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 5045A1E5207; Fri, 19 Dec 2025 06:18:34 +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=1766125114; cv=none; b=eNT2Kec9mhsgdxwP15gAuqCxV6lgsRfsMAdi/EIBgAlgWg7WJ8D8Q+nLTKjCtGQtzBleYslWdUAwT8qN6/0Y0LvC4CtovxoH368dOEzz40qhXOVPFX32SJMpcXXYhDGJRimMkbvV1+qx9kJD6iTQDWm56ZCV0N2D5Gzvsizh5dE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766125114; c=relaxed/simple; bh=xMlIHS8RXqYBLJVVEzUZMFwENBOT//REo0D9YNRvPpU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MI13eLjmYkaXGO45ZejUT3fV6TN3z/mrfRL3vsheFSwGi/haSHJrDNdZjh+NVCUD4f+q+F4VzcGKnat7kRvWBCWoD6e06yEDlY19QsPfMPcJo6GAmPU2VH7gSZ95Mi5xteoriqULkUlhZf+hb4Oc1CdzQdUhOLWyjV7tjUZiv34= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gqgGZDtI; 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="gqgGZDtI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 32648C4CEF1; Fri, 19 Dec 2025 06:18:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766125113; bh=xMlIHS8RXqYBLJVVEzUZMFwENBOT//REo0D9YNRvPpU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=gqgGZDtI1zhIVleIDAu0xskWmE2Fgbb2lH2Cz0pdmMBcNwjk/LP2OLp6DR8sE/ncy JtEBvuEw8OFnUCfpw+z7srVGdjs1qv3OQ1JkobH4OzPRQfbisNpbB05U3+sd8bOwI/ JcuuwoAMEzD3Svwp/O/KZ1rxsdZkIWAWzS4W5RQ+QwBUf6+rh5TEC6X2ivhlRod2cy tWyT+yEe0d+uJ+WHQo415+9ft7U2x7LtPu22tFVp/1GYVihMOWQlepnAPcCAsJM70e NOxy3x+GSmfieI7dfWpIOeUmfBv4H7puXAAU4YwJiJHg0gXzyKhS0Z7zqTCKPAtruT ED/lw+zbKuf1w== Message-ID: <1b87adbb-a34d-4c7e-98d4-664ccf71fc60@kernel.org> Date: Fri, 19 Dec 2025 07:18:21 +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: Harry Yoo , Qi Zheng Cc: 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, 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, 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> <49341ca3-1fc9-43d9-abbd-ecaabdda6ce0@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 12/19/25 05:12, Harry Yoo wrote: > On Thu, Dec 18, 2025 at 09:16:11PM +0800, Qi Zheng wrote: >> >> >> On 12/18/25 9:04 PM, David Hildenbrand (Red Hat) wrote: >>> 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. >> >> Or simple use folio_memcg_begin(memcg)/folio_memcg_end(memcg) in all cases. >> >> Or add a parameter to them: >> >> struct mem_cgroup *folio_memcg_begin(struct folio *folio, bool get_refcnt) >> { >> struct mem_cgroup *memcg; >> >> if (get_refcnt) >> memcg = get_mem_cgroup_from_folio(folio); >> else { >> rcu_read_lock(); >> memcg = folio_memcg(folio); >> } >> >> return memcg; >> } >> >> void folio_memcg_end(struct mem_cgroup *memcg, bool get_refcnt) >> { >> if (get_refcnt) >> mem_cgroup_put(memcg); >> else >> rcu_read_unlock(); >> } > > I would like to vote for open coding as we do now, because I think hiding > the RCU lock / refcount acquisition into a less obvious API doesn't make > it more readable. I wouldn't do it in an API as proposed above. I prefer to not have magical RCU locking in every caller. Easy to get wrong. See how we did something similar in the pte_*map*() vs. pte_unmap() API, without requiring all callers to open-code this. -- Cheers David