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Thu, 18 Dec 2025 19:56:01 -0800 (PST) Received: from localhost ([2603:7000:c01:2716:365a:60ff:fe62:ff29]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-88d997aed21sm12322946d6.30.2025.12.18.19.56.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 18 Dec 2025 19:56:00 -0800 (PST) Date: Thu, 18 Dec 2025 22:56:00 -0500 From: Johannes Weiner To: Shakeel Butt 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-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 81B95140006 X-Stat-Signature: stpa6zb5zj1qxsohh5bnz7ef6j41usfu X-HE-Tag: 1766116562-781882 X-HE-Meta: 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 c09lBgjw 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 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 Thu, Dec 18, 2025 at 10:53:57PM -0500, Johannes Weiner wrote: > On Thu, Dec 18, 2025 at 06:09:50PM -0800, Shakeel Butt wrote: > > On Wed, Dec 17, 2025 at 04:45:06PM -0500, Johannes Weiner wrote: > > > 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. > > Sorry, I did mean freeing. > > But in the new scheme, they will happen much closer together than > before, since charges don't hold a reference to the css anymore. > > So when css_killed_work_fn() does > > offline_css(css); > css_put(css); > > on rmdir, that's now the css_put() we expect to drop the refcount to 0 > even with folios in circulation. > > The race is then: > > get_mem_cgroup_from_folio() cgroup_rmdir() > memcg = folio_memcg(folio); > folio->objcg->memcg > offline_css() > reparent_objcgs() > objcg->memcg = objcg->memcg->parent > css_put() -> 0 > !css_tryget(&memcg->css) > > and the retry ensures we'll look up objcg->memcg again and find the > live parent and new owner of the folio. But yes, none of this happens until patch 27.