[PATCH v2] mm: memcg/slab: Fix root memcg vmstats

[PATCH v2] mm: memcg/slab: Fix root memcg vmstats

[Muchun Song]

If we reparent the slab objects to the root memcg, when we free
the slab object, we need to update the per-memcg vmstats to keep
it correct for the root memcg. Now this at least affects the vmstat
of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
size is smaller than the PAGE_SIZE.

Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
---
 v1->v2:
 - Just resend the patch.

 mm/memcontrol.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2dde734df7d1..bbd40c5af61e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -875,8 +875,13 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
 	rcu_read_lock();
 	memcg = mem_cgroup_from_obj(p);
 
-	/* Untracked pages have no memcg, no lruvec. Update only the node */
-	if (!memcg || memcg == root_mem_cgroup) {
+	/*
+	 * Untracked pages have no memcg, no lruvec. Update only the
+	 * node. If we reparent the slab objects to the root memcg,
+	 * when we free the slab object, we need to update the per-memcg
+	 * vmstats to keep it correct for the root memcg.
+	 */
+	if (!memcg) {
 		__mod_node_page_state(pgdat, idx, val);
 	} else {
 		lruvec = mem_cgroup_lruvec(memcg, pgdat);
-- 
2.20.1

Re: [PATCH v2] mm: memcg/slab: Fix root memcg vmstats

[Shakeel Butt]

On Mon, Nov 9, 2020 at 7:10 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> If we reparent the slab objects to the root memcg, when we free
> the slab object, we need to update the per-memcg vmstats to keep
> it correct for the root memcg. Now this at least affects the vmstat
> of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
> size is smaller than the PAGE_SIZE.
>
> Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Roman Gushchin <guro@fb.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

Re: [PATCH v2] mm: memcg/slab: Fix root memcg vmstats

[Johannes Weiner]

On Tue, Nov 10, 2020 at 11:10:15AM +0800, Muchun Song wrote:
> If we reparent the slab objects to the root memcg, when we free
> the slab object, we need to update the per-memcg vmstats to keep
> it correct for the root memcg. Now this at least affects the vmstat
> of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
> size is smaller than the PAGE_SIZE.
> 
> Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Roman Gushchin <guro@fb.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH v2] mm: memcg/slab: Fix root memcg vmstats

[David Rientjes]

On Tue, 10 Nov 2020, Muchun Song wrote:

> If we reparent the slab objects to the root memcg, when we free
> the slab object, we need to update the per-memcg vmstats to keep
> it correct for the root memcg. Now this at least affects the vmstat
> of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
> size is smaller than the PAGE_SIZE.
> 
> Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Roman Gushchin <guro@fb.com>

Acked-by: David Rientjes <rientjes@google.com>

I assume that without this fix that the root memcg's vmstat would always 
be inflated if we reparented?  If that's accurate, perhaps this is 
deserving of a

Cc: stable@vger.kernel.org # 5.3+