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Thu, 25 Jun 2026 11:41:46 -0700 (PDT) Date: Thu, 25 Jun 2026 14:41:45 -0400 From: Johannes Weiner To: Qi Zheng Cc: akpm@linux-foundation.org, david@kernel.org, kasong@tencent.com, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, 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 v3] mm: mglru: fix stale batch updates after memcg reparenting Message-ID: References: <20260625151554.55105-1-qi.zheng@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260625151554.55105-1-qi.zheng@linux.dev> On Thu, Jun 25, 2026 at 11:15:54PM +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 > --- > Changes in v3: > - re-implement lock_batch_lruvec() by checking CSS_DYING under the RCU lock > (suggested by Harry) > - update the commit message (suggested by Harry) > - temporarily drop the previous Reviewed-by tags > (since the sync method has changed) > - rebase onto the next-20260624 > > Changes in v2: > - update the commit message (pointed by Barry) > - collect Reviewed-by > > mm/vmscan.c | 45 ++++++++++++++++++++++++++++++++++++++------- > 1 file changed, 38 insertions(+), 7 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 35c3bb15ae96..1ec8c23c72b9 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -3262,10 +3262,44 @@ static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio, > walk->nr_pages[new_gen][type][zone] += delta; > } > > +#ifdef CONFIG_MEMCG > +static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec) > +{ > + struct pglist_data *pgdat = lruvec_pgdat(lruvec); > + struct mem_cgroup *memcg = lruvec_memcg(lruvec); > + > + rcu_read_lock(); Where is this unlocked? > + /* > + * The memcg can be NULL when the memory controller is disabled. > + * Otherwise, the caller keeps the memcg owning @lruvec alive. > + */ > + if (!memcg || !css_is_dying(&memcg->css)) > + goto lock; > + > + do { > + memcg = parent_mem_cgroup(memcg); > + } while (memcg && css_is_dying(&memcg->css)); > + lruvec = mem_cgroup_lruvec(memcg, pgdat); while (unlikely(memcg && css_is_dying(&memcg->css))) { memcg = parent_mem_cgroup(memcg); lruvec = mem_cgroup_lruvec(memcg, pgdat); }