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From: Shakeel Butt To: Johannes Weiner Cc: Qi Zheng , 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 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio() Message-ID: References: <29e5c116de15e55be082a544e3f24d8ddb6b3476.1765956025.git.zhengqi.arch@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Stat-Signature: bk16j3fmtdjkpj9bu4gd165d1agkeomq X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 88406C0008 X-Rspam-User: X-HE-Tag: 1766110208-506332 X-HE-Meta: U2FsdGVkX1+Olgqjn0fs3gssepYoo3a1PsQjbQWRq3Ofciu18Gnl1NmCOapImYfpQKucTxxgFuu7f/Nm285/MR8Q+a9yWBzgC+tyKxlXZ8dPV4kP2NRazx+yOEjf9HSfZRrru5s0ido4xblVAL3+gSVQvbofHvDN9lC9m5apTWZt/IOngCiyLZ5FvzMOoe3daVLzv053t8BI9QkTs0VA5RUpABUlavXXnItbQ8sAAoZu3vKu5AxnLedMJeughZoTFQZs6hhYHNueHna+QkGSKK12bzK6TSqD7h1TDdtzIOZph4GDBbIAKass8hXcbwg0+Oh/mVfEY40e5HvhOHg/8S8Di0dHD9v7eU92Cu7who2N1UeSl4y2yJ6IvsVsVAxXTfYvD1CiUkbrMkweT0n2LtuEFsPg5VcuMI55ia3da85b741zU5FKveYR8/fYDsYfxPCxiEDq4863ucfZnYnZNVz4OaXBDkqG266iE+B1RNKNVVetC/eO7IxZKkpei+RyHlQnp0mlmlBxneJJR5ZlOpt69Ukjz9vwd6/ANCXuBxBkijTLuo7IbKcHCpE7CSWl1W0pIfXX/pWbOZ9/na7v+jRAW2yvKiZ2WhcPemzX08gwfQ3Wp505oZZS8CTdxmZW/cZdc9+lhFtCmnxgXKg7pAyH/saYD9H90inJQiqLkYxRgMQ4MW4hkc2S3MCCWM/Fcly2U13PQlzvjusZDWPkxK5Klp3LQLKeMXagf+OhNWRjxNM9HSMqGOUjk90PsIZMF2lvdgkOwip9eNV/hWqcTJ3exoTyroRUqCPS26FXnJ+ZvBkEaMp/N1TE3JB4DAH9BMcW3050vJSl3ELb30vyvqnbnRz2Ew3IJKiHWCt94duzxhdZW89f5QuJjTnRRq9MNvDphmadkcjU4hw+JeUFcvjh6CcMobFIoaoIIfp23DmPtxvFJvf0ZlcMpsF3yQeSLUSgNNUj5VH3DuCuIwz ySvG4SOa YmTjmt7TAKjPRnbuFOLaHfX3qA1XPw0G5bcZrWdMnofHSwkcF2JoEJ3+b1UdbooweaTQRSD4NaJbKqJK7exkx5iCbcy9cg3VlElJ8zbDrNyZHv9Xu33inXytbQuAXbN23qXpo36VgMn8I0r6S1P35usaBkoARsCHNPBEjiIcJPFkcwg3C7zbcqcQluTum2fFNvJMp1uRieUR+z3iiAzeszo2DbJcoRspeZJpXbTs4GYEtjQ1kKqMwp3cw0BjT3qeXjhC3zrf06eFUub/GdffiP4VV3euVmp+hXKFjGiUIfQC8Nwg= 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 04:45:06PM -0500, Johannes Weiner wrote: > On Wed, Dec 17, 2025 at 03:27:32PM +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 rcu read lock is employed to safeguard > > against the release of the memory cgroup in get_mem_cgroup_from_folio(). > > > > 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/memcontrol.c | 11 ++++++++--- > > 1 file changed, 8 insertions(+), 3 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 21b5aad34cae7..431b3154c70c5 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void) > > */ > > struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio) > > { > > - struct mem_cgroup *memcg = folio_memcg(folio); > > + struct mem_cgroup *memcg; > > > > if (mem_cgroup_disabled()) > > return NULL; > > > > + if (!folio_memcg_charged(folio)) > > + return root_mem_cgroup; > > + > > rcu_read_lock(); > > - if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css))) > > - memcg = root_mem_cgroup; > > +retry: > > + memcg = folio_memcg(folio); > > + if (unlikely(!css_tryget(&memcg->css))) > > + goto retry; > > So starting in patch 27, the tryget can fail if the memcg is offlined, offlined or on its way to free? It is css_tryget() without online. > and the folio's objcg is reparented concurrently. We'll retry until we > find a memcg that isn't dead yet. There's always root_mem_cgroup. > > It makes sense, but a loop like this begs the question of how it is > bounded. I pieced it together looking ahead. Since this is a small > diff, it would be nicer to fold it into 27. I didn't see anything in > between depending on it, but correct me if I'm wrong. I agree to fold it in the patch where it is needed. Currently at this point in series I don't see how css_tryget() can fail here. > > Minor style preference: > > /* Comment explaining the above */ > do { > memcg = folio_memcg(folio); > } while (!css_tryget(&memcg->css));