[PATCH] mm: vmstat: add some comments on internal storage of byte items

[Johannes Weiner <hannes@cmpxchg.org>]

Byte-accounted items are used for slab object accounting at the cgroup
level, because the objects in a slab page can belong to different
cgroups. At the global level these items always change in multiples of
whole slab pages. The vmstat code exploits this and stores these items
as pages internally, which allows for more compact per-cpu data.

This optimization isn't self-evident from the asserts and the division
in the stat update functions. Provide the reader with some context.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/vmstat.h |  6 ++++++
 mm/vmstat.c            | 12 ++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index 773135fc6e19..506d625163a1 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -313,6 +313,12 @@ static inline void __mod_node_page_state(struct pglist_data *pgdat,
 			enum node_stat_item item, int delta)
 {
 	if (vmstat_item_in_bytes(item)) {
+		/*
+		 * Only cgroups use subpage accounting right now; at
+		 * the global level, these items still change in
+		 * multiples of whole pages. Store them as pages
+		 * internally to keep the per-cpu counters compact.
+		 */
 		VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
 		delta >>= PAGE_SHIFT;
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 1cf549dd703e..eff67397301b 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -346,6 +346,12 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 	long t;
 
 	if (vmstat_item_in_bytes(item)) {
+		/*
+		 * Only cgroups use subpage accounting right now; at
+		 * the global level, these items still change in
+		 * multiples of whole pages. Store them as pages
+		 * internally to keep the per-cpu counters compact.
+		 */
 		VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
 		delta >>= PAGE_SHIFT;
 	}
@@ -555,6 +561,12 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 	long o, n, t, z;
 
 	if (vmstat_item_in_bytes(item)) {
+		/*
+		 * Only cgroups use subpage accounting right now; at
+		 * the global level, these items still change in
+		 * multiples of whole pages. Store them as pages
+		 * internally to keep the per-cpu counters compact.
+		 */
 		VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
 		delta >>= PAGE_SHIFT;
 	}
-- 
2.30.0