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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rk7Ld-00000003gRr-0sAR; Tue, 12 Mar 2024 18:59:37 +0000 Date: Tue, 12 Mar 2024 18:59:37 +0000 From: Matthew Wilcox To: Roman Gushchin Cc: Vlastimil Babka , Linus Torvalds , Josh Poimboeuf , Jeff Layton , Chuck Lever , Kees Cook , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , Alexander Viro , Christian Brauner , Jan Kara , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH RFC 1/4] mm, slab: move memcg charging to post-alloc hook Message-ID: References: <20240301-slab-memcg-v1-0-359328a46596@suse.cz> <20240301-slab-memcg-v1-1-359328a46596@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 18C2F4001C X-Rspam-User: X-Stat-Signature: 483ke1eqam41qc5ff3nq8u1fgne4qn96 X-Rspamd-Server: rspam03 X-HE-Tag: 1710269989-80788 X-HE-Meta: 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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 Tue, Mar 12, 2024 at 11:52:46AM -0700, Roman Gushchin wrote: > On Fri, Mar 01, 2024 at 06:07:08PM +0100, Vlastimil Babka wrote: > > @@ -1926,71 +1939,51 @@ static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s, > > return false; > > } > > > > - if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s))) > > + if (obj_cgroup_charge(objcg, flags, size * obj_full_size(s))) > > return false; > > > > - *objcgp = objcg; > > + for (i = 0; i < size; i++) { > > + slab = virt_to_slab(p[i]); > > Not specific to this change, but I wonder if it makes sense to introduce virt_to_slab() > variant without any extra checks for this and similar cases, where we know for sure > that p resides on a slab page. What do you think? You'd only save a single test_bit() ... is it really worth doing? Cache misses are the expensive thing, not instructions. And debugging time: if somehow p[i] becomes not-on-a-slab-anymore, getting a NULL pointer splat here before we go any further might be worth all the CPU time wasted doing that test_bit().