From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0B840245008; Wed, 1 Jul 2026 15:36:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782920210; cv=none; b=tU6FQfwGqARCdqZpA8zxy38wDplm9pyb3YQf069tch5GuDP+M+jWQH8fCeJiIjw/SeDas+UNDbrpyDGlVmxHxEC/Vq/2IsXCFiEtDUG0rYkK2s9QhB6+K4cNvTogN50HI0bJxtqhNfAWxvPctiyvFQg2BrhYeXelg4nG6owh7m4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782920210; c=relaxed/simple; bh=Jj8RwJGsh68t6Q7Bpd3C3QxzgSE/g/sizzaOjDmksEI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GZayKZGTwOCGCu+8B8Ka0RmlcHmsj+ixsb23mR9asXdpMBkmAL6NPPE4NZnAAyqTarjnt4D/YAWJJjN9gwssNqsWchbiPCmzbuDJc0wp3sunhLzG+OpO7TpjInjSWH0npYCOw14JYXmgCc7vrSLr6OBuc1k8th7un5M5Jc5DaRE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=haHWV0Ul; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="haHWV0Ul" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50CE01F000E9; Wed, 1 Jul 2026 15:36:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782920208; bh=2ufqJUnCyvc+hknRtS5Siyc2qfmJS++aIpAUseDEK30=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=haHWV0UlWNsepxaEgoprUycVKUGfqF+k2KN2GFRSdpl7iWO34OFCmMQ+ZfZ5FjqM1 7zK7OYNRO/7CL+M7SYNokSmxdSFLdQ1DF+effczbWaWx+0QrOQkssBOIVBp/4m6FrN KUzjkt/gcrjQ8c7XSTlhKl/03Md167sdSVLDbvaGBW2HePO3IlXxTYd2WHwZBN3hcI iemgqXDYHxNDCmGDTzbS1B4uOOK6HHFk4h8CP1HD+2UXkOwAbUwC7SWHpPxjkFoUmm fiH9ucvCNzXD7N8P/xkaTE63ncimR5RCAPlH46Eo+YPIvB7a362fwZrBulhfvMyWby 44VKltThKbhUA== Message-ID: <2fb5ce53-666b-4b0a-a4ad-2b3a28c54768@kernel.org> Date: Thu, 2 Jul 2026 00:36:42 +0900 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting To: Usama Arif , Qi Zheng 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, hannes@cmpxchg.org, 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> Content-Language: en-US From: Harry Yoo In-Reply-To: <20260701145736.3785016-1-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. -- Cheers, Harry / Hyeonggon