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[91.12.106.18]) by smtp.gmail.com with ESMTPSA id l20sm3150399wmq.42.2021.10.12.04.09.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Oct 2021 04:09:28 -0700 (PDT) Subject: Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0 To: Vasily Averin , Vlastimil Babka , Michal Hocko Cc: Johannes Weiner , Vladimir Davydov , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel@openvz.org, Mel Gorman , Uladzislau Rezki References: <29155011-f884-b0e5-218e-911039568acb@suse.cz> From: David Hildenbrand Organization: Red Hat Message-ID: <0a707990-f12c-3c60-2a96-e1d531e100a6@redhat.com> Date: Tue, 12 Oct 2021 13:09:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12.10.21 12:42, Vasily Averin wrote: > On 08.10.2021 14:47, Vlastimil Babka wrote: >> On 10/8/21 11:24, Vasily Averin wrote: >>> __alloc_pages_bulk can call __count_zid_vm_events and zone_statistics >>> with nr_account = 0. >> >> But that's not a bug, right? Just an effective no-op that's not commonly >> happening, so is it worth the check? > > Why not? > > Yes, it's not a bug, it just makes the kernel a bit more efficient in a very unlikely case. > However, it looks strange and makes uninformed code reviewers like me worry about possible > problems inside the affected functions. No one else calls these functions from 0. > If it's not a BUG we'd better leave "Fixes:" tags away., it tends to confuse people looking for actual BUGs. I'm also not sure if this micro-optimization is worth it. "bit more efficient in a very unlikely case" doesn't sound very compelling ... and personally I'd assume accounting functions can deal with a delta of 0. -- Thanks, David / dhildenb