Re: [patch] mm: page_alloc: fix zone allocation fairness on UP

[Mel Gorman]

On Mon, Sep 08, 2014 at 08:35:51AM -0400, Johannes Weiner wrote:
> The zone allocation batches can easily underflow due to higher-order
> allocations or spills to remote nodes.  On SMP that's fine, because
> underflows are expected from concurrency and dealt with by returning
> 0.  But on UP, zone_page_state will just return a wrapped unsigned
> long, which will get past the <= 0 check and then consider the zone
> eligible until its watermarks are hit.
> 
> 3a025760fc15 ("mm: page_alloc: spill to remote nodes before waking
> kswapd") already made the counter-resetting use atomic_long_read() to
> accomodate underflows from remote spills, but it didn't go all the way
> with it.  Make it clear that these batches are expected to go negative
> regardless of concurrency, and use atomic_long_read() everywhere.
> 
> Fixes: 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy")
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: "3.12+" <stable@xxxxxxxxxx>

This is better than changing the behaviour of zone_page_state().

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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