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From: Shakeel Butt To: Qi Zheng Cc: 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, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng Subject: Re: [PATCH v2 10/28] writeback: prevent memory cgroup release in writeback module Message-ID: <5e2l35gcurnfclqtyqycylpcferjsitwjoa6ixbp5u6fng4b52@gfdmi4elp75m> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 82252C000B X-Rspamd-Server: rspam04 X-Stat-Signature: hwca6gsbz38pp63ppf16buptq5wz1ize X-HE-Tag: 1766111430-507344 X-HE-Meta: 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 LW0j8UHA 7Hjs7re3bXjffWgu6zN2gFw/ugvwV1KvodNyFzoGgGcDQyb0v7EDaIs4f55PtTD9I67y/jknziJ8jm1uZ8gVsfNobkJggI9J0fYz+NGODVKj+tbdC9fdwvJXyUhkpKBx93873qN0z4VyhtTYKqllt17iQZB06MkYBviwxlHGRzemoNgETkfZB8j7fuUtcgdS7BTcyk5gR5JQERBcyH/pb4EpPtIWBZ8pJX6zm1H2A7SCrR797dzMQHN7IpWIvKGDgkjlO 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 Wed, Dec 17, 2025 at 03:27:34PM +0800, 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 function get_mem_cgroup_css_from_folio() > and the rcu read lock are employed to safeguard against the release > of the memory cgroup. > > 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 > --- [...] > @@ -1549,9 +1549,14 @@ static inline void mem_cgroup_track_foreign_dirty(struct folio *folio, > if (mem_cgroup_disabled()) > return; > > + if (!folio_memcg_charged(folio)) > + return; > + > + rcu_read_lock(); > memcg = folio_memcg(folio); > - if (unlikely(memcg && &memcg->css != wb->memcg_css)) > + if (unlikely(&memcg->css != wb->memcg_css)) > mem_cgroup_track_foreign_dirty_slowpath(folio, wb); The slowpath in the name gave me a pause but it seems like it is safe to be called within rcu lock. Acked-by: Shakeel Butt