From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id AAB57CD4F26 for ; Tue, 23 Jun 2026 09:15:32 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 975AA6B0092; Tue, 23 Jun 2026 05:15:31 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 94D666B0093; Tue, 23 Jun 2026 05:15:31 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 863306B0095; Tue, 23 Jun 2026 05:15:31 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 5DEA56B0092 for ; Tue, 23 Jun 2026 05:15:31 -0400 (EDT) Received: from smtpin05.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay06.hostedemail.com (Postfix) with ESMTP id DCBEC1C5847 for ; Tue, 23 Jun 2026 09:15:30 +0000 (UTC) X-FDA: 84910619220.05.38F8288 Received: from out-181.mta1.migadu.com (out-181.mta1.migadu.com [95.215.58.181]) by imf30.hostedemail.com (Postfix) with ESMTP id CD1CC80009 for ; Tue, 23 Jun 2026 09:15:26 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=LObtBUp1; spf=pass (imf30.hostedemail.com: domain of qi.zheng@linux.dev designates 95.215.58.181 as permitted sender) smtp.mailfrom=qi.zheng@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1782206128; b=x8ipEpwCTSoZN/w4fk3R4tKVnRhd9DLs1cbyDi/6fqBQTnYGWqJxY55PSUPa6GHff8Cmw5 PTUxdQDfNncN0R2AkZudAcn9II9yZK26ciyQt76L6X0wDVHnC5KDcIXzbpAsznOKk0aMwt kVREBMB7zR3JJgw0t9p4fBSCheT9uvg= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1782206128; h=from:from:sender: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:dkim-signature; bh=tpZginHVqkNU49eIJBOmNB7unJDksJ/MhSRIVKp8tno=; b=ErdRsFMwerYolnPFGIwhJ51mC7LK8707v/TTzuHfQZ0+2hbXZ157Q/Oxh7s7QAe0nFUcoj o77IPZh3WnS19FFti3g73h5LcCywE4kLF5ScLVtVoigq0Gy+K/4ckN2zgND5UhIKx4jTNp S83zuzM2Rvaec+/dqZn8hohWGhO/Bno= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=LObtBUp1; spf=pass (imf30.hostedemail.com: domain of qi.zheng@linux.dev designates 95.215.58.181 as permitted sender) smtp.mailfrom=qi.zheng@linux.dev; dmarc=pass (policy=none) header.from=linux.dev Message-ID: <7946da94-dc1d-4cf2-986e-466c378665b6@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782206124; 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=tpZginHVqkNU49eIJBOmNB7unJDksJ/MhSRIVKp8tno=; b=LObtBUp15MZd/tRyF7vr3WJJOlnw8HBAHu6hLIba57mzs7Zah7zHFXPQeIrqlzSiWm2SPx cRxNmiT21o/E/q/mMWXss0B5GOBio155kO+Qie4Ve7AXwTvhf7RPepF5ZGbFjhtkDD/LSQ W+oRfd2s5LCelV8dP+jrElKGpU8Tx4U= Date: Tue, 23 Jun 2026 17:14:17 +0800 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> <8a76aefd-629c-41f3-b365-aefd4cc1411e@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: <8a76aefd-629c-41f3-b365-aefd4cc1411e@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: 1x5n6ype7emfzrp1o7dnt16y8jihskwk X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: CD1CC80009 X-HE-Tag: 1782206126-540132 X-HE-Meta: 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 BZEuxGTo MSoIoCARm3zCeS6M/CeiSTb6a1RsItK5TZLiTIo5wL8fIL32iluZM9CNr4UAevrn3Mi3oX8QS/etbd+56SeOn5+F+77H4tPld/gC895bnlR13xYxEI3Ay4dkLR/ZDKD+byr+6e47sbQXrzGUdlEwDD2p2aSnR9LITeyR8WgqaVVb9PaLEBGhUDy7/dH5RBQQV2AASQbwdnrBClYYz5c3WpeOlrqTJ8NG2NiSX67yiNX8BAKc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Harry, On 6/23/26 4:18 PM, Harry Yoo wrote: > > > On 6/23/26 4:16 PM, Qi Zheng wrote: >> Hi Harry, > > Hi Qi! > >> 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...". > > Thanks! > >>>> 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. > > Right. > >> I also considered using mem_cgroup_tryget_online(), but that only prevent >> the memcg from being freed. It's doesn't prevent the offlining. > > Right. > > I think checking CSS_DYING under RCU and grabbing the lruvec > of the first non-dying memcg should work (this pattern is already > used where we use RCU to guarantee memcgs are not freed). > > If we do not observe CSS_DYING flag, it is safe to charge deltas > to the lruvec because RCU guarantees that reparenting cannot happen > under us. > > If we do observe CSS_DYING, we can walk up the hierarchy and charge > deltas to the first non-dying memcg. Checking CSS_DYING looks feasible, but the rcu lock alone cannot prevent reparenting. We should recheck CSS_DYING after acquiring the lruvec lock, otherwise we might run into the following race: CPU0 reset_batch_size CPU1 memcg teardown ===================== ================== read !CSS_DYING set CSS_DYING memcg_reparent_objcgs() lock child lruvec move child to parent zero child nr_pages unlock child lruvec lock child lruvec charge stale delta to child So it seems lock_batch_lruvec() should be implemented like this: static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec) { struct mem_cgroup *memcg = lruvec_memcg(lruvec); rcu_read_lock(); retry: while (memcg && css_is_dying(&memcg->css)) memcg = parent_mem_cgroup(memcg); lruvec = mem_cgroup_lruvec(memcg, pgdat); spin_lock_irq(&lruvec->lru_lock); if (memcg && unlikely(css_is_dying(&memcg->css))) { spin_unlock_irq(&lruvec->lru_lock); goto retry; } rcu_read_unlock(); return lruvec; } This way, there is no need to add lrugen->reparented, right? Thanks, Qi