From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AB9C92FD69D for ; Wed, 1 Jul 2026 07:54:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782892456; cv=none; b=tPYaH7J/e35zK5Avvp28eg7TvGAVpD1hOOUE8l3SCLaV9GKjTQryYXifjPYLsD5Ezn8lRqr05A1NeBeV45KQT5DVOYPjINTODjobimvpP3CrAcCnda4e4IOnNwMASGZyKOQ7WTG+p3qTAYDZyGPyURLKI4XNd1/afoluxHNC8v0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782892456; c=relaxed/simple; bh=J4t5aefE64W35ClXcLr3mAtchUoWtgPq8OEqYUBGtB4=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=LDToLtCrNb0PGle8f5VQZ4GAT0YjiDWHOz9Zp+k66dia9OUOSdrwU6+n3TeE0cVuMCSByi44AywMgYmcfywTYbdRSDbK2y5gxm98oc1SWB11qdrHClZ52ql44YeT+8jHyxb7g8J+59HdljN4hK8YVPv3JAZxa8u2Xvvm7Yh459g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=gtC108Cj; arc=none smtp.client-ip=95.215.58.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="gtC108Cj" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782892442; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=3h7CGvVoalB/WWwlHPJ/8L12bWTkcsodPzdppqEOSlE=; b=gtC108CjJNVRGfhHG3RKkQAts4kg9aEW1C4fta5PvdlcJFWdsxKhyc5MDoAVC+TJsvhYq9 el42sk6fHOI4mNYDlHkVyL1NPiZXzMgltpKb+XA0Fnr/UfLwbQebwz4eHsSXR9JBV1fNf1 kBHcR2u4vhCRr6IG8Tqm59CGsZDDkQA= From: Qi Zheng To: 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, 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng , stable@vger.kernel.org Subject: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting Date: Wed, 1 Jul 2026 15:52:51 +0800 Message-ID: <20260701075251.56413-1-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-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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); + + return lruvec; +} +#else +static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec) +{ + lruvec_lock_irq(lruvec); + + return lruvec; +} +#endif + static void reset_batch_size(struct lru_gen_mm_walk *walk) { int gen, type, zone; - struct lruvec *lruvec = walk->lruvec; + struct lruvec *lruvec = lock_batch_lruvec(walk->lruvec); struct lru_gen_folio *lrugen = &lruvec->lrugen; walk->batched = 0; @@ -3285,6 +3315,8 @@ static void reset_batch_size(struct lru_gen_mm_walk *walk) lru += LRU_ACTIVE; __update_lru_size(lruvec, lru, zone, delta); } + + lruvec_unlock_irq(lruvec); } static int should_skip_vma(unsigned long start, unsigned long end, struct mm_walk *args) @@ -3779,11 +3811,8 @@ static void walk_mm(struct mm_struct *mm, struct lru_gen_mm_walk *walk) mmap_read_unlock(mm); } - if (walk->batched) { - lruvec_lock_irq(lruvec); + if (walk->batched) reset_batch_size(walk); - lruvec_unlock_irq(lruvec); - } cond_resched(); } while (err == -EAGAIN); @@ -4867,9 +4896,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec, walk = current->reclaim_state->mm_walk; if (walk && walk->batched) { walk->lruvec = lruvec; - lruvec_lock_irq(lruvec); reset_batch_size(walk); - lruvec_unlock_irq(lruvec); } mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(sc), -- 2.54.0