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 B19FE25B0BD for ; Tue, 23 Jun 2026 07:17:04 +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=1782199026; cv=none; b=SgDVCrk8RWzliUFaU2Dgyca/xZVkce74m55CC1V0TZ+unyqheeCAhqfsO1iMtojIuz4THdmLYoAhcN/GIFek1fCWKK37nXcGSmtUk8gy2UaXzGgUcpJKTdnzYvo2x5gxn1UrCEfxOezwv0Rj7gMNYA2LtYTCl11vJW4lmOcR+5s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782199026; c=relaxed/simple; bh=l1gQ6oU5AxLIn/0A7p5EEoFlikm6dyZM/gbIW61sjS8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=i0JNI9Dy4aNm+QRaZQV3e8fShhZ5nYneuWXKmI0Fjvf7aCgr2jqOcKYRTg2dGMkuOjfIGfOTi6uloJKguZhBBmDD37kbBV+7KBVhK4cZzNtbL36U8AtgPviqiRwLJ2e3JD2/e5VMrpc8BC5OCyqv7c3YS9siT0wISozpY/gSefE= 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=jQrd2K6W; 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="jQrd2K6W" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782199012; 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=zZ+uAaSGd3OZS1aU2uMCCmFDj6tXs2rYH2kiB6ZZtOs=; b=jQrd2K6Wcjb+o/3i0dgciyF91g4QWNDhUnuaOH0U/WqJXuqyfCODDSAdXGvqwnfORs9/pp NoJIu+nDxRDWNjuYV3koTQWxNHMwTCB4948DzxfGQiXU4rgwzn1XJNWgfBp0ZlycHlj8Bs 6FPTSMSFVXKDjpvJFk6zfgKzttDwzSc= Date: Tue, 23 Jun 2026 15:16:16 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2] mm: mglru: fix stale batch updates after memcg reparenting To: Harry Yoo , 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng , stable@vger.kernel.org References: <20260623024237.45990-1-qi.zheng@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 Hi Harry, On 6/23/26 2:17 PM, Harry Yoo wrote: > > > On 6/23/26 11:42 AM, 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. > > Ouch. > > IIRC 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. > > Perhaps worth mentioning in the changelog? Maybe this should be placed before "To fix it...". > >> 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 > > The problem here is that, while grabbing a reference to memcg > (via mem_cgroup_iter(), for example) makes sure that the memcg is not > freed, it does not prevent offlining happening, and reset_batch_size() > doesn't check whether the lruvec has been reparented, or the lruvec > is going to be reparented. > >> 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))); >> >> To fix it, add lrugen->reparented to remember the new owner of a >> reparented lruvec, and make reset_batch_size() charge pending deltas to >> that owner. > > Could you please explain why it is unavoidable to introduce the new > field and why checking whether the cgroup is dying (and charging deltas > to non-dying parent) doesn't work? Peiyang tried doing this [1], but it doesn't work because ss->css_offline() is called before clearing the CSS_ONLINE flag. I also considered using mem_cgroup_tryget_online(), but that only prevent the memcg from being freed. It's doesn't prevent the offlining. So in the end, I chose the approach used in this patch. Simply adding a new field to mglru to track its reparenting status seems to be the most straightforward and effective approach. Thanks, Qi [1]. https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn > >> 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: Barry Song >> --- >