linux-kernel - Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone

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Date:   Tue, 20 Dec 2016 12:31:22 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Jia He <hejianet@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Taku Izumi <izumi.taku@...fujitsu.com>
Subject: Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone_statistics
 data

On Mon, Dec 12, 2016 at 01:59:07PM +0800, Jia He wrote:
> In commit b9f00e147f27 ("mm, page_alloc: reduce branches in
> zone_statistics"), it reconstructed codes to reduce the branch miss rate.
> Compared with the original logic, it assumed if !(flag & __GFP_OTHER_NODE)
>  z->node would not be equal to preferred_zone->node. That seems to be
> incorrect.
> 
> Fixes: commit b9f00e147f27 ("mm, page_alloc: reduce branches in
> zone_statistics")
> 
> Signed-off-by: Jia He <hejianet@...il.com>

This is slightly curious. It appear it would only occur if a process was
running on a node that was outside the memory policy. Can you confirm
that is the case?

If so, your patch is a a semantic curiousity because it's actually
impossible for a NUMA allocation to be local and the definition of "HIT"
is fuzzy enough to be useless.

I won't object to the patch but it makes me trust "hit" even less than I
already do for any analysis.

Note that after this mail that I'll be unavailable by mail until early
new years.

-- 
Mel Gorman
SUSE Labs