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Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wvnp8-00000009PbM-0xVL; Mon, 17 Aug 2026 03:15:42 +0000 Date: Mon, 17 Aug 2026 04:15:42 +0100 From: Matthew Wilcox To: Song Hu Cc: akpm@linux-foundation.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, zhuhui@kylinos.cn, joshua.hahnjy@gmail.com, audra@redhat.com, bingfangguo@tencent.com Subject: Re: [PATCH] mm: memcg: flush empty per-cpu stock slots on memcg offlining Message-ID: References: <20260817025917.66233-1-husong@kylinos.cn> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260817025917.66233-1-husong@kylinos.cn> X-Stat-Signature: eg3k4yq79c8g5wm5nzw5epy48eq7exr8 X-Rspamd-Queue-Id: 74D14180005 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1786936552-275853 X-HE-Meta: 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 1T/nerYH KbX1DmAJ7dDKOZ5R/ms3CCu/3OrbXOmtVy91HQH3yXOAXXGYkkiYwC0/OhBiUOsQdIYN48f2TY6eFSy7KvyNqAvwLvI02JJ/fJhghoAVOSwCiY23CA1lDUkQQ40qV52q6+GjQQU+ooCNOAqqGGTmarOjA5elbbAY8Tkg+/TCQmmNfKgKkATnXTOkmo+/Eow5dCSn/kUNlDZ3FDNL2xW7J0ix7t1bUcooLMfjtwjUEDwSXUDlHxNocUBt2vgKT8GZGIfB7tt48/jzzDqJPkE6cZvY94NngBkPUo/uuMZUpNuxJceh3K0lUx+mp8fVRIjj0X5vV/EhI0yld5O798KcL9wfZsJoQsnoveLFuqTOU3GGdUpG4rccF2ruC0w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 17, 2026 at 10:59:17AM +0800, Song Hu wrote: > refill_stock() takes a css reference for each cached memcg slot and > mem_cgroup_css_offline() relies on drain_all_stock() to release those > references before the cgroup can finish dying. > > consume_stock() can drive a slot's nr_pages to zero while its cached[] > pointer stays set. is_memcg_drain_needed() skips such empty slots, so > the offlining drain is a no-op for them: the pinned css reference is > released only if the slot happens to be evicted by later unrelated > charges on the same CPU, or by CPU hotplug. The memcg then lingers > in the dying state indefinitely. On a CPU with container churn, a > per-cpu stock can pin up to NR_MEMCG_STOCK (7) zombie memcgs. > > The sibling obj_stock_flush_required() has no such gate: a cached > objcgs slot is flushed regardless of its byte count. Drop the > nr_pages gate from is_memcg_drain_needed() accordingly. Draining an > empty slot is a single css_put() and a NULL store, and it happens at > most once per slot per drain, so the extra work is negligible. > > This is easily reproduced with short-lived cgroups pinned to one CPU: > 12 charge/exit/rmdir cycles leave nr_dying_subsys_memory at +2, stable > across later slot displacement. > > Fixes: d1a05b6973c7 ("memcg: do not try to drain per-cpu caches without pages") > Signed-off-by: Song Hu > --- > mm/memcontrol.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 17da1f43b7d3..b931ec16bb82 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2291,8 +2291,12 @@ static bool is_memcg_drain_needed(struct memcg_stock_pcp *stock, > if (!memcg) > continue; > > - if (READ_ONCE(stock->nr_pages[i]) && > - mem_cgroup_is_descendant(memcg, root_memcg)) { > + /* > + * An empty slot still pins a css reference which > + * mem_cgroup_css_offline() relies on drain_all_stock() > + * to release. > + */ I don't think the comment adds any value. It explains why something _isn't_ there which makes no sense to someone reading the code. > + if (mem_cgroup_is_descendant(memcg, root_memcg)) { > flush = true; > break; > } > -- > 2.43.0 > >