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From: Shakeel Butt To: Qi Zheng Cc: akpm@linux-foundation.org, david@kernel.org, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, harry@kernel.org, muchun.song@linux.dev, peiyang_he@smail.nju.edu.cn, mhocko@kernel.org, roman.gushchin@linux.dev, ljs@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng , stable@vger.kernel.org Subject: Re: [PATCH v5] mm: mglru: fix stale batch updates after memcg reparenting Message-ID: References: <20260710154318.75388-1-qi.zheng@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260710154318.75388-1-qi.zheng@linux.dev> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: C02D314000B X-Stat-Signature: h4hwc4iycm7rjzz33hpopu3dza7s9yh5 X-HE-Tag: 1784059529-270797 X-HE-Meta: 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 9WA03ZZg TRfewcJAJPqk0eKxhY9vTKEu7UdHBaSRkNu6Rl6OAHTnPwTNK27By8T4eewzYo+SO1gMX2y7QTjImALw5stdPyg9VluQvyVdRyYT1sulKaiurT9MHiFIpbprxNczIf7UldysLuVjsY7LQGrnWaJtq8PWzh7CtKwKTPTiNE6XhYEClGZs9QPH6NsYGwIwgG9Z7E+KlrcW3KY6BeV8G1aGEyj4oZ2/aC71T8L0Rnlw9KPqjYsrNfhBYq90ASHM75TyBnaT+ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 10, 2026 at 11:43:18PM +0800, Qi Zheng wrote: > From: Qi Zheng > > The mglru page table walker batches per-generation size deltas in > walk->nr_pages while walking page tables without holding the lruvec lock. > The reset_batch_size() later folds those deltas into walk->lruvec under > the lruvec lock. > > The page table walker can run concurrently with the memcg reparenting path > as follows: > > CPU0 CPU1 > ==== ==== > > walk_mm > --> walk_page_range > --> update_batch_size > --> walk->nr_pages += delta > > mem_cgroup_css_offline > --> memcg_reparent_objcgs > --> lock lruvec > lru_gen_reparent_memcg > --> reparent child folios to parent > unlock lruvec > > lock lruvec > reset_batch_size > --> child lrugen->nr_pages += delta > > This will trigger the following warning in lru_gen_exit_memcg(): > > VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0, > sizeof(lruvec->lrugen.nr_pages))); > > And the user-visible impact of underestimated nr_pages in MGLRU was > premature OOMs because MGLRU does not try to reclaim memory when nr_pages > reaches zero, but there are still more pages. > > To fix it, make reset_batch_size() check CSS_DYING under RCU before > flushing the pending batch. A non-dying memcg keeps the original lruvec > stable against RCU-delayed offlining; a dying memcg redirects the deltas > to the first non-dying ancestor. > > Reported-by: Peiyang He > Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn > Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios") > Cc: > Signed-off-by: Qi Zheng > Reviewed-by: Harry Yoo (Oracle) Acked-by: Shakeel Butt