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From: Shakeel Butt To: Muchun Song Cc: hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, muchun.song@linux.dev, akpm@linux-foundation.org, vbabka@kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] mm: kmem: add lockdep assertion to obj_cgroup_memcg Message-ID: References: <20240725094330.72537-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240725094330.72537-1-songmuchun@bytedance.com> X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: CB9EC2002A X-Stat-Signature: ju9x8nykgztgw9zmpnxjeif84kax5szm X-HE-Tag: 1721923107-193579 X-HE-Meta: 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 3O4Kcu7W BEaN+R/amvFjOr+EDmUOTODjzhynwMJc/OiacT2HUWk9ik7cnc82k+9n2Pg0dVTjinFBSOnOek6UdUca8IDkbMcro103h5NWrI+PZD0rpJV4Qw2VbGgrHQJ+4AGzg+DClu/FPYPSrTA2C+eB6nMToX6kr5TQnGBNb7p4sM1GL4hZZoSh9zuVMpDrJ06Oeza6AEjfqJe+i4s2+olgmFOkg31/q9c3h2y01/ZZnzhtB+k8ZaLZj8TBVtb399YjbHCU++TL1eWe4eptumsWxe3MvRhbxzQ== 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, Jul 25, 2024 at 05:43:30PM GMT, Muchun Song wrote: > The obj_cgroup_memcg() is supposed to safe to prevent the returned > memory cgroup from being freed only when the caller is holding the > rcu read lock or objcg_lock or cgroup_mutex. It is very easy to > ignore thoes conditions when users call some upper APIs which call > obj_cgroup_memcg() internally like mem_cgroup_from_slab_obj() (See > the link below). So it is better to add lockdep assertion to > obj_cgroup_memcg() to find those issues ASAP. > > Because there is no user of obj_cgroup_memcg() holding objcg_lock > to make the returned memory cgroup safe, do not add objcg_lock > assertion (We should export objcg_lock if we really want to do). > Additionally, this is some internal implementation detail of memcg > and should not be accessible outside memcg code. > > Some users like __mem_cgroup_uncharge() do not care the lifetime > of the returned memory cgroup, which just want to know if the > folio is charged to a memory cgroup, therefore, they do not need > to hold the needed locks. In which case, introduce a new helper > folio_memcg_charged() to do this. Compare it to folio_memcg(), it > could eliminate a memory access of objcg->memcg for kmem, actually, > a really small gain. > > Link: https://lore.kernel.org/all/20240718083607.42068-1-songmuchun@bytedance.com/ > Signed-off-by: Muchun Song Acked-by: Shakeel Butt