Documentation for /proc/sys/fs/*	kernel version 2.2.10
  	(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
  	(c) 2009,        Shen Feng<shen@cn.fujitsu.com>
  
  For general info and legal blurb, please look in README.
  
  ==============================================================
  
  This file contains documentation for the sysctl files in
  /proc/sys/fs/ and is valid for Linux kernel version 2.2.
  
  The files in this directory can be used to tune and monitor
  miscellaneous and general things in the operation of the Linux
  kernel. Since some of the files _can_ be used to screw up your
  system, it is advisable to read both documentation and source
  before actually making adjustments.
  
  1. /proc/sys/fs
  ----------------------------------------------------------
  
  Currently, these files are in /proc/sys/fs:
  - aio-max-nr
  - aio-nr
  - dentry-state
  - dquot-max
  - dquot-nr
  - file-max
  - file-nr
  - inode-max
  - inode-nr
  - inode-state
  - nr_open
  - overflowuid
  - overflowgid
  - protected_hardlinks
  - protected_symlinks
  - suid_dumpable
  - super-max
  - super-nr
  
  ==============================================================
  
  aio-nr & aio-max-nr:
  
  aio-nr is the running total of the number of events specified on the
  io_setup system call for all currently active aio contexts.  If aio-nr
  reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
  raising aio-max-nr does not result in the pre-allocation or re-sizing
  of any kernel data structures.
  
  ==============================================================
  
  dentry-state:
  
  From linux/fs/dentry.c:
  --------------------------------------------------------------
  struct {
          int nr_dentry;
          int nr_unused;
          int age_limit;         /* age in seconds */
          int want_pages;        /* pages requested by system */
          int dummy[2];
  } dentry_stat = {0, 0, 45, 0,};
  -------------------------------------------------------------- 
  
  Dentries are dynamically allocated and deallocated, and
  nr_dentry seems to be 0 all the time. Hence it's safe to
  assume that only nr_unused, age_limit and want_pages are
  used. Nr_unused seems to be exactly what its name says.
  Age_limit is the age in seconds after which dcache entries
  can be reclaimed when memory is short and want_pages is
  nonzero when shrink_dcache_pages() has been called and the
  dcache isn't pruned yet.
  
  ==============================================================
  
  dquot-max & dquot-nr:
  
  The file dquot-max shows the maximum number of cached disk
  quota entries.
  
  The file dquot-nr shows the number of allocated disk quota
  entries and the number of free disk quota entries.
  
  If the number of free cached disk quotas is very low and
  you have some awesome number of simultaneous system users,
  you might want to raise the limit.
  
  ==============================================================
  
  file-max & file-nr:
  
  The value in file-max denotes the maximum number of file-
  handles that the Linux kernel will allocate. When you get lots
  of error messages about running out of file handles, you might
  want to increase this limit.
  
  Historically,the kernel was able to allocate file handles
  dynamically, but not to free them again. The three values in
  file-nr denote the number of allocated file handles, the number
  of allocated but unused file handles, and the maximum number of
  file handles. Linux 2.6 always reports 0 as the number of free
  file handles -- this is not an error, it just means that the
  number of allocated file handles exactly matches the number of
  used file handles.
  
  Attempts to allocate more file descriptors than file-max are
  reported with printk, look for "VFS: file-max limit <number>
  reached".
  ==============================================================
  
  nr_open:
  
  This denotes the maximum number of file-handles a process can
  allocate. Default value is 1024*1024 (1048576) which should be
  enough for most machines. Actual limit depends on RLIMIT_NOFILE
  resource limit.
  
  ==============================================================
  
  inode-max, inode-nr & inode-state:
  
  As with file handles, the kernel allocates the inode structures
  dynamically, but can't free them yet.
  
  The value in inode-max denotes the maximum number of inode
  handlers. This value should be 3-4 times larger than the value
  in file-max, since stdin, stdout and network sockets also
  need an inode struct to handle them. When you regularly run
  out of inodes, you need to increase this value.
  
  The file inode-nr contains the first two items from
  inode-state, so we'll skip to that file...
  
  Inode-state contains three actual numbers and four dummies.
  The actual numbers are, in order of appearance, nr_inodes,
  nr_free_inodes and preshrink.
  
  Nr_inodes stands for the number of inodes the system has
  allocated, this can be slightly more than inode-max because
  Linux allocates them one pageful at a time.
  
  Nr_free_inodes represents the number of free inodes (?) and
  preshrink is nonzero when the nr_inodes > inode-max and the
  system needs to prune the inode list instead of allocating
  more.
  
  ==============================================================
  
  overflowgid & overflowuid:
  
  Some filesystems only support 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 a fixed value before being written to disk.
  
  These sysctls allow you to change the value of the fixed UID and GID.
  The default is 65534.
  
  ==============================================================
  
  protected_hardlinks:
  
  A long-standing class of security issues is the hardlink-based
  time-of-check-time-of-use race, most commonly seen in world-writable
  directories like /tmp. The common method of exploitation of this flaw
  is to cross privilege boundaries when following a given hardlink (i.e. a
  root process follows a hardlink created by another user). Additionally,
  on systems without separated partitions, this stops unauthorized users
  from "pinning" vulnerable setuid/setgid files against being upgraded by
  the administrator, or linking to special files.
  
  When set to "0", hardlink creation behavior is unrestricted.
  
  When set to "1" hardlinks cannot be created by users if they do not
  already own the source file, or do not have read/write access to it.
  
  This protection is based on the restrictions in Openwall and grsecurity.
  
  ==============================================================
  
  protected_symlinks:
  
  A long-standing class of security issues is the symlink-based
  time-of-check-time-of-use race, most commonly seen in world-writable
  directories like /tmp. The common method of exploitation of this flaw
  is to cross privilege boundaries when following a given symlink (i.e. a
  root process follows a symlink belonging to another user). For a likely
  incomplete list of hundreds of examples across the years, please see:
  http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
  
  When set to "0", symlink following behavior is unrestricted.
  
  When set to "1" symlinks are permitted to be followed only when outside
  a sticky world-writable directory, or when the uid of the symlink and
  follower match, or when the directory owner matches the symlink's owner.
  
  This protection is based on the restrictions in Openwall and grsecurity.
  
  ==============================================================
  
  suid_dumpable:
  
  This value can be used to query and set the core dump mode for setuid
  or otherwise protected/tainted binaries. The modes are
  
  0 - (default) - traditional behaviour. Any process which has changed
  	privilege levels or is execute only will not be dumped.
  1 - (debug) - all processes dump core when possible. The core dump is
  	owned by the current user and no security is applied. This is
  	intended for system debugging situations only. Ptrace is unchecked.
  	This is insecure as it allows regular users to examine the memory
  	contents of privileged processes.
  2 - (suidsafe) - any binary which normally would not be dumped is dumped
  	anyway, but only if the "core_pattern" kernel sysctl is set to
  	either a pipe handler or a fully qualified path. (For more details
  	on this limitation, see CVE-2006-2451.) This mode is appropriate
  	when administrators are attempting to debug problems in a normal
  	environment, and either have a core dump pipe handler that knows
  	to treat privileged core dumps with care, or specific directory
  	defined for catching core dumps. If a core dump happens without
  	a pipe handler or fully qualifid path, a message will be emitted
  	to syslog warning about the lack of a correct setting.
  
  ==============================================================
  
  super-max & super-nr:
  
  These numbers control the maximum number of superblocks, and
  thus the maximum number of mounted filesystems the kernel
  can have. You only need to increase super-max if you need to
  mount more filesystems than the current value in super-max
  allows you to.
  
  ==============================================================
  
  aio-nr & aio-max-nr:
  
  aio-nr shows the current system-wide number of asynchronous io
  requests.  aio-max-nr allows you to change the maximum value
  aio-nr can grow to.
  
  ==============================================================
  
  
  2. /proc/sys/fs/binfmt_misc
  ----------------------------------------------------------
  
  Documentation for the files in /proc/sys/fs/binfmt_misc is
  in Documentation/binfmt_misc.txt.
  
  
  3. /proc/sys/fs/mqueue - POSIX message queues filesystem
  ----------------------------------------------------------
  
  The "mqueue"  filesystem provides  the necessary kernel features to enable the
  creation of a  user space  library that  implements  the  POSIX message queues
  API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
  Interfaces specification.)
  
  The "mqueue" filesystem contains values for determining/setting  the amount of
  resources used by the file system.
  
  /proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
  maximum number of message queues allowed on the system.
  
  /proc/sys/fs/mqueue/msg_max  is  a  read/write file  for  setting/getting  the
  maximum number of messages in a queue value.  In fact it is the limiting value
  for another (user) limit which is set in mq_open invocation. This attribute of
  a queue must be less or equal then msg_max.
  
  /proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
  maximum  message size value (it is every  message queue's attribute set during
  its creation).
  
  /proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
  default number of messages in a queue value if attr parameter of mq_open(2) is
  NULL. If it exceed msg_max, the default value is initialized msg_max.
  
  /proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
  the default message size value if attr parameter of mq_open(2) is NULL. If it
  exceed msgsize_max, the default value is initialized msgsize_max.
  
  4. /proc/sys/fs/epoll - Configuration options for the epoll interface
  --------------------------------------------------------
  
  This directory contains configuration options for the epoll(7) interface.
  
  max_user_watches
  ----------------
  
  Every epoll file descriptor can store a number of files to be monitored
  for event readiness. Each one of these monitored files constitutes a "watch".
  This configuration option sets the maximum number of "watches" that are
  allowed for each user.
  Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
  on a 64bit one.
  The current default value for  max_user_watches  is the 1/32 of the available
  low memory, divided for the "watch" cost in bytes.