[patch 023/158] Documentation/admin-guide/cgroup-v2.rst: document why inactive_X + active_X may not equal X

[patch 023/158] Documentation/admin-guide/cgroup-v2.rst: document why inactive_X + active_X may not equal X

[akpm]

From: Chris Down <chris@chrisdown.name>
Subject: Documentation/admin-guide/cgroup-v2.rst: document why inactive_X + active_X may not equal X

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>
---

 Documentation/admin-guide/cgroup-v2.rst |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/Documentation/admin-guide/cgroup-v2.rst~docs-cgroup-mm-document-why-inactive_x-active_x-may-not-equal-x
+++ a/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
_