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

[Leon Romanovsky <leon@leon.nu>]

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Hi Johaness,


On Tue, Sep 9, 2014 at 4:15 PM, Johannes Weiner <hannes@cmpxchg.org> 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")
> Reported-by: Vlastimil Babka <vbabka@suse.cz>
> Reported-by: Leon Romanovsky <leon@leon.nu>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Mel Gorman <mgorman@suse.de>
> Cc: "3.12+" <stable@kernel.org>
> ---
>  mm/page_alloc.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> Sorry I forgot to CC you, Leon.  Resend with updated Tags.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 18cee0d4c8a2..eee961958021 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1612,7 +1612,7 @@ again:
>         }
>
>         __mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
> -       if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&
> +       if (atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]) <= 0 &&
>             !zone_is_fair_depleted(zone))
>                 zone_set_flag(zone, ZONE_FAIR_DEPLETED);
>
> @@ -5701,9 +5701,8 @@ static void __setup_per_zone_wmarks(void)
>                 zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp
> >> 1);
>
>                 __mod_zone_page_state(zone, NR_ALLOC_BATCH,
> -                                     high_wmark_pages(zone) -
> -                                     low_wmark_pages(zone) -
> -                                     zone_page_state(zone,
> NR_ALLOC_BATCH));
> +                       high_wmark_pages(zone) - low_wmark_pages(zone) -
> +                       atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]));
>
>                 setup_zone_migrate_reserve(zone);
>                 spin_unlock_irqrestore(&zone->lock, flags);
>

I think the better way will be to apply Mel's patch
https://lkml.org/lkml/2014/9/8/214 which fix zone_page_state shadow casting
issue and convert all atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH])) to
zone_page__state(zone, NR_ALLOC_BATCH). This move will unify access to
vm_stat.



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-- 
Leon Romanovsky | Independent Linux Consultant
        www.leon.nu | leon@leon.nu

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