From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (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 55A69309F08 for ; Fri, 30 Jan 2026 07:22:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769757767; cv=none; b=UHVDlmlzgpYqQ6s43qzFP1Lw5xDE/gRrA7IEYzQBD7v5KkNrUkxjaUU85pXvhFZ1EBIs6Cco6v7wGNnB6CYOyKqiyif7lsksd0Qdi6vNsjN8gIesnEPYJHcXgbqHJc+RTDZf+I3ops8bkyXtK1Wr+eLCUpepvZCwBYeEC+ouY0M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769757767; c=relaxed/simple; bh=IdO5Ic3HMWIxTFWF8FWK5Joh8ZBNw0ajx0cCeF2DaHk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OsrkdW5vyn7rds3Zwj3Y1uRtIV9gC141w4RrKKknnmlQ+lDxgeok0h/06CSlyPl7F4qg/Eg+2fK9TxORN6qbCxN8VBcx0OInXi0v6wlrPTdjnvHScAh0v7eCoWwdrwHbVbgzSkSHuiufT2lNPNi2EkcU6kpDMEcUHuRzlPC928o= 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=ktmqdqo8; arc=none smtp.client-ip=95.215.58.171 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="ktmqdqo8" Message-ID: <4535d53c-68aa-4c3d-b95e-6fbafdb83881@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1769757753; 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=DMcRu4QwA2GjVjU5hQW3p3XdUu94fizF7UvrxMPIJbU=; b=ktmqdqo8hEUIvmBaLB5kEAOvoWyJMBhFHdYM1Niq9JxZhSbD4k8N9it1RiFmkRH4pg8tme WBV6lOxiVdEE4gKY79ZHnJE8JbmRG8lJ7b6x6ffnoo0YlP/LAIo6RJLQ5VrWn3bBQ3mT1S 39DregSZ+wfBsxMMJldJMaI/oqsQ050= Date: Fri, 30 Jan 2026 15:22:20 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v3 28/30] mm: memcontrol: prepare for reparenting state_local To: Harry Yoo Cc: Shakeel Butt , hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, muchun.song@linux.dev, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, yosry.ahmed@linux.dev, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chenridong@huaweicloud.com, mkoutny@suse.com, akpm@linux-foundation.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Qi Zheng References: <9b9057f8-4c4c-4067-b6ba-0791888c25e8@linux.dev> <6860146b-be12-4d5f-bec1-bbcec1dffbc6@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 On 1/29/26 8:23 PM, Harry Yoo wrote: > On Thu, Jan 29, 2026 at 04:50:39PM +0800, Qi Zheng wrote: >> >> >> On 1/29/26 10:10 AM, Harry Yoo wrote: >>> On Mon, Jan 19, 2026 at 11:34:53AM +0800, Qi Zheng wrote: >>>> >>>> >>>> On 1/18/26 11:20 AM, Shakeel Butt wrote: >>>>> On Wed, Jan 14, 2026 at 07:32:55PM +0800, Qi Zheng wrote: >>>>>> From: Qi Zheng >>>>>> >>>>>> To resolve the dying memcg issue, we need to reparent LRU folios of child >>>>>> memcg to its parent memcg. The following counts are all non-hierarchical >>>>>> and need to be reparented to prevent the counts of parent memcg overflow. >>>>>> >>>>>> 1. memcg->vmstats->state_local[i] >>>>>> 2. pn->lruvec_stats->state_local[i] >>>>>> >>>>>> This commit implements the specific function, which will be used during >>>>>> the reparenting process. >>>>> >>>>> Please add more explanation which was discussed in the email chain at >>>>> https://lore.kernel.org/all/5dsb6q2r4xsi24kk5gcnckljuvgvvp6nwifwvc4wuho5hsifeg@5ukg2dq6ini5/ >>>> >>>> OK, will do. >>>> >>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>>>>> index 70583394f421f..7aa32b97c9f17 100644 >>>>>> --- a/mm/memcontrol.c >>>>>> +++ b/mm/memcontrol.c >>>>>> @@ -225,6 +225,28 @@ static inline struct obj_cgroup *__memcg_reparent_objcgs(struct mem_cgroup *memc >>>>>> return objcg; >>>>>> } >>>>>> +#ifdef CONFIG_MEMCG_V1 >>>>>> +static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force); >>>>>> + >>>>>> +static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent) >>>>>> +{ >>>>>> + if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) >>>>>> + return; >>>>>> + >>>>>> + synchronize_rcu(); >>>>> >>>>> Hmm synchrinuze_rcu() is a heavy hammer here. Also you would need rcu >>>>> read lock in mod_memcg_state() & mod_memcg_lruvec_state() for this >>>>> synchronize_rcu(). >>>> >>>> Since these two functions require memcg or lruvec, they are already >>>> within the critical section of the RCU lock. >>> >>> What happens if someone grabbed a refcount and then release the rcu read >>> lock before percpu refkill and then call mod_memcg[_lruvec]_state()? >>> >>> In this case, can we end up reparenting in the middle of non-hierarchical >>> stat update because they don't have RCU grace period? >>> >>> Something like >>> >>> T1 T2 >>> >>> - rcu_read_lock() >>> - get memcg refcnt >>> - rcu_read_unlock() >>> >>> - call mod_memcg_state() >>> - CSS_IS_DYING is not set >>> - Set CSS_IS_DYING >>> - Trigger percpu refkill >>> >>> - Trigger offline_css() >>> -> reparent non-hierarchical - update non-hierarchical stats >>> stats >>> - put memcg refcount >> >> Good catch, I think you are right. >> >> The rcu lock should be added to mod_memcg_state() and >> mod_memcg_lruvec_state(). > > Thanks for confirming! > > Because it's quite confusing, let me ask few more questions... > > Q1. Yosry mentioned [1] [2] that stat updates should be done in the same > RCU section that is used to grab a refcount of the cgroup. > > But I don't think your work is relying on that. Instead, I guess, it's > relying on the CSS_DYING check from reader side to determine whether it Only relying the CSS_DYING check is insufficient. Otherwise, the following race may occur: T1 T2 memcg_is_dying is false Set CSS_IS_DYING reparent non-hierarchical update non-hierarchical stats for child So IIUC we should add rcu lock, then: T1 T2 rcu_read_lock memcg_is_dying is false Set CSS_IS_DYING update non-hierarchical stats for child rcu_read_unlock synchronize_rcu or rcu work --> reparent non-hierarchical > should update stats of the child or parent memcg, right? > > -> That being said, when rcu_read_lock() is called _after_ stats are > reparented, the reader must observe that the CSS_DYING flag is set. > > [1] https://lore.kernel.org/all/utl6esq7jz5e4f7kwgrpwdjc2rm3yi33ljb6xkm5nxzufa4o7s@hblq2piu3vnz > [2] https://lore.kernel.org/all/ebdhvcwygvnfejai5azhg3sjudsjorwmlcvmzadpkhexoeq3tb@5gj5y2exdhpn > > Q2. When a reader checks CSS_DYING flag, how is the flag change > guaranteed to be visible to the reader without any lock, memory barrier, > or atomic ops involved? The main situation requiring CSS_DYING check is as follow: T1 T2 Set CSS_IS_DYING synchronize_rcu or rcu work --> reparent non-hierarchical rcu_read_lock() memcg_is_dying is true update non-hierarchical stats for parent Referring to the "Memory-Barrier Guarantees" section in [1], synchronize_rcu() can guarantee that T2 can see CSS_IS_DYING. Right? [1]. https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.rst Thanks, Qi > > As Shakeel mentioned elsewhere, I hope some explanations for correctness > to be included in the commit message :) > >> I will update to v4 as soon as possible. > > Thanks a lot! > > I'll wait for that and will review carefully to make sure it's correct ;) > >> Thanks, >> Qi >> >>>>> Hmm instead of synchronize_rcu() here, we can use queue_rcu_work() in >>>>> css_killed_ref_fn(). It would be as simple as the following: >>>> >>>> It does look much simpler, will do. >>>> >>>> Thanks, >>>> Qi >