Documentation/admin-guide/cgroup-v2.rst: document why inactive_X + active_X may not...

This has confused a significant number of people using cgroups inside
Facebook, and some of those outside as well judging by posts like
this[0] (although it's not a problem unique to cgroup v2).

If shmem handling in particular becomes more coherent at some point in
the future -- although that seems unlikely now -- we can change the
wording here.

[0]: https://unix.stackexchange.com/q/525092/10762

Link: http://lkml.kernel.org/r/20191111144958.GA11914@chrisdown.name
Signed-off-by: Chris Down <chris@chrisdown.name>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 007ba86..6d13f2d 100644 (file)
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1288,7 +1288,12 @@ PAGE_SIZE multiple when read back.
          inactive_anon, active_anon, inactive_file, active_file, unevictable
                Amount of memory, swap-backed and filesystem-backed,
                on the internal memory management lists used by the
-               page reclaim algorithm
+               page reclaim algorithm.
+
+               As these represent internal list state (eg. shmem pages are on anon
+               memory management lists), inactive_foo + active_foo may not be equal to
+               the value for the foo counter, since the foo counter is type-based, not
+               list-based.
 
          slab_reclaimable
                Part of "slab" that might be reclaimed, such as