From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (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 712D0328602; Thu, 18 Dec 2025 11:41:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766058083; cv=none; b=Fsxd04vBnvy2LmkVKn9qDMUusX24rIjLQjgg8aCJYEF95kEfG7bPoXsa8uvd/OwbQGaiZM4Lp4J+rcr6BmNEkYANralxqigocVVTbEFAiFSTvfjpAcMiAY6tg8HyIEu7A99Qys/PTHnti5sKrB6uEVGKMvPld9+//RXd0KNp/FY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766058083; c=relaxed/simple; bh=m+Frw1gPIt8tDFtm76USz7dxD4XOJp5bAs+/tb1DLzU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ay7UvISU+/NfJDQlpNSorq/v+3+iA/Pl2wdq5xo/Reqr5hPw6DN9K00KGeqdBZz4tOKR1kK7rivDiqai7LgY0u/VcFP/TbbKZ1wjPF87j84fNo5TcyhJh3oMOdVMNLDCWrF6pYjHv1t+40Bsks6Kjg1KVJAXWU0qC3UCKdwqqIU= 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=E7H2Crh7; arc=none smtp.client-ip=91.218.175.180 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="E7H2Crh7" Message-ID: <4effa243-bae3-45e4-8662-dca86a7e5d12@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766058071; 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=5bv+aDdmLUwbT/Rl2vd0tHpuESUGfI/xwE/ZSUQEBgM=; b=E7H2Crh7Tjtpdw3/pt1bEzDFr+XY5jMekcgGa0dJx6pR8/ey2yQI9Q3z9khKF/MXx1eOrT wZtnaWkFqPthwPwTH3GtZ36Msdu2nBZ71cmvI2IZ3C+jixvKGkBR54aZuNXH/ExEc2lguI Hyjjb81j7HxKPFwmarfVQOLO3jGFnvo= Date: Thu, 18 Dec 2025 19:40:43 +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 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping() To: "David Hildenbrand (Red Hat)" , hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, lorenzo.stoakes@oracle.com, ziy@nvidia.com, harry.yoo@oracle.com, 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng References: <1554459c705a46324b83799ede617b670b9e22fb.1765956025.git.zhengqi.arch@bytedance.com> <3a6ab69e-a2cc-4c61-9de1-9b0958c72dda@kernel.org> <02c3be32-4826-408d-8b96-1db51dcababf@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: 8bit X-Migadu-Flow: FLOW_OUT On 12/18/25 5:43 PM, David Hildenbrand (Red Hat) wrote: > On 12/18/25 10:36, Qi Zheng wrote: >> >> >> On 12/18/25 5:09 PM, David Hildenbrand (Red Hat) wrote: >>> On 12/17/25 08:27, Qi Zheng wrote: >>>> From: Muchun Song >>>> >>>> In the near future, a folio will no longer pin its corresponding >>>> memory cgroup. To ensure safety, it will only be appropriate to >>>> hold the rcu read lock or acquire a reference to the memory cgroup >>>> returned by folio_memcg(), thereby preventing it from being released. >>>> >>>> In the current patch, the rcu read lock is employed to safeguard >>>> against the release of the memory cgroup in folio_migrate_mapping(). >>> >>> We usually avoid talking about "patches". >> >> Got it. >> >>> >>> In __folio_migrate_mapping(), the rcu read lock ... >> >> Will do. >> >>> >>>> >>>> This serves as a preparatory measure for the reparenting of the >>>> LRU pages. >>>> >>>> Signed-off-by: Muchun Song >>>> Signed-off-by: Qi Zheng >>>> Reviewed-by: Harry Yoo >>>> --- >>>>    mm/migrate.c | 2 ++ >>>>    1 file changed, 2 insertions(+) >>>> >>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>> index 5169f9717f606..8bcd588c083ca 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct >>>> address_space *mapping, >>>>            struct lruvec *old_lruvec, *new_lruvec; >>>>            struct mem_cgroup *memcg; >>>> +        rcu_read_lock(); >>>>            memcg = folio_memcg(folio); >>> >>> In general, LGTM >>> >>> I wonder, though, whether we should embed that in the ABI. >>> >>> Like "lock RCU and get the memcg" in one operation, to the "return memcg >>> and unock rcu" in another operation. >> >> Do you mean adding a helper function like get_mem_cgroup_from_folio()? > > Right, something like > > memcg = folio_memcg_begin(folio); > folio_memcg_end(memcg); For some longer or might-sleep critical sections (such as those pointed by Johannes), perhaps it can be defined like this: struct mem_cgroup *folio_memcg_begin(struct folio *folio) { return get_mem_cgroup_from_folio(folio); } void folio_memcg_end(struct mem_cgroup *memcg) { mem_cgroup_put(memcg); } But for some short critical sections, using RCU lock directly might be the most convention option? > > Maybe someone reading along has a better idea. Then you can nicely > document the requirements in the kerneldocs, and it is clear why the RCU > lock is used (internally). >