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Based on kernel version 4.8. Page generated on 2016-10-06 23:19 EST.

1	Documentation for /proc/sys/fs/*	kernel version 2.2.10
2		(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
3		(c) 2009,        Shen Feng<shen@cn.fujitsu.com>
4	
5	For general info and legal blurb, please look in README.
6	
7	==============================================================
8	
9	This file contains documentation for the sysctl files in
10	/proc/sys/fs/ and is valid for Linux kernel version 2.2.
11	
12	The files in this directory can be used to tune and monitor
13	miscellaneous and general things in the operation of the Linux
14	kernel. Since some of the files _can_ be used to screw up your
15	system, it is advisable to read both documentation and source
16	before actually making adjustments.
17	
18	1. /proc/sys/fs
19	----------------------------------------------------------
20	
21	Currently, these files are in /proc/sys/fs:
22	- aio-max-nr
23	- aio-nr
24	- dentry-state
25	- dquot-max
26	- dquot-nr
27	- file-max
28	- file-nr
29	- inode-max
30	- inode-nr
31	- inode-state
32	- nr_open
33	- overflowuid
34	- overflowgid
35	- pipe-user-pages-hard
36	- pipe-user-pages-soft
37	- protected_hardlinks
38	- protected_symlinks
39	- suid_dumpable
40	- super-max
41	- super-nr
42	
43	==============================================================
44	
45	aio-nr & aio-max-nr:
46	
47	aio-nr is the running total of the number of events specified on the
48	io_setup system call for all currently active aio contexts.  If aio-nr
49	reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
50	raising aio-max-nr does not result in the pre-allocation or re-sizing
51	of any kernel data structures.
52	
53	==============================================================
54	
55	dentry-state:
56	
57	From linux/fs/dentry.c:
58	--------------------------------------------------------------
59	struct {
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	
68	Dentries are dynamically allocated and deallocated, and
69	nr_dentry seems to be 0 all the time. Hence it's safe to
70	assume that only nr_unused, age_limit and want_pages are
71	used. Nr_unused seems to be exactly what its name says.
72	Age_limit is the age in seconds after which dcache entries
73	can be reclaimed when memory is short and want_pages is
74	nonzero when shrink_dcache_pages() has been called and the
75	dcache isn't pruned yet.
76	
77	==============================================================
78	
79	dquot-max & dquot-nr:
80	
81	The file dquot-max shows the maximum number of cached disk
82	quota entries.
83	
84	The file dquot-nr shows the number of allocated disk quota
85	entries and the number of free disk quota entries.
86	
87	If the number of free cached disk quotas is very low and
88	you have some awesome number of simultaneous system users,
89	you might want to raise the limit.
90	
91	==============================================================
92	
93	file-max & file-nr:
94	
95	The value in file-max denotes the maximum number of file-
96	handles that the Linux kernel will allocate. When you get lots
97	of error messages about running out of file handles, you might
98	want to increase this limit.
99	
100	Historically,the kernel was able to allocate file handles
101	dynamically, but not to free them again. The three values in
102	file-nr denote the number of allocated file handles, the number
103	of allocated but unused file handles, and the maximum number of
104	file handles. Linux 2.6 always reports 0 as the number of free
105	file handles -- this is not an error, it just means that the
106	number of allocated file handles exactly matches the number of
107	used file handles.
108	
109	Attempts to allocate more file descriptors than file-max are
110	reported with printk, look for "VFS: file-max limit <number>
111	reached".
112	==============================================================
113	
114	nr_open:
115	
116	This denotes the maximum number of file-handles a process can
117	allocate. Default value is 1024*1024 (1048576) which should be
118	enough for most machines. Actual limit depends on RLIMIT_NOFILE
119	resource limit.
120	
121	==============================================================
122	
123	inode-max, inode-nr & inode-state:
124	
125	As with file handles, the kernel allocates the inode structures
126	dynamically, but can't free them yet.
127	
128	The value in inode-max denotes the maximum number of inode
129	handlers. This value should be 3-4 times larger than the value
130	in file-max, since stdin, stdout and network sockets also
131	need an inode struct to handle them. When you regularly run
132	out of inodes, you need to increase this value.
133	
134	The file inode-nr contains the first two items from
135	inode-state, so we'll skip to that file...
136	
137	Inode-state contains three actual numbers and four dummies.
138	The actual numbers are, in order of appearance, nr_inodes,
139	nr_free_inodes and preshrink.
140	
141	Nr_inodes stands for the number of inodes the system has
142	allocated, this can be slightly more than inode-max because
143	Linux allocates them one pageful at a time.
144	
145	Nr_free_inodes represents the number of free inodes (?) and
146	preshrink is nonzero when the nr_inodes > inode-max and the
147	system needs to prune the inode list instead of allocating
148	more.
149	
150	==============================================================
151	
152	overflowgid & overflowuid:
153	
154	Some filesystems only support 16-bit UIDs and GIDs, although in Linux
155	UIDs and GIDs are 32 bits. When one of these filesystems is mounted
156	with writes enabled, any UID or GID that would exceed 65535 is translated
157	to a fixed value before being written to disk.
158	
159	These sysctls allow you to change the value of the fixed UID and GID.
160	The default is 65534.
161	
162	==============================================================
163	
164	pipe-user-pages-hard:
165	
166	Maximum total number of pages a non-privileged user may allocate for pipes.
167	Once this limit is reached, no new pipes may be allocated until usage goes
168	below the limit again. When set to 0, no limit is applied, which is the default
169	setting.
170	
171	==============================================================
172	
173	pipe-user-pages-soft:
174	
175	Maximum total number of pages a non-privileged user may allocate for pipes
176	before the pipe size gets limited to a single page. Once this limit is reached,
177	new pipes will be limited to a single page in size for this user in order to
178	limit total memory usage, and trying to increase them using fcntl() will be
179	denied until usage goes below the limit again. The default value allows to
180	allocate up to 1024 pipes at their default size. When set to 0, no limit is
181	applied.
182	
183	==============================================================
184	
185	protected_hardlinks:
186	
187	A long-standing class of security issues is the hardlink-based
188	time-of-check-time-of-use race, most commonly seen in world-writable
189	directories like /tmp. The common method of exploitation of this flaw
190	is to cross privilege boundaries when following a given hardlink (i.e. a
191	root process follows a hardlink created by another user). Additionally,
192	on systems without separated partitions, this stops unauthorized users
193	from "pinning" vulnerable setuid/setgid files against being upgraded by
194	the administrator, or linking to special files.
195	
196	When set to "0", hardlink creation behavior is unrestricted.
197	
198	When set to "1" hardlinks cannot be created by users if they do not
199	already own the source file, or do not have read/write access to it.
200	
201	This protection is based on the restrictions in Openwall and grsecurity.
202	
203	==============================================================
204	
205	protected_symlinks:
206	
207	A long-standing class of security issues is the symlink-based
208	time-of-check-time-of-use race, most commonly seen in world-writable
209	directories like /tmp. The common method of exploitation of this flaw
210	is to cross privilege boundaries when following a given symlink (i.e. a
211	root process follows a symlink belonging to another user). For a likely
212	incomplete list of hundreds of examples across the years, please see:
213	http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
214	
215	When set to "0", symlink following behavior is unrestricted.
216	
217	When set to "1" symlinks are permitted to be followed only when outside
218	a sticky world-writable directory, or when the uid of the symlink and
219	follower match, or when the directory owner matches the symlink's owner.
220	
221	This protection is based on the restrictions in Openwall and grsecurity.
222	
223	==============================================================
224	
225	suid_dumpable:
226	
227	This value can be used to query and set the core dump mode for setuid
228	or otherwise protected/tainted binaries. The modes are
229	
230	0 - (default) - traditional behaviour. Any process which has changed
231		privilege levels or is execute only will not be dumped.
232	1 - (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.
235		This is insecure as it allows regular users to examine the memory
236		contents of privileged processes.
237	2 - (suidsafe) - any binary which normally would not be dumped is dumped
238		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.
247	
248	==============================================================
249	
250	super-max & super-nr:
251	
252	These numbers control the maximum number of superblocks, and
253	thus the maximum number of mounted filesystems the kernel
254	can have. You only need to increase super-max if you need to
255	mount more filesystems than the current value in super-max
256	allows you to.
257	
258	==============================================================
259	
260	aio-nr & aio-max-nr:
261	
262	aio-nr shows the current system-wide number of asynchronous io
263	requests.  aio-max-nr allows you to change the maximum value
264	aio-nr can grow to.
265	
266	==============================================================
267	
268	
269	2. /proc/sys/fs/binfmt_misc
270	----------------------------------------------------------
271	
272	Documentation for the files in /proc/sys/fs/binfmt_misc is
273	in Documentation/binfmt_misc.txt.
274	
275	
276	3. /proc/sys/fs/mqueue - POSIX message queues filesystem
277	----------------------------------------------------------
278	
279	The "mqueue"  filesystem provides  the necessary kernel features to enable the
280	creation of a  user space  library that  implements  the  POSIX message queues
281	API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
282	Interfaces specification.)
283	
284	The "mqueue" filesystem contains values for determining/setting  the amount of
285	resources used by the file system.
286	
287	/proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
288	maximum number of message queues allowed on the system.
289	
290	/proc/sys/fs/mqueue/msg_max  is  a  read/write file  for  setting/getting  the
291	maximum number of messages in a queue value.  In fact it is the limiting value
292	for another (user) limit which is set in mq_open invocation. This attribute of
293	a queue must be less or equal then msg_max.
294	
295	/proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
296	maximum  message size value (it is every  message queue's attribute set during
297	its creation).
298	
299	/proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
300	default number of messages in a queue value if attr parameter of mq_open(2) is
301	NULL. If it exceed msg_max, the default value is initialized msg_max.
302	
303	/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
304	the default message size value if attr parameter of mq_open(2) is NULL. If it
305	exceed msgsize_max, the default value is initialized msgsize_max.
306	
307	4. /proc/sys/fs/epoll - Configuration options for the epoll interface
308	--------------------------------------------------------
309	
310	This directory contains configuration options for the epoll(7) interface.
311	
312	max_user_watches
313	----------------
314	
315	Every epoll file descriptor can store a number of files to be monitored
316	for event readiness. Each one of these monitored files constitutes a "watch".
317	This configuration option sets the maximum number of "watches" that are
318	allowed for each user.
319	Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
320	on a 64bit one.
321	The current default value for  max_user_watches  is the 1/32 of the available
322	low memory, divided for the "watch" cost in bytes.
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