From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta0.migadu.com (out-177.mta0.migadu.com [91.218.175.177]) (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 84A04370AC9 for ; Fri, 10 Jul 2026 06:19:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783664380; cv=none; b=RFLzd30dveoWYvg4YWBqVlZWyYoiE3+0xfufPRyBWZNJaU/j+aDmeBWsWYBS1K1UtUnxA8hbTVqX2YpZ0eseZrDn2jo7KKa6YutbGENYdGW7flVsfJnGTQ6Hs9B73GJ6KQ4yfTFvQZdlt8tvSK/k+d26qQ0e+cjFBW92ICpHa6w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783664380; c=relaxed/simple; bh=bHkFgh0vSTy5meqHu0zl/Wu1jpOMi5o1E5ThFUnPwdI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=klJBMyVSfdBzcyo0MDaRNj+6ayxatNYxvejdCHlDgs/8jibrOoLKY1P74vfM5+tImCOo1aGeVdxYOcRFrOO528ahpieZFe/tRiHkBYAtl5ZAkHyBYK6iQ0luzR5NTwKcNcsVdNUsPvJQ2vIiCI6M+KDSMiACkbg14xWnk0qPuO4= 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=K4rbRarg; arc=none smtp.client-ip=91.218.175.177 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="K4rbRarg" Message-ID: <843f6d0d-e893-43ef-9cb3-1df21fb64b8d@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783664364; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=DrsFrXe2gSCY+3C8A2OllD1L02A+ogsgIvQEHN3LEIs=; b=K4rbRargGu/1m7H7J1KlcEYwGgrEC/POAKdRi4IsE48sycrNl9n81QN99X5HnfmKV6pmcQ fysIWrFwHA85OWdrNsqN2XQV4xNxuSTFJBomRbesKeVzqYeissHyYD0AuQwjYYtYqTpCTE WW8SbGhVvnSDt+IBBP6XoZbP7zvPIcE= Date: Fri, 10 Jul 2026 14:19:15 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting To: Johannes Weiner Cc: Harry Yoo , Usama Arif , 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 References: <20260701145736.3785016-1-usama.arif@linux.dev> <2fb5ce53-666b-4b0a-a4ad-2b3a28c54768@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 7/10/26 9:51 AM, Johannes Weiner wrote: > On Thu, Jul 02, 2026 at 09:38:41AM +0800, Qi Zheng wrote: >> >> >> On 7/1/26 11:59 PM, Johannes Weiner wrote: >>> 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()? >> >> But lruvec_lock_irq() grabs the rcu lock too. :( > > Yes, but it's self-explanatory if you put it with those definitions: > > static inline void lruvec_lock_irq(struct lruvec *lruvec) > { > rcu_read_lock(); > spin_lock_irq(&lruvec->lru_lock); > } > > static struct lruvec *lruvec_live_lock_irq(struct lruvec *lruvec) > { > struct mem_cgroup *memcg = lruvec_memcg(lruvec); > > rcu_read_lock(); > while (unlikely(memcg && css_is_dying(&memcg->css))) { > memcg = parent_mem_cgroup(memcg); > lruvec = mem_cgroup_lruvec(memcg, lruvec_pgdat(lruvec)); > } > spin_lock_irq(&lruvec->lru_lock); > } All right, should the implementation for !CONFIG_MEMCG be placed here too? Will send the v5. Thanks, Qi > > static inline void lruvec_unlock_irq(struct lruvec *lruvec) > { > spin_unlock_irq(&lruvec->lru_lock); > rcu_read_unlock(); > } > > etc.