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 3E7D6CFD2F6 for ; Thu, 27 Nov 2025 03:49:38 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0F76D6B000C; Wed, 26 Nov 2025 22:49:37 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0D0236B000E; Wed, 26 Nov 2025 22:49:37 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F27206B0022; Wed, 26 Nov 2025 22:49:36 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id E1CF86B000E for ; Wed, 26 Nov 2025 22:49:36 -0500 (EST) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 7CF1013BEFB for ; Thu, 27 Nov 2025 03:49:36 +0000 (UTC) X-FDA: 84155007552.13.2B25323 Received: from out-184.mta0.migadu.com (out-184.mta0.migadu.com [91.218.175.184]) by imf10.hostedemail.com (Postfix) with ESMTP id 64173C0005 for ; Thu, 27 Nov 2025 03:49:34 +0000 (UTC) Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=MprLPJcp; spf=pass (imf10.hostedemail.com: domain of qi.zheng@linux.dev designates 91.218.175.184 as permitted sender) smtp.mailfrom=qi.zheng@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1764215374; 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=hQ0ZXYaEsqXDUL5h8p3v+NwU49BKhktpBBnYsZDc89I=; b=fQ3G4AOaLV3a+aVGhriv+U56xJTj1aVcvqqKpN3L/nJQ1CZ0ZWNKTNL+dw/vYkRvOA1x9S JZVZtuT4RI5Su/MTgtgUCrNrN8E/3dLnqXmm9gYnAgLLPx8K2KHXR4fHiToGbT3W5Wuiii prhvJ/ioOLmxTIkzFAAAf6rBWsHDgPU= ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=MprLPJcp; spf=pass (imf10.hostedemail.com: domain of qi.zheng@linux.dev designates 91.218.175.184 as permitted sender) smtp.mailfrom=qi.zheng@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1764215374; a=rsa-sha256; cv=none; b=MNDI9CvNdWee6QRkR9oLZXwnkdyIMKAxia3/yHKf9h+L4bu2eaiYjo6b8G3U9WcNMEV++A ntt68NBzLI4ApTDhSozxwmdKYZ0wtQmtfPzkhgzsDM0UP4/6yDRgrr4RNJ3fL4zLAXmc99 Koh7HH2xXEf8I12U/r9PcNSWwt132jg= Message-ID: <363eac01-4dd3-44c2-835a-1c843c9ad176@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1764215371; 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=hQ0ZXYaEsqXDUL5h8p3v+NwU49BKhktpBBnYsZDc89I=; b=MprLPJcpPK1AcE4rMa8EQXQpeJ8xaMTQGBfWxH6LezFLbZf+L6bnUsNF/Ns+IavWjiNpuv JbTSaofPAHEMDRMzawPDXRQ1/ZUX4WK2W+QJBN1PXCrUoZOQlS1cCeNa0kSnICyk5aim5W nZ9cof3nDUTQf6/5y1Fjdl4Jhb4qi3Q= Date: Thu, 27 Nov 2025 11:48:30 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v1 23/26] mm: vmscan: prepare for reparenting MGLRU folios To: Harry Yoo Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Qi Zheng References: 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 X-Rspamd-Queue-Id: 64173C0005 X-Rspamd-Server: rspam02 X-Stat-Signature: ezx3fzgtxr86iz1p9qi7dsmnbzb4xnn9 X-Rspam-User: X-HE-Tag: 1764215374-946653 X-HE-Meta: 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 imbkaKuc 9oRPeiA/LT8m/906BsYBuUacjUx71Ha5tqUP/fT6xaEfRGjV2B4DATvHuxEc7Fw4rbTZHlm41uN4cRLB1mgFY+kkGQE+/j/Kn6UqlRVxN++6PPv3VR1QwrnMBIsreT6qx85PAsh7t9nVMqjBDi5Ph6FQw7VdlYH5k6+U/8yiFB/SGreiqlSgtk7uqyYYvcPxV36KUirSjrUtTEfDrfp0WgVV39Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/26/25 9:48 PM, Harry Yoo wrote: > On Tue, Oct 28, 2025 at 09:58:36PM +0800, Qi Zheng wrote: >> From: Qi Zheng >> >> Similar to traditional LRU folios, in order to solve the dying memcg >> problem, we also need to reparenting MGLRU folios to the parent memcg when >> memcg offline. >> >> However, there are the following challenges: >> >> 1. Each lruvec has between MIN_NR_GENS and MAX_NR_GENS generations, the >> number of generations of the parent and child memcg may be different, >> so we cannot simply transfer MGLRU folios in the child memcg to the >> parent memcg as we did for traditional LRU folios. >> 2. The generation information is stored in folio->flags, but we cannot >> traverse these folios while holding the lru lock, otherwise it may >> cause softlockup. >> 3. In walk_update_folio(), the gen of folio and corresponding lru size >> may be updated, but the folio is not immediately moved to the >> corresponding lru list. Therefore, there may be folios of different >> generations on an LRU list. >> 4. In lru_gen_del_folio(), the generation to which the folio belongs is >> found based on the generation information in folio->flags, and the >> corresponding LRU size will be updated. Therefore, we need to update >> the lru size correctly during reparenting, otherwise the lru size may >> be updated incorrectly in lru_gen_del_folio(). >> >> Finally, this patch chose a compromise method, which is to splice the lru >> list in the child memcg to the lru list of the same generation in the >> parent memcg during reparenting. And in order to ensure that the parent >> memcg has the same generation, we need to increase the generations in the >> parent memcg to the MAX_NR_GENS before reparenting. >> >> Of course, the same generation has different meanings in the parent and >> child memcg, this will cause confusion in the hot and cold information of >> folios. But other than that, this method is simple enough, the lru size >> is correct, and there is no need to consider some concurrency issues (such >> as lru_gen_del_folio()). >> >> To prepare for the above work, this commit implements the specific >> functions, which will be used during reparenting. >> >> Suggested-by: Harry Yoo >> Suggested-by: Imran Khan >> Signed-off-by: Qi Zheng >> --- >> include/linux/mmzone.h | 16 ++++++++ >> mm/vmscan.c | 86 ++++++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 102 insertions(+) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 7aa8e1472d10d..3ee7fb96b8aeb 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -4468,6 +4468,92 @@ void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid) >> lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD); >> } >> >> +bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg) >> +{ >> + int nid; >> + >> + for_each_node(nid) { >> + struct lruvec *lruvec = get_lruvec(memcg, nid); >> + int type; >> + >> + for (type = 0; type < ANON_AND_FILE; type++) { >> + if (get_nr_gens(lruvec, type) != MAX_NR_GENS) >> + return false; >> + } >> + } >> + >> + return true; >> +} >> + >> +/* >> + * We need to ensure that the folios of child memcg can be reparented to the >> + * same gen of the parent memcg, so the gens of the parent memcg needed be >> + * incremented to the MAX_NR_GENS before reparenting. >> + */ >> +void max_lru_gen_memcg(struct mem_cgroup *memcg) >> +{ >> + int nid; >> + >> + for_each_node(nid) { >> + struct lruvec *lruvec = get_lruvec(memcg, nid); >> + int type; >> + > > I was testing this series and observed two warnings... > >> + for (type = 0; type < ANON_AND_FILE; type++) { >> + while (get_nr_gens(lruvec, type) < MAX_NR_GENS) { >> + DEFINE_MAX_SEQ(lruvec); >> + >> + inc_max_seq(lruvec, max_seq, mem_cgroup_swappiness(memcg)); >> + cond_resched(); > > Warning 1) Here we increment max_seq but we skip updating mm_state->seq. > (try_to_inc_max_seq() iterates the mm list and update mm_state->seq after > an iteration, but since we directly call inc_max_seq(), we don't update it) > > When mm_state->seq is more than one generation behind walk->seq, a warning is > triggered in iterate_mm_list(): > > VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq); > > Warning 2) In try_to_inc_max_seq(), the last walker of mm list > is supposed to succeed to increment max_seq by calling inc_max_seq(): > > if (success) { > success = inc_max_seq(lruvec, seq, swappiness); > WARN_ON_ONCE(!success); > } > > But with this patch it may observe the max_seq is already advanced due to > reparenting and thus inc_max_seq() returns false, triggering the warning. Got it. Thanks for testing and reporting! > > I'm learning MGLRU internals to see whether we can simply remove the warnings > or if we need to do something to advance max_seq without actually iterating > over the mm list. Thanks! I will also check on this. >