From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 6AE552F83A2; Thu, 18 Dec 2025 13:05:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766063107; cv=none; b=Nh1nD75vsHo7QDHNXINJCE3pvSgnW8f0aGITBEzao36RNmL52EjWdYp51N2ZIxVjZhaFDrYofu26DZonZSthlto1pKc3A0jxyDmNk+WOAcMvPszuw0NIZEf8who1zYje/y2LsHDt0UqCc6eifvnqJ2NpCs3iMdRFixly9Y2f85M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766063107; c=relaxed/simple; bh=wlrGBAWuPnDkrqpZ3OpcV2IRiPY3keO1pBIgIrsLYXE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Z385aT1orOVZposaMhtN5uG/0hemamGt3wKODa4Fv61yX1hgBC+LHt2xJ77R7kcrpOX1jfGCtdBW+HWRYQFDLCZ4z3aDyKm5dE8tF9yF5GrDgagHmgBvMpp3mRaoblGvlBWafr/L/neVoF5EQdj/22bWxauu5B+87MT3SxxaiCA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MCE/KsJk; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="MCE/KsJk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D0B91C4CEFB; Thu, 18 Dec 2025 13:04:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766063106; bh=wlrGBAWuPnDkrqpZ3OpcV2IRiPY3keO1pBIgIrsLYXE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=MCE/KsJkd9f5ZrdGW1PLNun0XeCg+Xh8HbqQz+yv/3v0MYk96s7RrtTDXtLgPZ9RM c4IWPMVpcydxnrnRdx7aYdEoDAOfXET6HnqQTCCg9k5OS0pXgNjVumFXVJWqaAPjC2 g1+GjcIaoltN5KwMKBW7WSQi41HH15lg09vE2q+vdNJKJYBkGeLGmiqPv13KWRpANB /L+9U48s4svbJDE3vOEobqtgg/JwZ0N0WksL9xYxPjJzbl9vR/wjRjSv8KQLRyNKs6 MnEu8di+BbAhXRzd06agcIeS703vdtZ2osdRTfk7JpLtVsNMtsaUnQnpAmgXmUN3cs QJ7UOuedxQ51Q== Message-ID: <49341ca3-1fc9-43d9-abbd-ecaabdda6ce0@kernel.org> Date: Thu, 18 Dec 2025 14:04:54 +0100 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping() To: Qi Zheng , 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> <4effa243-bae3-45e4-8662-dca86a7e5d12@linux.dev> <11a60eba-3447-47de-9d59-af5842f5dc5e@kernel.org> <3c32d80a-ba0e-4ed2-87ae-fb80fc3374f7@linux.dev> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <3c32d80a-ba0e-4ed2-87ae-fb80fc3374f7@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 12/18/25 14:00, Qi Zheng wrote: > > > On 12/18/25 7:56 PM, David Hildenbrand (Red Hat) wrote: >> On 12/18/25 12:40, Qi Zheng wrote: >>> >>> >>> 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? >>> >> >> Then put the rcu read locking in there instead? > > So for some longer or might-sleep critical sections, using: > > memcg = folio_memcg_begin(folio); > do_some_thing(memcg); > folio_memcg_end(folio); > > for some short critical sections, using: > > rcu_read_lock(); > memcg = folio_memcg(folio); > do_some_thing(memcg); > rcu_read_unlock(); > > Right? What I mean is: memcg = folio_memcg_begin(folio); do_some_thing(memcg); folio_memcg_end(folio); but do the rcu_read_lock() in folio_memcg_begin() and the rcu_read_unlock() in folio_memcg_end(). You could also have (expensive) variants, as you describe, that mess with getting/dopping the memcg. But my points was about hiding the rcu details in a set of helpers. Sorry if what I say is confusing. -- Cheers David