Re: [PATCH] mm/memcg: Fix incorrect flushing of lruvec data in obj_stock

On 8/2/21 2:28 AM, Michal Hocko wrote:

On Sun 01-08-21 22:28:27, Waiman Long wrote:

quoted

When mod_objcg_state() is called with a pgdat that is different from
that in the obj_stock, the old lruvec data cached in obj_stock are
flushed out. Unfortunately, they were flushed to the new pgdat and
hence the wrong node, not the one cached in obj_stock.

It would be great to explicitly mention user observable problems here. I
do assume this will make slab stats skewed but the effect wouldn't be
very big, right?

It is the /sys/devices/system/node/node*/meminfo that will get skewed. Not /proc/meminfo. So it is a relatively minor issue. Will update the patch to mention that.

quoted

Fix that by flushing the data to the cached pgdat instead.

Fixes: 68ac5b3c8db2 ("mm/memcg: cache vmstat data in percpu memcg_stock_pcp")
Signed-off-by: Waiman Long <redacted>

Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

quoted

---
  mm/memcontrol.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ae1f5d0cb581..881ec4ddddcd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -3106,17 +3106,19 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
  		stock->cached_pgdat = pgdat;
  	} else if (stock->cached_pgdat != pgdat) {
  		/* Flush the existing cached vmstat data */
+		struct pglist_data *oldpg = stock->cached_pgdat;
+
+		stock->cached_pgdat = pgdat;
  		if (stock->nr_slab_reclaimable_b) {
-			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
+			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
  					  stock->nr_slab_reclaimable_b);
  			stock->nr_slab_reclaimable_b = 0;
  		}
  		if (stock->nr_slab_unreclaimable_b) {
-			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
+			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_UNRECLAIMABLE_B,
  					  stock->nr_slab_unreclaimable_b);
  			stock->nr_slab_unreclaimable_b = 0;
  		}
-		stock->cached_pgdat = pgdat;

Minor nit. Is there any reason to move the cached_pgdat? TBH I found the
original way better from the readability POV.

Right. Will move it back to its original place.

Cheers,
Longman