Documentation/sysctl/fs.txt

   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 - protected_hardlinks
  36 - protected_symlinks
  37 - suid_dumpable
  38 - super-max
  39 - super-nr
  40
  41 ==============================================================
  42
  43 aio-nr & aio-max-nr:
  44
  45 aio-nr is the running total of the number of events specified on the
  46 io_setup system call for all currently active aio contexts.  If aio-nr
  47 reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
  48 raising aio-max-nr does not result in the pre-allocation or re-sizing
  49 of any kernel data structures.
  50
  51 ==============================================================
  52
  53 dentry-state:
  54
  55 From linux/fs/dentry.c:
  56 --------------------------------------------------------------
  57 struct {
  58         int nr_dentry;
  59         int nr_unused;
  60         int age_limit;         /* age in seconds */
  61         int want_pages;        /* pages requested by system */
  62         int dummy[2];
  63 } dentry_stat = {0, 0, 45, 0,};
  64 --------------------------------------------------------------
  65
  66 Dentries are dynamically allocated and deallocated, and
  67 nr_dentry seems to be 0 all the time. Hence it's safe to
  68 assume that only nr_unused, age_limit and want_pages are
  69 used. Nr_unused seems to be exactly what its name says.
  70 Age_limit is the age in seconds after which dcache entries
  71 can be reclaimed when memory is short and want_pages is
  72 nonzero when shrink_dcache_pages() has been called and the
  73 dcache isn't pruned yet.
  74
  75 ==============================================================
  76
  77 dquot-max & dquot-nr:
  78
  79 The file dquot-max shows the maximum number of cached disk
  80 quota entries.
  81
  82 The file dquot-nr shows the number of allocated disk quota
  83 entries and the number of free disk quota entries.
  84
  85 If the number of free cached disk quotas is very low and
  86 you have some awesome number of simultaneous system users,
  87 you might want to raise the limit.
  88
  89 ==============================================================
  90
  91 file-max & file-nr:
  92
  93 The value in file-max denotes the maximum number of file-
  94 handles that the Linux kernel will allocate. When you get lots
  95 of error messages about running out of file handles, you might
  96 want to increase this limit.
  97
  98 Historically,the kernel was able to allocate file handles
  99 dynamically, but not to free them again. The three values in
 100 file-nr denote the number of allocated file handles, the number
 101 of allocated but unused file handles, and the maximum number of
 102 file handles. Linux 2.6 always reports 0 as the number of free
 103 file handles -- this is not an error, it just means that the
 104 number of allocated file handles exactly matches the number of
 105 used file handles.
 106
 107 Attempts to allocate more file descriptors than file-max are
 108 reported with printk, look for "VFS: file-max limit <number>
 109 reached".
 110 ==============================================================
 111
 112 nr_open:
 113
 114 This denotes the maximum number of file-handles a process can
 115 allocate. Default value is 1024*1024 (1048576) which should be
 116 enough for most machines. Actual limit depends on RLIMIT_NOFILE
 117 resource limit.
 118
 119 ==============================================================
 120
 121 inode-max, inode-nr & inode-state:
 122
 123 As with file handles, the kernel allocates the inode structures
 124 dynamically, but can't free them yet.
 125
 126 The value in inode-max denotes the maximum number of inode
 127 handlers. This value should be 3-4 times larger than the value
 128 in file-max, since stdin, stdout and network sockets also
 129 need an inode struct to handle them. When you regularly run
 130 out of inodes, you need to increase this value.
 131
 132 The file inode-nr contains the first two items from
 133 inode-state, so we'll skip to that file...
 134
 135 Inode-state contains three actual numbers and four dummies.
 136 The actual numbers are, in order of appearance, nr_inodes,
 137 nr_free_inodes and preshrink.
 138
 139 Nr_inodes stands for the number of inodes the system has
 140 allocated, this can be slightly more than inode-max because
 141 Linux allocates them one pageful at a time.
 142
 143 Nr_free_inodes represents the number of free inodes (?) and
 144 preshrink is nonzero when the nr_inodes > inode-max and the
 145 system needs to prune the inode list instead of allocating
 146 more.
 147
 148 ==============================================================
 149
 150 overflowgid & overflowuid:
 151
 152 Some filesystems only support 16-bit UIDs and GIDs, although in Linux
 153 UIDs and GIDs are 32 bits. When one of these filesystems is mounted
 154 with writes enabled, any UID or GID that would exceed 65535 is translated
 155 to a fixed value before being written to disk.
 156
 157 These sysctls allow you to change the value of the fixed UID and GID.
 158 The default is 65534.
 159
 160 ==============================================================
 161
 162 protected_hardlinks:
 163
 164 A long-standing class of security issues is the hardlink-based
 165 time-of-check-time-of-use race, most commonly seen in world-writable
 166 directories like /tmp. The common method of exploitation of this flaw
 167 is to cross privilege boundaries when following a given hardlink (i.e. a
 168 root process follows a hardlink created by another user). Additionally,
 169 on systems without separated partitions, this stops unauthorized users
 170 from "pinning" vulnerable setuid/setgid files against being upgraded by
 171 the administrator, or linking to special files.
 172
 173 When set to "0", hardlink creation behavior is unrestricted.
 174
 175 When set to "1" hardlinks cannot be created by users if they do not
 176 already own the source file, or do not have read/write access to it.
 177
 178 This protection is based on the restrictions in Openwall and grsecurity.
 179
 180 ==============================================================
 181
 182 protected_symlinks:
 183
 184 A long-standing class of security issues is the symlink-based
 185 time-of-check-time-of-use race, most commonly seen in world-writable
 186 directories like /tmp. The common method of exploitation of this flaw
 187 is to cross privilege boundaries when following a given symlink (i.e. a
 188 root process follows a symlink belonging to another user). For a likely
 189 incomplete list of hundreds of examples across the years, please see:
 190 http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
 191
 192 When set to "0", symlink following behavior is unrestricted.
 193
 194 When set to "1" symlinks are permitted to be followed only when outside
 195 a sticky world-writable directory, or when the uid of the symlink and
 196 follower match, or when the directory owner matches the symlink's owner.
 197
 198 This protection is based on the restrictions in Openwall and grsecurity.
 199
 200 ==============================================================
 201
 202 suid_dumpable:
 203
 204 This value can be used to query and set the core dump mode for setuid
 205 or otherwise protected/tainted binaries. The modes are
 206
 207 0 - (default) - traditional behaviour. Any process which has changed
 208         privilege levels or is execute only will not be dumped
 209 1 - (debug) - all processes dump core when possible. The core dump is
 210         owned by the current user and no security is applied. This is
 211         intended for system debugging situations only. Ptrace is unchecked.
 212 2 - (suidsafe) - any binary which normally would not be dumped is dumped
 213         readable by root only. This allows the end user to remove
 214         such a dump but not access it directly. For security reasons
 215         core dumps in this mode will not overwrite one another or
 216         other files. This mode is appropriate when administrators are
 217         attempting to debug problems in a normal environment.
 218
 219 ==============================================================
 220
 221 super-max & super-nr:
 222
 223 These numbers control the maximum number of superblocks, and
 224 thus the maximum number of mounted filesystems the kernel
 225 can have. You only need to increase super-max if you need to
 226 mount more filesystems than the current value in super-max
 227 allows you to.
 228
 229 ==============================================================
 230
 231 aio-nr & aio-max-nr:
 232
 233 aio-nr shows the current system-wide number of asynchronous io
 234 requests.  aio-max-nr allows you to change the maximum value
 235 aio-nr can grow to.
 236
 237 ==============================================================
 238
 239
 240 2. /proc/sys/fs/binfmt_misc
 241 ----------------------------------------------------------
 242
 243 Documentation for the files in /proc/sys/fs/binfmt_misc is
 244 in Documentation/binfmt_misc.txt.
 245
 246
 247 3. /proc/sys/fs/mqueue - POSIX message queues filesystem
 248 ----------------------------------------------------------
 249
 250 The "mqueue"  filesystem provides  the necessary kernel features to enable the
 251 creation of a  user space  library that  implements  the  POSIX message queues
 252 API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
 253 Interfaces specification.)
 254
 255 The "mqueue" filesystem contains values for determining/setting  the amount of
 256 resources used by the file system.
 257
 258 /proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
 259 maximum number of message queues allowed on the system.
 260
 261 /proc/sys/fs/mqueue/msg_max  is  a  read/write file  for  setting/getting  the
 262 maximum number of messages in a queue value.  In fact it is the limiting value
 263 for another (user) limit which is set in mq_open invocation. This attribute of
 264 a queue must be less or equal then msg_max.
 265
 266 /proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
 267 maximum  message size value (it is every  message queue's attribute set during
 268 its creation).
 269
 270 /proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
 271 default number of messages in a queue value if attr parameter of mq_open(2) is
 272 NULL. If it exceed msg_max, the default value is initialized msg_max.
 273
 274 /proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
 275 the default message size value if attr parameter of mq_open(2) is NULL. If it
 276 exceed msgsize_max, the default value is initialized msgsize_max.
 277
 278 4. /proc/sys/fs/epoll - Configuration options for the epoll interface
 279 --------------------------------------------------------
 280
 281 This directory contains configuration options for the epoll(7) interface.
 282
 283 max_user_watches
 284 ----------------
 285
 286 Every epoll file descriptor can store a number of files to be monitored
 287 for event readiness. Each one of these monitored files constitutes a "watch".
 288 This configuration option sets the maximum number of "watches" that are
 289 allowed for each user.
 290 Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
 291 on a 64bit one.
 292 The current default value for  max_user_watches  is the 1/32 of the available
 293 low memory, divided for the "watch" cost in bytes.
 294