[PATCH mm-hotfixes] tmpfs: fix Documentation of noswap and huge mount options

[PATCH mm-hotfixes] tmpfs: fix Documentation of noswap and huge mount options

[Hugh Dickins]

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).

Fixes: d0f5a85442d1 ("shmem: update documentation")
Fixes: 2c6efe9cf2d7 ("shmem: add support to ignore swap")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 Documentation/filesystems/tmpfs.rst | 45 ++++++++++++-----------------
 1 file changed, 19 insertions(+), 26 deletions(-)

diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
index f18f46be5c0c..28aeaeea47d0 100644
--- a/Documentation/filesystems/tmpfs.rst
+++ b/Documentation/filesystems/tmpfs.rst
@@ -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
-- 
2.35.3

Re: [PATCH mm-hotfixes] tmpfs: fix Documentation of noswap and huge mount options

[Luis Chamberlain]

On Sun, Jul 23, 2023 at 01:55:00PM -0700, Hugh Dickins wrote:
> 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).
> 
> Fixes: d0f5a85442d1 ("shmem: update documentation")
> Fixes: 2c6efe9cf2d7 ("shmem: add support to ignore swap")
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org> 

Thanks for fixing this and addressing the preference you wanted for
master documentation, I'll follow up again with the style fixes for the
man pages.

  Luis