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From: Shakeel Butt To: Sebastian Andrzej Siewior Cc: Vlastimil Babka , Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [RFC PATCH 06/10] memcg: do obj_cgroup_put inside drain_obj_stock Message-ID: References: <20250314061511.1308152-1-shakeel.butt@linux.dev> <20250314061511.1308152-7-shakeel.butt@linux.dev> <0b3ab5e5-e684-44ce-b6ed-276ad37784e6@suse.cz> <20250314113533.jNrVXeyr@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250314113533.jNrVXeyr@linutronix.de> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: B4B72C0011 X-Stat-Signature: 7ufixt4cyqdokecaksdsdtmwrhx1ekmx X-Rspamd-Server: rspam06 X-HE-Tag: 1741966170-818862 X-HE-Meta: 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 iTOieOWM BYVaD/YO3uBiOQhNHr+kMOxoOlAqH87TO/Zx7OtpsxvkHdTF7gDr67V1q9KX4r7XEzx7mA1lIpKzzgnKkuwwl1s8Ewt7rpY6h92SaVrPsQSnXBFlb6W7oVXbVg1wtvrGOLSS/5QLxHRjA8SR/mzEFmxJuVdUzGtsp5yt/V/ejoEohw2L8pg7jOHGucv6tDYReiqL9Ff7wLa0tGz3P19fLCbDQsnX1nfK/sCJ4 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 Fri, Mar 14, 2025 at 12:35:33PM +0100, Sebastian Andrzej Siewior wrote: > On 2025-03-14 11:17:28 [+0100], Vlastimil Babka wrote: > > On 3/14/25 07:15, Shakeel Butt wrote: > > > Previously we could not call obj_cgroup_put() inside the local lock > > > because on the put on the last reference, the release function > > > obj_cgroup_release() may try to re-acquire the local lock. However that > > > chain has been broken. Now simply do obj_cgroup_put() inside > > > drain_obj_stock() instead of returning the old objcg. > > > > > > Signed-off-by: Shakeel Butt > > > > Hm is this really safe? I can see obj_cgroup_release() doing > > percpu_ref_exit() -> kfree(), do we have guaranteed that allocation won't be > > also in a kmemcg and recurse? > > This was like this until commit > 5675114623872 ("mm/memcg: protect memcg_stock with a local_lock_t") > > at which point the put had to happen outside. This "percpu_ref_exit() -> > kfree()" was also prior this commit. Yes, as Sebastian said, this is as safe as before commit 567511462387. Also the ref->data which is getting kfree()'ed from percpu_ref_exit() is not a __GFP_ACCOUNT allocation, so can't recurse.