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From: Roman Gushchin To: Muchun Song Cc: hannes@cmpxchg.org, mhocko@kernel.org, shakeel.butt@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-Rspam-User: X-Rspamd-Queue-Id: 138D710001F X-Rspamd-Server: rspam01 X-Stat-Signature: hoj1bwskrhutu1dxh5b5a4ih6nw3rk9u X-HE-Tag: 1721929195-241905 X-HE-Meta: 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 zE3oPdq6 Ex6J8Yx3Ub9g2x2e4gTnuxRmmlwerMX+tGG3JSPGEFFUq47kbe4x84l8JvsK/7HkpnCejCyeeTRr3X+joAkd8QNgXx3yv0b8D8LZrXRYaXtIly+cQqKX99ysPQwfPLOJ/G3WZdpAJ7wKHam2pHfhJICjbSGinoRHJt1U28HBT9OQnoU2fd0g1CPnnLkwUPlfr7GRLegsEIU5jNEXivVIHV46RGfqZFq08jX2Brqw5N9RbUHwE9OCQx0+EZvfWqpWGnsxRIRk7bBygBofhNypgR6ggCd334TKAD3cV8f2nKwpb4jDlWZdR6sSJKYw9uEL7fU4z 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 +0800, 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: Roman Gushchin Thanks!