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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 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 X-Rspam-User: X-Rspamd-Queue-Id: 1A72310000E X-Rspamd-Server: rspam04 X-Stat-Signature: z93carycsqnoe4hmohj6facsocdfpr1p X-HE-Tag: 1766063107-722592 X-HE-Meta: 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 Q5+f3QRM IZ6uk0uPtGdC/s+q9VNE8Q04vvHeACY5x+udr/XYbQuOs0TevR6DVapS+tyRp7WnjbUYOsAIvd2Vym0+ydANa8UXbUWx31qJcnXz42+V4Et01//tpfT2EJEb186bXnfNjVVl63S2yCnJUnEgE5JzVhmCW//vQv0xkqPbY4nk5F4lrfZWeytPdsNJrCO2ULQE2epRAojlLHYiV43W4IssjdQ+1OhOrr09cL2Mc3amgr6QDljhTPPRneQgsJklFYMAQsnIp1htpv1aWBuK0qfz8u2UQXOrH/fNPNB87aewYIwijXdD2tZztU84fB5C9rqErtgIfuK8jvm3zMv67q9WQWm18PmYS8FrzWLXB+uZEJjnP2Heb7IwVDEsPMR0N34VUaozw 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 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