Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 mount-max: 269 270 This denotes the maximum number of mounts that may exist 271 in a mount namespace. 272 273 ============================================================== 274 275 276 2. /proc/sys/fs/binfmt_misc 277 ---------------------------------------------------------- 278 279 Documentation for the files in /proc/sys/fs/binfmt_misc is 280 in Documentation/admin-guide/binfmt-misc.rst. 281 282 283 3. /proc/sys/fs/mqueue - POSIX message queues filesystem 284 ---------------------------------------------------------- 285 286 The "mqueue" filesystem provides the necessary kernel features to enable the 287 creation of a user space library that implements the POSIX message queues 288 API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System 289 Interfaces specification.) 290 291 The "mqueue" filesystem contains values for determining/setting the amount of 292 resources used by the file system. 293 294 /proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the 295 maximum 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 298 maximum number of messages in a queue value. In fact it is the limiting value 299 for another (user) limit which is set in mq_open invocation. This attribute of 300 a 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 303 maximum message size value (it is every message queue's attribute set during 304 its creation). 305 306 /proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the 307 default number of messages in a queue value if attr parameter of mq_open(2) is 308 NULL. 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 311 the default message size value if attr parameter of mq_open(2) is NULL. If it 312 exceed msgsize_max, the default value is initialized msgsize_max. 313 314 4. /proc/sys/fs/epoll - Configuration options for the epoll interface 315 -------------------------------------------------------- 316 317 This directory contains configuration options for the epoll(7) interface. 318 319 max_user_watches 320 ---------------- 321 322 Every epoll file descriptor can store a number of files to be monitored 323 for event readiness. Each one of these monitored files constitutes a "watch". 324 This configuration option sets the maximum number of "watches" that are 325 allowed for each user. 326 Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes 327 on a 64bit one. 328 The current default value for max_user_watches is the 1/32 of the available 329 low memory, divided for the "watch" cost in bytes.