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

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. Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Johannes Weiner <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
jix · Feb 26, 2021 · 629484a · 629484a
1 parent 2bbd00a
commit 629484a
Show file tree

Hide file tree

Showing 2 changed files with 18 additions and 0 deletions.
diff --git 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
@@ -342,6 +342,12 @@ 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;
 	}
@@ -551,6 +557,12 @@ 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;
 	}