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Wed, 17 Dec 2025 14:27:18 -0800 (PST) Date: Wed, 17 Dec 2025 17:27:17 -0500 From: Johannes Weiner To: Qi Zheng Cc: hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@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, Qi Zheng Subject: Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in folio_split_queue_lock{_irqsave}() Message-ID: References: <4cb81ea06298a3b41873b7086bfc68f64b2ba8be.1765956025.git.zhengqi.arch@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4cb81ea06298a3b41873b7086bfc68f64b2ba8be.1765956025.git.zhengqi.arch@bytedance.com> On Wed, Dec 17, 2025 at 03:27:41PM +0800, Qi Zheng wrote: > From: Qi Zheng > > 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_split_queue_lock{_irqsave}(). > > Signed-off-by: Qi Zheng > Reviewed-by: Harry Yoo > --- > mm/huge_memory.c | 16 ++++++++++++++-- > 1 file changed, 14 insertions(+), 2 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 12b46215b30c1..b9e6855ec0b6a 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1154,13 +1154,25 @@ split_queue_lock_irqsave(int nid, struct mem_cgroup *memcg, unsigned long *flags > > static struct deferred_split *folio_split_queue_lock(struct folio *folio) > { > - return split_queue_lock(folio_nid(folio), folio_memcg(folio)); > + struct deferred_split *queue; > + > + rcu_read_lock(); > + queue = split_queue_lock(folio_nid(folio), folio_memcg(folio)); > + rcu_read_unlock(); Ah, the memcg destruction path is acquiring the split queue lock for reparenting. Once you have it locked, it's safe to drop the rcu lock. Acked-by: Johannes Weiner