Re: [PATCH] mm, memcg: fix wrong mem cgroup protection

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 24, 2020 at 09:14:52AM -0400, Johannes Weiner wrote:
> However, mem_cgroup_protected() never expected anybody to look at the
> effective protection values when it indicated that the cgroup is above
> its protection. As a result, a query during limit reclaim may return
> stale protection values that were calculated by a previous reclaim
> cycle in which the cgroup did have siblings.

Btw, I think there is opportunity to make this a bit less error prone.

We have a mem_cgroup_protected() that returns yes or no, essentially,
but protection isn't a binary state anymore.

It's also been a bit iffy that it looks like a simple predicate
function, but it indeed needs to run procedurally for each cgroup in
order for the calculations throughout the tree to be correct.

It might be better to have a

	mem_cgroup_calculate_protection()

that runs for every cgroup we visit and sets up the internal state;
then have more self-explanatory query functions on top of that:

	mem_cgroup_below_min()
	mem_cgroup_below_low()
	mem_cgroup_protection()

What do you guys think?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e0f502b5fca6..dbd3f75d39b9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2615,14 +2615,15 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		unsigned long reclaimed;
 		unsigned long scanned;
 
-		switch (mem_cgroup_protected(target_memcg, memcg)) {
-		case MEMCG_PROT_MIN:
+		mem_cgroup_calculate_protection(target_memcg, memcg);
+
+		if (mem_cgroup_below_min(memcg)) {
 			/*
 			 * Hard protection.
 			 * If there is no reclaimable memory, OOM.
 			 */
 			continue;
-		case MEMCG_PROT_LOW:
+		} else if (mem_cgroup_below_low(memcg)) {
 			/*
 			 * Soft protection.
 			 * Respect the protection only as long as
@@ -2634,16 +2635,6 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 				continue;
 			}
 			memcg_memory_event(memcg, MEMCG_LOW);
-			break;
-		case MEMCG_PROT_NONE:
-			/*
-			 * All protection thresholds breached. We may
-			 * still choose to vary the scan pressure
-			 * applied based on by how much the cgroup in
-			 * question has exceeded its protection
-			 * thresholds (see get_scan_count).
-			 */
-			break;
 		}
 
 		reclaimed = sc->nr_reclaimed;