[PATCH 5/6] shmem: update documentation

[Luis Chamberlain]

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.

Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
---
 Documentation/filesystems/tmpfs.rst | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
index 0408c245785e..e77ebdacadd0 100644
--- a/Documentation/filesystems/tmpfs.rst
+++ b/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 ...'
-
-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.
+unneeded pages out to swap space.
+
+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
-- 
2.39.1