From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta0.migadu.com (out-170.mta0.migadu.com [91.218.175.170]) (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 57B4C29ACC5 for ; Thu, 2 Jul 2026 01:39:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782956351; cv=none; b=ox4DCyQWok4jx80NgOhMqauZ5hq4dR5Au53qJEN8RYwUyIAOr8KCMAHFJETK+v+4UJgz7oXjFQuEVgGRFvfAOcaHYX+M/LSSNapYKtepObatyqctvo3UYDHhU6DqTLlaUhd6P6FA5+as3vcc3ONJ3qTph+pyPCwmGwoRRL1osAc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782956351; c=relaxed/simple; bh=66DydrvzAjI033LmDdftprpH7jAqg26PyF9qX1g/7OA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=p1gi2Fz2pKbUSXErfrc91RPXj6FQ2cGKPwLgEbl/WMsYYHO1LY9IAzDL065+v64ruGRlGu/ok6ffj1iUL6skIx3jW3VbtwHTf+ifIBm3jUvOrDuw6GAFtZaLs0ae1LHieT9BPoSdjIGRLp46uqnn4wDUDLIcEJlRttadE8rNdWk= 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=FWVIAyHU; arc=none smtp.client-ip=91.218.175.170 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="FWVIAyHU" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782956346; 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=oy1rhcxFbJPb8dTCKU8Gfj8AxUhJZJdRCaNcIMoi9hk=; b=FWVIAyHUAhvVuzDPPF9CTLQ/MP/m+BXRk2M/5Nyym94ltS0gGjlgX50dne/NMoifnWgzcL 25c+3QkD3raK6VNU9Ug5FwpGDYFrIlsdiqG7oo17tMmezBIn4UU1GY2ixBYrCYci/6LVMM rpd6LCAqwTU5etYfN9G8gEudOanqpPw= Date: Thu, 2 Jul 2026 09:38:41 +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 , Harry Yoo Cc: 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 7/1/26 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. :(