmall (since Linux 2.2)
This file contains the system-wide limit on the total number
of pages of System V shared memory.
/proc/sys/kernel/shmmax (since Linux 2.2)
This file can be used to query and set the run-time limit on
the maximum (System V IPC) shared memory segment size that can
be created. Shared memory segments up to 1GB are now sup‐
ported in the kernel. This value defaults to SHMMAX.
/proc/sys/kernel/shmmni (since Linux 2.4)
This file specifies the system-wide maximum number of System V
shared memory segments that can be created.
/proc/sys/kernel/sysctl_writes_strict (since Linux 3.16)
The value in this file determines how the file offset affects
the behavior of updating entries in files under /proc/sys.
The file has three possible values:
-1 This provides legacy handling, with no printk warnings.
Each write(2) must fully contain the value to be written,
and multiple writes on the same file descriptor will over‐
write the entire value, regardless of the file position.
0 (default) This provides the same behavior as for -1, but
printk warnings are written for processes that perform
writes when the file offset is not 0.
1 Respect the file offset when writing strings into
/proc/sys files. Multiple writes will append to the value
buffer. Anything written beyond the maximum length of the
value buffer will be ignored. Writes to numeric /proc/sys
entries must always be at file offset 0 and the value must
be fully contained in the buffer provided to write(2).
/proc/sys/kernel/sysrq
This file controls the functions allowed to be invoked by the
SysRq key. By default, the file contains 1 meaning that every
possible SysRq request is allowed (in older kernel versions,
SysRq was disabled by default, and you were required to
specifically enable it at run-time, but this is not the case
any more). Possible values in this file are:
0 Disable sysrq completely
1 Enable all functions of sysrq
> 1 Bit mask of allowed sysrq functions, as follows:
2 Enable control of console logging level
4 Enable control of keyboard (SAK, unraw)
8 Enable debugging dumps of processes etc.
16 Enable sync command
32 Enable remount read-only
64 Enable signaling of processes (term, kill, oom-kill)
128 Allow reboot/poweroff
256 Allow nicing of all real-time tasks
This file is present only if the CONFIG_MAGIC_SYSRQ kernel
configuration option is enabled. For further details see the
Linux kernel source file Documentation/admin-guide/sysrq.rst
(or Documentation/sysrq.txt before Linux 4.10).
/proc/sys/kernel/version
This file contains a string such as:
#5 Wed Feb 25 21:49:24 MET 1998
The "#5" means that this is the fifth kernel built from this
source base and the date following it indicates the time the
kernel was built.
/proc/sys/kernel/threads-max (since Linux 2.3.11)
This file specifies the system-wide limit on the number of
threads (tasks) that can be created on the system.
Since Linux 4.1, the value that can be written to threads-max
is bounded. The minimum value that can be written is 20. The
maximum value that can be written is given by the constant
FUTEX_TID_MASK (0x3fffffff). If a value outside of this range
is written to threads-max, the error EINVAL occurs.
The value written is checked against the available RAM pages.
If the thread structures would occupy too much (more than
1/8th) of the available RAM pages, threads-max is reduced
accordingly.
/proc/sys/kernel/yama/ptrace_scope (since Linux 3.5)
See ptrace(2).
/proc/sys/kernel/zero-paged (PowerPC only)
This file contains a flag. When enabled (nonzero), Linux-PPC
will pre-zero pages in the idle loop, possibly speeding up
get_free_pages.
/proc/sys/net
This directory contains networking stuff. Explanations for
some of the files under this directory can be found in tcp(7)
and ip(7).
/proc/sys/net/core/bpf_jit_enable
See bpf(2).
/proc/sys/net/core/somaxconn
This file defines a ceiling value for the backlog argument of
listen(2); see the listen(2) manual page for details.
/proc/sys/proc
This directory may be empty.
/proc/sys/sunrpc
This directory supports Sun remote procedure call for network
filesystem (NFS). On some systems, it is not present.
/proc/sys/user (since Linux 4.9)
See namespaces(7).
/proc/sys/vm
This directory contains files for memory management tuning,
buffer and cache management.
/proc/sys/vm/admin_reserve_kbytes (since Linux 3.10)
This file defines the amount of free memory (in KiB) on the
system that should be reserved for users with the capability
CAP_SYS_ADMIN.
The default value in this file is the minimum of [3% of free
pages, 8MiB] expressed as KiB. The default is intended to
provide enough for the superuser to log in and kill a process,
if necessary, under the default overcommit 'guess' mode (i.e.,
0 in /proc/sys/vm/overcommit_memory).
Systems running in "overcommit never" mode (i.e., 2 in
/proc/sys/vm/overcommit_memory) should increase the value in
this file to account for the full virtual memory size of the
programs used to recover (e.g., login(1) ssh(1), and top(1))
Otherwise, the superuser may not be able to log in to recover
the system. For example, on x86-64 a suitable value is 131072
(128MiB reserved).
Changing the value in this file takes effect whenever an
application requests memory.
/proc/sys/vm/compact_memory (since Linux 2.6.35)
When 1 is written to this file, all zones are compacted such
that free memory is available in contiguous blocks where pos‐
sible. The effect of this action can be seen by examining
/proc/buddyinfo.
Present only if the kernel was configured with CONFIG_COM‐
PACTION.
/proc/sys/vm/drop_caches (since Linux 2.6.16)
Writing to this file causes the kernel to drop clean caches,
dentries, and inodes from memory, causing that memory to
become free. This can be useful for memory management testing
and performing reproducible filesystem benchmarks. Because
writing to this file causes the benefits of caching to be
lost, it can degrade overall system performance.
To free pagecache, use:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes, use:
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes, use:
echo 3 > /proc/sys/vm/drop_caches
Because writing to this file is a nondestructive operation and
dirty objects are not freeable, the user should run sync(1)
first.
/proc/sys/vm/legacy_va_layout (since Linux 2.6.9)
If nonzero, this disables the new 32-bit memory-mapping lay‐
out; the kernel will use the legacy (2.4) layout for all pro‐
cesses.
/proc/sys/vm/memory_failure_early_kill (since Linux 2.6.32)
Control how to kill processes when an uncorrected memory error
(typically a 2-bit error in a memory module) that cannot be
handled by the kernel is detected in the background by hard‐
ware. In some cases (like the page still having a valid copy
on disk), the kernel will handle the failure transparently
without affecting any applications. But if there is no other
up-to-date copy of the data, it will kill processes to prevent
any data corruptions from propagating.
The file has one of the following values:
1: Kill all processes that have the corrupted-and-not-reload‐
able page mapped as soon as the corruption is detected.
Note that this is not supported for a few types of pages,
such as kernel internally allocated data or the swap
cache, but works for the majority of user pages.
0: Unmap the corrupted page from all processes and kill a
process only if it tries to access the page.
The kill is performed using a SIGBUS signal with si_code set
to BUS_MCEERR_AO. Processes can handle this if they want to;
see sigaction(2) for more details.
This feature is active only on architectures/platforms with
advanced machine check handling and depends on the hardware
capabilities.
Applications can override the memory_failure_early_kill set‐
ting individually with the prctl(2) PR_MCE_KILL operation.
Present only if the kernel was configured with CONFIG_MEM‐
ORY_FAILURE.
/proc/sys/vm/memory_failure_recovery (since Linux 2.6.32)
Enable memory failure recovery (when supported by the plat‐
form)
1: Attempt recovery.
0: Always panic on a memory failure.
Present only if the kernel was configured with CONFIG_MEM‐
ORY_FAILURE.
/proc/sys/vm/oom_dump_tasks (since Linux 2.6.25)
Enables a system-wide task dump (excluding kernel threads) to
be produced when the kernel performs an OOM-killing. The dump
includes the following information for each task (thread,
process): thread ID, real user ID, thread group ID (process
ID), virtual memory size, resident set size, the CPU that the
task is scheduled on, oom_adj score (see the description of
/proc/[pid]/oom_adj), and command name. This is helpful to
determine why the OOM-killer was invoked and to identify the
rogue task that caused it.
If this contains the value zero, this information is sup‐
pressed. On very large systems with thousands of tasks, it
may not be feasible to dump the memory state information for
each one. Such systems should not be forced to incur a per‐
formance penalty in OOM situations when the information may
not be desired.
If this is set to nonzero, this information is shown whenever
the OOM-killer actually kills a memory-hogging task.
The default value is 0.
/proc/sys/vm/oom_kill_allocating_task (since Linux 2.6.24)
This enables or disables killing the OOM-triggering task in
out-of-memory situations.
If this is set to zero, the OOM-killer will scan through the
entire tasklist and select a task based on heuristics to kill.
This normally selects a rogue memory-hogging task that frees
up a large amount of memory when killed.
If this is set to nonzero, the OOM-killer simply kills the
task that triggered the out-of-memory condition. This avoids
a possibly expensive tasklist scan.
If /proc/sys/vm/panic_on_oom is nonzero, it takes precedence
over whatever value is used in /proc/sys/vm/oom_kill_allocat‐
ing_task.
The default value is 0.
/proc/sys/vm/overcommit_kbytes (since Linux 3.14)
This writable file provides an alternative to
/proc/sys/vm/overcommit_ratio for controlling the CommitLimit
when /proc/sys/vm/overcommit_memory has the value 2. It
allows the amount of memory overcommitting to be specified as
an absolute value (in kB), rather than as a percentage, as is
done with overcommit_ratio. This allows for finer-grained
control of CommitLimit on systems with extremely large memory
sizes.
Only one of overcommit_kbytes or overcommit_ratio can have an
effect: if overcommit_kbytes has a nonzero value, then it is
used to calculate CommitLimit, otherwise overcommit_ratio is
used. Writing a value to either of these files causes the
value in the other file to be set to zero.
/proc/sys/vm/overcommit_memory
This file contains the kernel virtual memory accounting mode.
Values are:
0: heuristic overcommit (this is the default)
1: always overcommit, never check
2: always check, never overcommit
In mode 0, calls of mmap(2) with MAP_NORESERVE are not
checked, and the default check is very weak, leading to the
risk of getting a process "OOM-killed".
In mode 1, the kernel pretends there is always enough memory,
until memory actually runs out. One use case for this mode is
scientific computing applications that employ large sparse
arrays. In Linux kernel versions before 2.6.0, any nonzero
value implies mode 1.
In mode 2 (available since Linux 2.6), the total virtual
address space that can be allocated (CommitLimit in /proc/mem‐
info) is calculated as
CommitLimit = (total_RAM - total_huge_TLB) *
overcommit_ratio / 100 + total_swap
where:
* total_RAM is the total amount of RAM on the system;
* total_huge_TLB is the amount of memory set aside for
huge pages;
* overcommit_ratio is the value in /proc/sys/vm/overcom‐
mit_ratio; and
* total_swap is the amount of swap space.
For example, on a system with 16GB of physical RAM, 16GB of
swap, no space dedicated to huge pages, and an overcom‐
mit_ratio of 50, this formula yields a CommitLimit of 24GB.
Since Linux 3.14, if the value in /proc/sys/vm/overcom‐
mit_kbytes is nonzero, then CommitLimit is instead calculated
as:
CommitLimit = overcommit_kbytes + total_swap
See also the description of /proc/sys/vm/admiin_reserve_kbytes
and /proc/sys/vm/user_reserve_kbytes.
/proc/sys/vm/overcommit_ratio (since Linux 2.6.0)
This writable file defines a percentage by which memory can be
overcommitted. The default value in the file is 50. See the
description of /proc/sys/vm/overcommit_memory.
/proc/sys/vm/panic_on_oom (since Linux 2.6.18)
This enables or disables a kernel panic in an out-of-memory
situation.
If this file is set to the value 0, the kernel's OOM-killer
will kill some rogue process. Usually, the OOM-killer is able
to kill a rogue process and the system will survive.
If this file is set to the value 1, then the kernel normally
panics when out-of-memory happens. However, if a process lim‐
its allocations to certain nodes using memory policies
(mbind(2) MPOL_BIND) or cpusets (cpuset(7)) and those nodes
reach memory exhaustion status, one process may be killed by
the OOM-killer. No panic occurs in this case: because other
nodes' memory may be free, this means the system as a whole
may not have reached an out-of-memory situation yet.
If this file is set to the value 2, the kernel always panics
when an out-of-memory condition occurs.
The default value is 0. 1 and 2 are for failover of cluster‐
ing. Select either according to your policy of failover.
/proc/sys/vm/swappiness
The value in this file controls how aggressively the kernel
will swap memory pages. Higher values increase aggressive‐
ness, lower values decrease aggressiveness. The default value
is 60.
/proc/sys/vm/user_reserve_kbytes (since Linux 3.10)
Specifies an amount of memory (in KiB) to reserve for user
processes, This is intended to prevent a user from starting a
single memory hogging process, such that they cannot recover
(kill the hog). The value in this file has an effect only
when /proc/sys/vm/overcommit_memory is set to 2 ("overcommit
never" mode). In this case, the system reserves an amount of
memory that is the minimum of [3% of current process size,
user_reserve_kbytes].
The default value in this file is the minimum of [3% of free
pages, 128MiB] expressed as KiB.
If the value in this file is set to zero, then a user will be
allowed to allocate all free memory with a single process
(minus the amount reserved by
/proc/sys/vm/admin_reserve_kbytes). Any subsequent attempts
to execute a command will result in "fork: Cannot allocate
memory".
Changing the value in this file takes effect whenever an
application requests memory.
/proc/sysrq-trigger (since Linux 2.4.21)
Writing a character to this file triggers the same SysRq func‐
tion as typing ALT-SysRq- (see the description of
/proc/sys/kernel/sysrq). This file is normally writable only
by root. For further details see the Linux kernel source file
Documentation/admin-guide/sysrq.rst (or Documenta‐
tion/sysrq.txt before Linux 4.10).
/proc/sysvipc
Subdirectory containing the pseudo-files msg, sem and shm.
These files list the System V Interprocess Communication (IPC)
objects (respectively: message queues, semaphores, and shared
memory) that currently exist on the system, providing similar
information to that available via ipcs(1). These files have
headers and are formatted (one IPC object per line) for easy
understanding. svipc(7) provides further background on the
information shown by these files.
/proc/thread-self (since Linux 3.17)
This directory refers to the thread accessing the /proc
filesystem, and is identical to the /proc/self/task/[tid]
directory named by the process thread ID ([tid]) of the same
thread.
/proc/timer_list (since Linux 2.6.21)
This read-only file exposes a list of all currently pending
(high-resolution) timers, all clock-event sources, and their
parameters in a human-readable form.
/proc/timer_stats (from Linux 2.6.21 until Linux 4.10)
This is a debugging facility to make timer (ab)use in a Linux
system visible to kernel and user-space developers. It can be
used by kernel and user-space developers to verify that their
code does not make undue use of timers. The goal is to avoid
unnecessary wakeups, thereby optimizing power consumption.
If enabled in the kernel (CONFIG_TIMER_STATS), but not used,
it has almost zero runtime overhead and a relatively small
data-structure overhead. Even if collection is enabled at
runtime, overhead is low: all the locking is per-CPU and
lookup is hashed.
The /proc/timer_stats file is used both to control sampling
facility and to read out the sampled information.
The timer_stats functionality is inactive on bootup. A sam‐
pling period can be started using the following command:
# echo 1 > /proc/timer_stats
The following command stops a sampling period:
# echo 0 > /proc/timer_stats
The statistics can be retrieved by:
$ cat /proc/timer_stats
While sampling is enabled, each readout from /proc/timer_stats
will see newly updated statistics. Once sampling is disabled,
the sampled information is kept until a new sample period is
started. This allows multiple readouts.
Sample output from /proc/timer_stats:
$ cat /proc/timer_stats
Timer Stats Version: v0.3
Sample period: 1.764 s
Collection: active
255, 0 swapper/3 hrtimer_start_range_ns (tick_sched_timer)
71, 0 swapper/1 hrtimer_start_range_ns (tick_sched_timer)
58, 0 swapper/0 hrtimer_start_range_ns (tick_sched_timer)
4, 1694 gnome-shell mod_delayed_work_on (delayed_work_timer_fn)
17, 7 rcu_sched rcu_gp_kthread (process_timeout)
...
1, 4911 kworker/u16:0 mod_delayed_work_on (delayed_work_timer_fn)
1D, 2522 kworker/0:0 queue_delayed_work_on (delayed_work_timer_fn)
1029 total events, 583.333 events/sec
The output columns are:
* a count of the number of events, optionally (since Linux
2.6.23) followed by the letter 'D' if this is a deferrable
timer;
* the PID of the process that initialized the timer;
* the name of the process that initialized the timer;
* the function where the timer was initialized; and
* (in parentheses) the callback function that is associated
with the timer.
During the Linux 4.11 development cycle, this file was
removed because of security concerns, as it exposes informa‐
tion across namespaces. Furthermore, it is possible to obtain
the same information via in-kernel tracing facilities such as
ftrace.
/proc/tty
Subdirectory containing the pseudo-files and subdirectories
for tty drivers and line disciplines.
/proc/uptime
This file contains two numbers: the uptime of the system (sec‐
onds), and the amount of time spent in idle process (seconds).
/proc/version
This string identifies the kernel version that is currently
running. It includes the contents of /proc/sys/kernel/ostype,
/proc/sys/kernel/osrelease and /proc/sys/kernel/version. For
example:
Linux version 1.0.9 (quinlan@phaze) #1 Sat May 14 01:51:54 EDT 1994
/proc/vmstat (since Linux 2.6.0)
This file displays various virtual memory statistics. Each
line of this file contains a single name-value pair, delimited
by white space. Some lines are present only if the kernel was
configured with suitable options. (In some cases, the options
required for particular files have changed across kernel ver‐
sions, so they are not listed here. Details can be found by
consulting the kernel source code.) The following fields may
be present:
nr_free_pages (since Linux 2.6.31)
nr_alloc_batch (since Linux 3.12)
nr_inactive_anon (since Linux 2.6.28)
nr_active_anon (since Linux 2.6.28)
nr_inactive_file (since Linux 2.6.28)
nr_active_file (since Linux 2.6.28)
nr_unevictable (since Linux 2.6.28)
nr_mlock (since Linux 2.6.28)
nr_anon_pages (since Linux 2.6.18)
nr_mapped (since Linux 2.6.0)
nr_file_pages (since Linux 2.6.18)
nr_dirty (since Linux 2.6.0)
nr_writeback (since Linux 2.6.0)
nr_slab_reclaimable (since Linux 2.6.19)
nr_slab_unreclaimable (since Linux 2.6.19)
nr_page_table_pages (since Linux 2.6.0)
nr_kernel_stack (since Linux 2.6.32)
Amount of memory allocated to kernel stacks.
nr_unstable (since Linux 2.6.0)
nr_bounce (since Linux 2.6.12)
nr_vmscan_write (since Linux 2.6.19)
nr_vmscan_immediate_reclaim (since Linux 3.2)
nr_writeback_temp (since Linux 2.6.26)
nr_isolated_anon (since Linux 2.6.32)
nr_isolated_file (since Linux 2.6.32)
nr_shmem (since Linux 2.6.32)
Pages used by shmem and tmpfs(5).
nr_dirtied (since Linux 2.6.37)
nr_written (since Linux 2.6.37)
nr_pages_scanned (since Linux 3.17)
numa_hit (since Linux 2.6.18)
numa_miss (since Linux 2.6.18)
numa_foreign (since Linux 2.6.18)
numa_interleave (since Linux 2.6.18)
numa_local (since Linux 2.6.18)
numa_other (since Linux 2.6.18)
workingset_refault (since Linux 3.15)
workingset_activate (since Linux 3.15)
workingset_nodereclaim (since Linux 3.15)
nr_anon_transparent_hugepages (since Linux 2.6.38)
nr_free_cma (since Linux 3.7)
Number of free CMA (Contiguous Memory Allocator) pages.
nr_dirty_threshold (since Linux 2.6.37)
nr_dirty_background_threshold (since Linux 2.6.37)
pgpgin (since Linux 2.6.0)
pgpgout (since Linux 2.6.0)
pswpin (since Linux 2.6.0)
pswpout (since Linux 2.6.0)
pgalloc_dma (since Linux 2.6.5)
pgalloc_dma32 (since Linux 2.6.16)
pgalloc_normal (since Linux 2.6.5)
pgalloc_high (since Linux 2.6.5)
pgalloc_movable (since Linux 2.6.23)
pgfree (since Linux 2.6.0)
pgactivate (since Linux 2.6.0)
pgdeactivate (since Linux 2.6.0)
pgfault (since Linux 2.6.0)
pgmajfault (since Linux 2.6.0)
pgrefill_dma (since Linux 2.6.5)
pgrefill_dma32 (since Linux 2.6.16)
pgrefill_normal (since Linux 2.6.5)
pgrefill_high (since Linux 2.6.5)
pgrefill_movable (since Linux 2.6.23)
pgsteal_kswapd_dma (since Linux 3.4)
pgsteal_kswapd_dma32 (since Linux 3.4)
pgsteal_kswapd_normal (since Linux 3.4)
pgsteal_kswapd_high (since Linux 3.4)
pgsteal_kswapd_movable (since Linux 3.4)
pgsteal_direct_dma
pgsteal_direct_dma32 (since Linux 3.4)
pgsteal_direct_normal (since Linux 3.4)
pgsteal_direct_high (since Linux 3.4)
pgsteal_direct_movable (since Linux 2.6.23)
pgscan_kswapd_dma
pgscan_kswapd_dma32 (since Linux 2.6.16)
pgscan_kswapd_normal (since Linux 2.6.5)
pgscan_kswapd_high
pgscan_kswapd_movable (since Linux 2.6.23)
pgscan_direct_dma
pgscan_direct_dma32 (since Linux 2.6.16)
pgscan_direct_normal
pgscan_direct_high
pgscan_direct_movable (since Linux 2.6.23)
pgscan_direct_throttle (since Linux 3.6)
zone_reclaim_failed (since linux 2.6.31)
pginodesteal (since linux 2.6.0)
slabs_scanned (since linux 2.6.5)
kswapd_inodesteal (since linux 2.6.0)
kswapd_low_wmark_hit_quickly (since 2.6.33)
kswapd_high_wmark_hit_quickly (since 2.6.33)
pageoutrun (since Linux 2.6.0)
allocstall (since Linux 2.6.0)
pgrotated (since Linux 2.6.0)
drop_pagecache (since Linux 3.15)
drop_slab (since Linux 3.15)
numa_pte_updates (since Linux 3.8)
numa_huge_pte_updates (since Linux 3.13)
numa_hint_faults (since Linux 3.8)
numa_hint_faults_local (since Linux 3.8)
numa_pages_migrated (since Linux 3.8)
pgmigrate_success (since Linux 3.8)
pgmigrate_fail (since Linux 3.8)
compact_migrate_scanned (since Linux 3.8)
compact_free_scanned (since Linux 3.8)
compact_isolated (since Linux 3.8)
compact_stall (since Linux 2.6.35)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
compact_fail (since Linux 2.6.35)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
compact_success (since Linux 2.6.35)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
htlb_buddy_alloc_success (since Linux 2.6.26)
htlb_buddy_alloc_fail (since Linux 2.6.26)
unevictable_pgs_culled (since Linux 2.6.28)
unevictable_pgs_scanned (since Linux 2.6.28)
unevictable_pgs_rescued (since Linux 2.6.28)
unevictable_pgs_mlocked (since Linux 2.6.28)
unevictable_pgs_munlocked (since Linux 2.6.28)
unevictable_pgs_cleared (since Linux 2.6.28)
unevictable_pgs_stranded (since Linux 2.6.28)
thp_fault_alloc (since Linux 2.6.39)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_fault_fallback (since Linux 2.6.39)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_collapse_alloc (since Linux 2.6.39)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_collapse_alloc_failed (since Linux 2.6.39)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_split (since Linux 2.6.39)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_zero_page_alloc (since Linux 3.8)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
thp_zero_page_alloc_failed (since Linux 3.8)
See the kernel source file Documentation/vm/tran‐
shuge.txt.
balloon_inflate (since Linux 3.18)
balloon_deflate (since Linux 3.18)
balloon_migrate (since Linux 3.18)
nr_tlb_remote_flush (since Linux 3.12)
nr_tlb_remote_flush_received (since Linux 3.12)
nr_tlb_local_flush_all (since Linux 3.12)
nr_tlb_local_flush_one (since Linux 3.12)
vmacache_find_calls (since Linux 3.16)
vmacache_find_hits (since Linux 3.16)
vmacache_full_flushes (since Linux 3.19)
/proc/zoneinfo (since Linux 2.6.13)
This file display information about memory zones. This is
useful for analyzing virtual memory behavior.
NOTES top
Many strings (i.e., the environment and command line) are in the
internal format, with subfields terminated by null bytes ('\0'), so
you may find that things are more readable if you use od -c or tr
"\000" "\n" to read them. Alternatively, echo `cat ` works
well.
This manual page is incomplete, possibly inaccurate, and is the kind
of thing that needs to be updated very often.
SEE ALSO top
cat(1), dmesg(1), find(1), free(1), init(1), ps(1), tr(1), uptime(1),
chroot(2), mmap(2), readlink(2), syslog(2), slabinfo(5), sysfs(5),
hier(7), namespaces(7), time(7), arp(8), hdparm(8), ifconfig(8),
lsmod(8), lspci(8), mount(8), netstat(8), procinfo(8), route(8),
sysctl(8)
The Linux kernel source files: Documentation/filesystems/proc.txt
Documentation/sysctl/fs.txt, Documentation/sysctl/kernel.txt,
Documentation/sysctl/net.txt, and Documentation/sysctl/vm.txt.
COLOPHON top
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 PROC(5)