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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Documentation for /proc/sys/fs/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
Shen Feng760df932009-04-02 16:57:20 -07003 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
Linus Torvalds1da177e2005-04-16 15:20:36 -07004
5For general info and legal blurb, please look in README.
6
7==============================================================
8
9This file contains documentation for the sysctl files in
10/proc/sys/fs/ and is valid for Linux kernel version 2.2.
11
12The files in this directory can be used to tune and monitor
13miscellaneous and general things in the operation of the Linux
14kernel. Since some of the files _can_ be used to screw up your
15system, it is advisable to read both documentation and source
16before actually making adjustments.
17
Shen Feng760df932009-04-02 16:57:20 -0700181. /proc/sys/fs
19----------------------------------------------------------
20
Linus Torvalds1da177e2005-04-16 15:20:36 -070021Currently, these files are in /proc/sys/fs:
Shen Feng760df932009-04-02 16:57:20 -070022- aio-max-nr
23- aio-nr
Linus Torvalds1da177e2005-04-16 15:20:36 -070024- dentry-state
25- dquot-max
26- dquot-nr
27- file-max
28- file-nr
29- inode-max
30- inode-nr
31- inode-state
Eric Dumazet9cfe0152008-02-06 01:37:16 -080032- nr_open
Linus Torvalds1da177e2005-04-16 15:20:36 -070033- overflowuid
34- overflowgid
Willy Tarreau759c0112016-01-18 16:36:09 +010035- pipe-user-pages-hard
36- pipe-user-pages-soft
Kees Cook800179c2012-07-25 17:29:07 -070037- protected_hardlinks
38- protected_symlinks
Alexey Dobriyana2e0b562006-08-27 01:23:28 -070039- suid_dumpable
Linus Torvalds1da177e2005-04-16 15:20:36 -070040- super-max
41- super-nr
42
Shen Feng760df932009-04-02 16:57:20 -070043==============================================================
44
45aio-nr & aio-max-nr:
46
47aio-nr is the running total of the number of events specified on the
48io_setup system call for all currently active aio contexts. If aio-nr
49reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
50raising aio-max-nr does not result in the pre-allocation or re-sizing
51of any kernel data structures.
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
53==============================================================
54
55dentry-state:
56
57From linux/fs/dentry.c:
58--------------------------------------------------------------
59struct {
60 int nr_dentry;
61 int nr_unused;
62 int age_limit; /* age in seconds */
63 int want_pages; /* pages requested by system */
64 int dummy[2];
65} dentry_stat = {0, 0, 45, 0,};
66--------------------------------------------------------------
67
68Dentries are dynamically allocated and deallocated, and
69nr_dentry seems to be 0 all the time. Hence it's safe to
70assume that only nr_unused, age_limit and want_pages are
71used. Nr_unused seems to be exactly what its name says.
72Age_limit is the age in seconds after which dcache entries
73can be reclaimed when memory is short and want_pages is
74nonzero when shrink_dcache_pages() has been called and the
75dcache isn't pruned yet.
76
77==============================================================
78
79dquot-max & dquot-nr:
80
81The file dquot-max shows the maximum number of cached disk
82quota entries.
83
84The file dquot-nr shows the number of allocated disk quota
85entries and the number of free disk quota entries.
86
87If the number of free cached disk quotas is very low and
88you have some awesome number of simultaneous system users,
89you might want to raise the limit.
90
91==============================================================
92
93file-max & file-nr:
94
Linus Torvalds1da177e2005-04-16 15:20:36 -070095The value in file-max denotes the maximum number of file-
96handles that the Linux kernel will allocate. When you get lots
97of error messages about running out of file handles, you might
98want to increase this limit.
99
Federica Teodorica3b78a2011-03-15 16:12:05 -0700100Historically,the kernel was able to allocate file handles
101dynamically, but not to free them again. The three values in
102file-nr denote the number of allocated file handles, the number
103of allocated but unused file handles, and the maximum number of
104file handles. Linux 2.6 always reports 0 as the number of free
105file handles -- this is not an error, it just means that the
106number of allocated file handles exactly matches the number of
107used file handles.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700108
Xiaotian Fengbcadbbd2009-09-23 15:56:13 -0700109Attempts to allocate more file descriptors than file-max are
110reported with printk, look for "VFS: file-max limit <number>
111reached".
Linus Torvalds1da177e2005-04-16 15:20:36 -0700112==============================================================
113
Eric Dumazet9cfe0152008-02-06 01:37:16 -0800114nr_open:
115
116This denotes the maximum number of file-handles a process can
117allocate. Default value is 1024*1024 (1048576) which should be
118enough for most machines. Actual limit depends on RLIMIT_NOFILE
119resource limit.
120
121==============================================================
122
Linus Torvalds1da177e2005-04-16 15:20:36 -0700123inode-max, inode-nr & inode-state:
124
125As with file handles, the kernel allocates the inode structures
126dynamically, but can't free them yet.
127
128The value in inode-max denotes the maximum number of inode
129handlers. This value should be 3-4 times larger than the value
130in file-max, since stdin, stdout and network sockets also
131need an inode struct to handle them. When you regularly run
132out of inodes, you need to increase this value.
133
134The file inode-nr contains the first two items from
135inode-state, so we'll skip to that file...
136
137Inode-state contains three actual numbers and four dummies.
138The actual numbers are, in order of appearance, nr_inodes,
139nr_free_inodes and preshrink.
140
141Nr_inodes stands for the number of inodes the system has
142allocated, this can be slightly more than inode-max because
143Linux allocates them one pageful at a time.
144
145Nr_free_inodes represents the number of free inodes (?) and
146preshrink is nonzero when the nr_inodes > inode-max and the
147system needs to prune the inode list instead of allocating
148more.
149
150==============================================================
151
152overflowgid & overflowuid:
153
154Some filesystems only support 16-bit UIDs and GIDs, although in Linux
155UIDs and GIDs are 32 bits. When one of these filesystems is mounted
156with writes enabled, any UID or GID that would exceed 65535 is translated
157to a fixed value before being written to disk.
158
159These sysctls allow you to change the value of the fixed UID and GID.
160The default is 65534.
161
162==============================================================
163
Willy Tarreau759c0112016-01-18 16:36:09 +0100164pipe-user-pages-hard:
165
166Maximum total number of pages a non-privileged user may allocate for pipes.
167Once this limit is reached, no new pipes may be allocated until usage goes
168below the limit again. When set to 0, no limit is applied, which is the default
169setting.
170
171==============================================================
172
173pipe-user-pages-soft:
174
175Maximum total number of pages a non-privileged user may allocate for pipes
176before the pipe size gets limited to a single page. Once this limit is reached,
177new pipes will be limited to a single page in size for this user in order to
178limit total memory usage, and trying to increase them using fcntl() will be
179denied until usage goes below the limit again. The default value allows to
180allocate up to 1024 pipes at their default size. When set to 0, no limit is
181applied.
182
183==============================================================
184
Kees Cook800179c2012-07-25 17:29:07 -0700185protected_hardlinks:
186
187A long-standing class of security issues is the hardlink-based
188time-of-check-time-of-use race, most commonly seen in world-writable
189directories like /tmp. The common method of exploitation of this flaw
190is to cross privilege boundaries when following a given hardlink (i.e. a
191root process follows a hardlink created by another user). Additionally,
192on systems without separated partitions, this stops unauthorized users
193from "pinning" vulnerable setuid/setgid files against being upgraded by
194the administrator, or linking to special files.
195
196When set to "0", hardlink creation behavior is unrestricted.
197
198When set to "1" hardlinks cannot be created by users if they do not
199already own the source file, or do not have read/write access to it.
200
201This protection is based on the restrictions in Openwall and grsecurity.
202
203==============================================================
204
205protected_symlinks:
206
207A long-standing class of security issues is the symlink-based
208time-of-check-time-of-use race, most commonly seen in world-writable
209directories like /tmp. The common method of exploitation of this flaw
210is to cross privilege boundaries when following a given symlink (i.e. a
211root process follows a symlink belonging to another user). For a likely
212incomplete list of hundreds of examples across the years, please see:
213http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
214
215When set to "0", symlink following behavior is unrestricted.
216
217When set to "1" symlinks are permitted to be followed only when outside
218a sticky world-writable directory, or when the uid of the symlink and
219follower match, or when the directory owner matches the symlink's owner.
220
221This protection is based on the restrictions in Openwall and grsecurity.
222
223==============================================================
224
Alexey Dobriyana2e0b562006-08-27 01:23:28 -0700225suid_dumpable:
226
227This value can be used to query and set the core dump mode for setuid
228or otherwise protected/tainted binaries. The modes are
229
2300 - (default) - traditional behaviour. Any process which has changed
Kees Cook95206282012-07-30 14:39:15 -0700231 privilege levels or is execute only will not be dumped.
Alexey Dobriyana2e0b562006-08-27 01:23:28 -07002321 - (debug) - all processes dump core when possible. The core dump is
233 owned by the current user and no security is applied. This is
234 intended for system debugging situations only. Ptrace is unchecked.
Kees Cook95206282012-07-30 14:39:15 -0700235 This is insecure as it allows regular users to examine the memory
236 contents of privileged processes.
Alexey Dobriyana2e0b562006-08-27 01:23:28 -07002372 - (suidsafe) - any binary which normally would not be dumped is dumped
Kees Cook95206282012-07-30 14:39:15 -0700238 anyway, but only if the "core_pattern" kernel sysctl is set to
239 either a pipe handler or a fully qualified path. (For more details
240 on this limitation, see CVE-2006-2451.) This mode is appropriate
241 when administrators are attempting to debug problems in a normal
242 environment, and either have a core dump pipe handler that knows
243 to treat privileged core dumps with care, or specific directory
244 defined for catching core dumps. If a core dump happens without
245 a pipe handler or fully qualifid path, a message will be emitted
246 to syslog warning about the lack of a correct setting.
Alexey Dobriyana2e0b562006-08-27 01:23:28 -0700247
248==============================================================
249
Linus Torvalds1da177e2005-04-16 15:20:36 -0700250super-max & super-nr:
251
252These numbers control the maximum number of superblocks, and
253thus the maximum number of mounted filesystems the kernel
254can have. You only need to increase super-max if you need to
255mount more filesystems than the current value in super-max
256allows you to.
257
258==============================================================
259
260aio-nr & aio-max-nr:
261
262aio-nr shows the current system-wide number of asynchronous io
263requests. aio-max-nr allows you to change the maximum value
264aio-nr can grow to.
265
266==============================================================
Shen Feng760df932009-04-02 16:57:20 -0700267
Eric W. Biedermand2921682016-09-28 00:27:17 -0500268mount-max:
269
270This denotes the maximum number of mounts that may exist
271in a mount namespace.
272
273==============================================================
274
Shen Feng760df932009-04-02 16:57:20 -0700275
2762. /proc/sys/fs/binfmt_misc
277----------------------------------------------------------
278
279Documentation for the files in /proc/sys/fs/binfmt_misc is
280in Documentation/binfmt_misc.txt.
281
282
2833. /proc/sys/fs/mqueue - POSIX message queues filesystem
284----------------------------------------------------------
285
286The "mqueue" filesystem provides the necessary kernel features to enable the
287creation of a user space library that implements the POSIX message queues
288API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
289Interfaces specification.)
290
291The "mqueue" filesystem contains values for determining/setting the amount of
292resources used by the file system.
293
294/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
295maximum number of message queues allowed on the system.
296
297/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
298maximum number of messages in a queue value. In fact it is the limiting value
299for another (user) limit which is set in mq_open invocation. This attribute of
300a queue must be less or equal then msg_max.
301
302/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
303maximum message size value (it is every message queue's attribute set during
304its creation).
305
KOSAKI Motohirocef01842012-05-31 16:26:33 -0700306/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the
307default number of messages in a queue value if attr parameter of mq_open(2) is
308NULL. If it exceed msg_max, the default value is initialized msg_max.
309
310/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
311the default message size value if attr parameter of mq_open(2) is NULL. If it
312exceed msgsize_max, the default value is initialized msgsize_max.
Shen Feng760df932009-04-02 16:57:20 -0700313
3144. /proc/sys/fs/epoll - Configuration options for the epoll interface
315--------------------------------------------------------
316
317This directory contains configuration options for the epoll(7) interface.
318
Shen Feng760df932009-04-02 16:57:20 -0700319max_user_watches
320----------------
321
322Every epoll file descriptor can store a number of files to be monitored
323for event readiness. Each one of these monitored files constitutes a "watch".
324This configuration option sets the maximum number of "watches" that are
325allowed for each user.
326Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
327on a 64bit one.
328The current default value for max_user_watches is the 1/32 of the available
329low memory, divided for the "watch" cost in bytes.
330