linux-kernel - Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

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Date:   Tue, 12 Oct 2021 13:09:27 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vasily Averin <vvs@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

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