From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 535A13D75CF for ; Fri, 26 Jun 2026 06:24:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782455084; cv=none; b=hsjxVeMfCWAtMRrmMqWmLGa1yjbSuotR43z0o6fkdXDaG4uaKY2sIiOdAwAMCVPA01/or+bZHEE7/KsdYzefgCOcNLrsY/7puJytXiI16bd/nPMtCqnME9V+CCs20tuAirJJlrdQqiivleP0+126AbEg2/h8jerLdXjitDoDXK8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782455084; c=relaxed/simple; bh=6z0pDXwG+WflR4vn/7DFQg0YvB/jKU1NelyQjvHQPyE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=d+3XezTEGr5l96kBIMP9Lkc6kY7zRBhDpUJovb5LshI0D0Xfr8uBIlYbOZpZA+pRIwWTBoRGNt4IJaQftAoij8qbsiH9oqKBBht9UlRjF0qCjLzx8KWJpdH82YpOke1dJJHv52B9OMtL14ZEqcbFHNd0wfB0U0jts+ePu0H3m8Y= 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=SCHSdyuL; arc=none smtp.client-ip=91.218.175.188 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="SCHSdyuL" Message-ID: <90fd5300-1016-42e7-abad-08ad85fb62b4@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782455079; 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=NgoXI/pWvN7GuJPUblNOuG55t1D6HkoC5HdaKGAVlw0=; b=SCHSdyuL3by3xzeUqlOpn5GvFi4n0Njtp7RwEDm2r/Afz/A856RT3d7CtPhFyZm2PnhQr/ 5JTnSnAZCirA9DjvwjvTbWEpPbIq4+iZzwWVGRWXFM3CrmNb6woZjfmpK2QcO0p/mmpUYo lS+NbkXXIQ04p2idLRZn2mDke96fKD0= Date: Fri, 26 Jun 2026 14:24:17 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v3] mm: mglru: fix stale batch updates after memcg reparenting To: Harry Yoo , Johannes Weiner 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, 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: <20260625151554.55105-1-qi.zheng@linux.dev> <4c7b0c46-14f0-4a62-893e-e50714e09b74@linux.dev> <46ac28bf-5be1-4600-b522-0a1aa76c28e6@kernel.org> <08cf8972-6cfc-4452-9a3c-88e0368dbbf9@linux.dev> 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 6/26/26 12:59 PM, Harry Yoo wrote: > > > On 6/26/26 1:48 PM, Qi Zheng wrote: >> >> >> On 6/26/26 12:43 PM, Harry Yoo wrote: >>> >>> >>> On 6/26/26 11:27 AM, Qi Zheng wrote: >>>> Hi Johannes, >>>> >>>> On 6/26/26 2:41 AM, Johannes Weiner wrote: >>>>> 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 lruvec_unlock_irq() in reset_batch_size() will handle the unlocking. >>>> >>>>> >>>>>> + /* >>>>>> + * 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); >>>> >>>> There is no need to acquire the lruvec before finding the first >>>> non-dying memcg. >>> >>> struct pglist_data *pgdat = lruvec_pgdat(lruvec); >>> struct mem_cgroup *memcg = lruvec_memcg(lruvec); >>> >>> rcu_read_lock() >>> >>> while (unlikely(memcg_is_dying(memcg))) >>> memcg = parent_mem_cgroup(memcg); >>> >>> lruvec = mem_cgroup_lruvec(memcg, pgdat); >> >> If the first memcg is already non-dying, there's no need to re-acquire >> the lruvec. ;) > > Oh, right :) > > Hmm but I still think Johannes' suggestion makes the code cleaner. I don't have a strong preference on which of the two coding styles is more readable. BTW, is there any kernel documentation I could refer to for this? > Observing a dying cgroup should be rare anyway, it's worth focusing > more on readability? While it's rare to encounter consecutive dying memcgs, it can still happen, right? Thanks, Qi >