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docs: tmpfs: update the large folios policy for tmpfs and shmem

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Update the large folios policy for tmpfs and shmem.

Link: https://lkml.kernel.org/r/9b7418af30e300d1eb05721b81d79074d0bb0ec9.1732779148.git.baolin.wang@linux.alibaba.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2024-11-28 15:40:43 +08:00 committed by Andrew Morton
parent d635ccdb43
commit 5a4550c96b
1 changed files with 41 additions and 17 deletions
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58
Documentation/admin-guide/mm/transhuge.rst
View File

@ -358,8 +358,21 @@ default to ``never``.
Hugepages in tmpfs/shmem
========================
You can control hugepage allocation policy in tmpfs with mount option
``huge=``. It can have following values:
Traditionally, tmpfs only supported a single huge page size ("PMD"). Today,
it also supports smaller sizes just like anonymous memory, often referred
to as "multi-size THP" (mTHP). Huge pages of any size are commonly
represented in the kernel as "large folios".
While there is fine control over the huge page sizes to use for the internal
shmem mount (see below), ordinary tmpfs mounts will make use of all available
huge page sizes without any control over the exact sizes, behaving more like
other file systems.
tmpfs mounts
------------
The THP allocation policy for tmpfs mounts can be adjusted using the mount
option: ``huge=``. It can have following values:
always
Attempt to allocate huge pages every time we need a new page;
@ -374,19 +387,19 @@ within_size
advise
Only allocate huge pages if requested with fadvise()/madvise();
The default policy is ``never``.
Remember, that the kernel may use huge pages of all available sizes, and
that no fine control as for the internal tmpfs mount is available.
The default policy in the past was ``never``, but it can now be adjusted
using the kernel parameter ``transparent_hugepage_tmpfs=<policy>``.
``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
``huge=never`` will not attempt to break up huge pages at all, just stop more
from being allocated.
There's also sysfs knob to control hugepage allocation policy for internal
shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
In addition to policies listed above, shmem_enabled allows two further
values:
In addition to policies listed above, the sysfs knob
/sys/kernel/mm/transparent_hugepage/shmem_enabled will affect the
allocation policy of tmpfs mounts, when set to the following values:
deny
For use in emergencies, to force the huge option off from
@ -394,13 +407,24 @@ deny
force
Force the huge option on for all - very useful for testing;
Shmem can also use "multi-size THP" (mTHP) by adding a new sysfs knob to
control mTHP allocation:
'/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled',
and its value for each mTHP is essentially consistent with the global
setting. An 'inherit' option is added to ensure compatibility with these
global settings. Conversely, the options 'force' and 'deny' are dropped,
which are rather testing artifacts from the old ages.
shmem / internal tmpfs
----------------------
The mount internal tmpfs mount is used for SysV SHM, memfds, shared anonymous
mmaps (of /dev/zero or MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
To control the THP allocation policy for this internal tmpfs mount, the
sysfs knob /sys/kernel/mm/transparent_hugepage/shmem_enabled and the knobs
per THP size in
'/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled'
can be used.
The global knob has the same semantics as the ``huge=`` mount options
for tmpfs mounts, except that the different huge page sizes can be controlled
individually, and will only use the setting of the global knob when the
per-size knob is set to 'inherit'.
The options 'force' and 'deny' are dropped for the individual sizes, which
are rather testing artifacts from the old ages.
always
Attempt to allocate <size> huge pages every time we need a new page;