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Wed, 01 Jul 2026 08:59:18 -0700 (PDT) Date: Wed, 1 Jul 2026 11:59:17 -0400 From: Johannes Weiner To: Harry Yoo Cc: Usama Arif , Qi Zheng , 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, 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 v4] mm: mglru: fix stale batch updates after memcg reparenting Message-ID: References: <20260701145736.3785016-1-usama.arif@linux.dev> <2fb5ce53-666b-4b0a-a4ad-2b3a28c54768@kernel.org> 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: <2fb5ce53-666b-4b0a-a4ad-2b3a28c54768@kernel.org> On Thu, Jul 02, 2026 at 12:36:42AM +0900, Harry Yoo wrote: > > > On 7/1/26 11:57 PM, Usama Arif wrote: > > On Wed, 1 Jul 2026 15:52:51 +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) > >> --- > >> Changes in v4: > >> - re-implement lock_batch_lruvec() in a simpler way > >> (suggested by Johannes and Harry) > >> - collect Reviewed-by > >> - rebase onto the next-20260630 > >> > >> 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 | 41 ++++++++++++++++++++++++++++++++++------- > >> 1 file changed, 34 insertions(+), 7 deletions(-) > >> > >> diff --git a/mm/vmscan.c b/mm/vmscan.c > >> index 35c3bb15ae96..ca1e2a870d51 100644 > >> --- a/mm/vmscan.c > >> +++ b/mm/vmscan.c > >> @@ -3262,10 +3262,40 @@ 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(); > >> + > >> + /* > >> + * The memcg can be NULL when the memory controller is disabled. > >> + * Otherwise, the caller keeps the memcg owning @lruvec alive. > >> + */ > >> + while (unlikely(memcg && css_is_dying(&memcg->css))) { > >> + memcg = parent_mem_cgroup(memcg); > >> + lruvec = mem_cgroup_lruvec(memcg, pgdat); > >> + } > >> + > >> + spin_lock_irq(&lruvec->lru_lock); > > > > Do we need an rcu_read_unlock() here? > > lruvec_unlock_irq() does that. Yeah, that tripped me up too. And it makes me think Shakeel was right after all: this should live next to the other lruvec_lock() primitives. Sure, MGLRU is the only user, but it's still much easier to understand this if the code sits next to the rest of the API (and the unlock!). lruvec_live_lock_irq()?