From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: mm: page_alloc: fix zone allocation fairness on UP
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@xxxxxxx>
Reported-by: Leon Romanovsky <leon@xxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.12+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff -puN mm/page_alloc.c~mm-page_alloc-fix-zone-allocation-fairness-on-up mm/page_alloc.c
--- a/mm/page_alloc.c~mm-page_alloc-fix-zone-allocation-fairness-on-up
+++ a/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);
_
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