mm: memcontrol: report slab usage in cgroup2 memory.stat

Show how much memory is used for storing reclaimable and unreclaimable
in-kernel data structures allocated from slab caches.

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index ff49cf9..e4e0c1d 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -843,6 +843,11 @@
 		Amount of memory used to cache filesystem data,
 		including tmpfs and shared memory.
 
+	  slab
+
+		Amount of memory used for storing in-kernel data
+		structures.
+
 	  sock
 
 		Amount of memory used in network transmission buffers
@@ -871,6 +876,16 @@
 		on the internal memory management lists used by the
 		page reclaim algorithm
 
+	  slab_reclaimable
+
+		Part of "slab" that might be reclaimed, such as
+		dentries and inodes.
+
+	  slab_unreclaimable
+
+		Part of "slab" that cannot be reclaimed on memory
+		pressure.
+
 	  pgfault
 
 		Total number of page faults incurred