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Documentation for /proc/sys/fs/* kernel version 2.2.10
(c) 1998, 1999, Rik van Riel <>
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.
Currently, these files are in /proc/sys/fs:
- dentry-state
- dquot-max
- dquot-nr
- file-max
- file-nr
- inode-max
- inode-nr
- inode-state
- overflowuid
- overflowgid
- suid_dumpable
- super-max
- super-nr
Documentation for the files in /proc/sys/fs/binfmt_misc is
in Documentation/binfmt_misc.txt.
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 kernel allocates file handles dynamically, but as yet it
doesn't free them again.
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.
The three values in file-nr denote the number of allocated
file handles, the number of unused file handles and the maximum
number of file handles. When the allocated file handles come
close to the maximum, but the number of unused file handles is
significantly greater than 0, you've encountered a peak in your
usage of file handles and you don't need to increase the maximum.
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
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.
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.
2 - (suidsafe) - any binary which normally would not be dumped is dumped
readable by root only. This allows the end user to remove
such a dump but not access it directly. For security reasons
core dumps in this mode will not overwrite one another or
other files. This mode is appropriate when adminstrators are
attempting to debug problems in a normal environment.
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.