Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0
To:     Vasily Averin <vvs@virtuozzo.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel@openvz.org,
        Mel Gorman <mgorman@techsingularity.net>,
        Uladzislau Rezki <urezki@gmail.com>
References: <b2371951-bb8a-e62e-8d33-10830bbf6275@virtuozzo.com>
 <29155011-f884-b0e5-218e-911039568acb@suse.cz>
 <f52c5cd3-9b74-0fd5-2b7b-83ca21c52b2a@virtuozzo.com>
From:   David Hildenbrand <david@redhat.com>
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: <f52c5cd3-9b74-0fd5-2b7b-83ca21c52b2a@virtuozzo.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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