shmem: update documentation

Update the docs to reflect a bit better why some folks prefer tmpfs over
ramfs and clarify a bit more about the difference between brd ramdisks.

While at it, add THP docs for tmpfs, both the mount options and the sysfs
file.

Link: https://lkml.kernel.org/r/20230309230545.2930737-6-mcgrof@kernel.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/tmpfs.rst

@@ -13,14 +13,25 @@ everything stored therein is lost.
 
 tmpfs puts everything into the kernel internal caches and grows and
 shrinks to accommodate the files it contains and is able to swap
-unneeded pages out to swap space. It has maximum size limits which can
-be adjusted on the fly via 'mount -o remount ...'
+unneeded pages out to swap space, and supports THP.
 
-If you compare it to ramfs (which was the template to create tmpfs)
-you gain swapping and limit checking. Another similar thing is the RAM
-disk (/dev/ram*), which simulates a fixed size hard disk in physical
-RAM, where you have to create an ordinary filesystem on top. Ramdisks
-cannot swap and you do not have the possibility to resize them.
+tmpfs extends ramfs with a few userspace configurable options listed and
+explained further below, some of which can be reconfigured dynamically on the
+fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs
+filesystem can be resized but it cannot be resized to a size below its current
+usage. tmpfs also supports POSIX ACLs, and extended attributes for the
+trusted.* and security.* namespaces. ramfs does not use swap and you cannot
+modify any parameter for a ramfs filesystem. The size limit of a ramfs
+filesystem is how much memory you have available, and so care must be taken if
+used so to not run out of memory.
+
+An alternative to tmpfs and ramfs is to use brd to create RAM disks
+(/dev/ram*), which allows you to simulate a block device disk in physical RAM.
+To write data you would just then need to create an regular filesystem on top
+this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also
+configured in size at initialization and you cannot dynamically resize them.
+Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the
+block layer at all.
 
 Since tmpfs lives completely in the page cache and on swap, all tmpfs
 pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
@@ -85,6 +96,36 @@ 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 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()
+======  ============================================================
+
+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
+==  ============================================================
 
 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