linux-kernel - [PATCH] memcg: avoid refill

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Message-ID: <20250313054812.2185900-1-shakeel.butt@linux.dev>
Date: Wed, 12 Mar 2025 22:48:12 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: [PATCH] memcg: avoid refill_stock for root memcg

We never charge the page counters of root memcg, so there is no need to
put root memcg in the memcg stock. At the moment, refill_stock() can be
called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
mem_cgroup_uncharge_skmem().

The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
with root memcg, so those are fine. However obj_cgroup_uncharge_pages()
can potentially call refill_stock() with root memcg if the objcg object
has been reparented over to the root memcg. Let's just avoid
refill_stock() from obj_cgroup_uncharge_pages() for root memcg.

Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e08ce52caabd..393b73aec6dd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2643,7 +2643,8 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,

 	mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
 	memcg1_account_kmem(memcg, -nr_pages);
-	refill_stock(memcg, nr_pages);
+	if (!mem_cgroup_is_root(memcg))
+		refill_stock(memcg, nr_pages);

 	css_put(&memcg->css);
 }
-- 
2.47.1