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tmpfs: fix Documentation of noswap and huge mount options

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The noswap mount option is surely not one of the three options for sizing:
move its description down.

The huge= mount option does not accept numeric values: those are just in
an internal enum.  Delete those numbers, and follow the manpage text more
closely (but there's not yet any fadvise() or fcntl() which applies here).

/sys/kernel/mm/transparent_hugepage/shmem_enabled is hard to describe, and
barely relevant to mounting a tmpfs: just refer to transhuge.rst (while
still using the words deny and force, to help as informal reminders).

[rdunlap@infradead.org: fixup Docs table for huge mount options]
  Link: https://lkml.kernel.org/r/20230725052333.26857-1-rdunlap@infradead.org
Link: https://lkml.kernel.org/r/986cb0bf-9780-354-9bb-4bf57aadbab@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: d0f5a85442 ("shmem: update documentation")
Fixes: 2c6efe9cf2 ("shmem: add support to ignore swap")
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org> 
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2023-07-23 13:55:00 -07:00 committed by Andrew Morton
parent dddfa05eb5
commit 253e5df8b8
1 changed files with 19 additions and 26 deletions
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45
Documentation/filesystems/tmpfs.rst
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@ -84,8 +84,6 @@ nr_inodes The maximum number of inodes for this instance. The default
is half of the number of your physical RAM pages, or (on a
machine with highmem) the number of lowmem RAM pages,
whichever is the lower.
noswap Disables swap. Remounts must respect the original settings.
By default swap is enabled.
========= ============================================================
These parameters accept a suffix k, m or g for kilo, mega and giga and
@ -99,36 +97,31 @@ mount with such options, since it allows any user with write access to
use up all the memory on the machine; but enhances the scalability of
that instance in a system with many CPUs making intensive use of it.
tmpfs blocks may be swapped out, when there is a shortage of memory.
tmpfs has a mount option to disable its use of swap:
====== ===========================================================
noswap Disables swap. Remounts must respect the original settings.
By default swap is enabled.
====== ===========================================================
tmpfs also supports Transparent Huge Pages which requires a kernel
configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for
your system (has_transparent_hugepage(), which is architecture specific).
The mount options for this are:
====== ============================================================
huge=0 never: disables huge pages for the mount
huge=1 always: enables huge pages for the mount
huge=2 within_size: only allocate huge pages if the page will be
fully within i_size, also respect fadvise()/madvise() hints.
huge=3 advise: only allocate huge pages if requested with
fadvise()/madvise()
====== ============================================================
================ ==============================================================
huge=never Do not allocate huge pages. This is the default.
huge=always Attempt to allocate huge page every time a new page is needed.
huge=within_size Only allocate huge page if it will be fully within i_size.
Also respect madvise(2) hints.
huge=advise Only allocate huge page if requested with madvise(2).
================ ==============================================================
There is a sysfs file which you can also use to control system wide THP
configuration for all tmpfs mounts, the file is:
/sys/kernel/mm/transparent_hugepage/shmem_enabled
This sysfs file is placed on top of THP sysfs directory and so is registered
by THP code. It is however only used to control all tmpfs mounts with one
single knob. Since it controls all tmpfs mounts it should only be used either
for emergency or testing purposes. The values you can set for shmem_enabled are:
== ============================================================
-1 deny: disables huge on shm_mnt and all mounts, for
emergency use
-2 force: enables huge on shm_mnt and all mounts, w/o needing
option, for testing
== ============================================================
See also Documentation/admin-guide/mm/transhuge.rst, which describes the
sysfs file /sys/kernel/mm/transparent_hugepage/shmem_enabled: which can
be used to deny huge pages on all tmpfs mounts in an emergency, or to
force huge pages on all tmpfs mounts for testing.
tmpfs has a mount option to set the NUMA memory allocation policy for
all files in that instance (if CONFIG_NUMA is enabled) - which can be