The watermark check consists of two sub-checks.
The first one is:
if (free_pages <= min + lowmem_reserve)
return false;
The check assures that there is minimal amount of RAM in the zone. If CMA is
used then the free_pages is reduced by the number of free pages in CMA prior
to the over-mentioned check.
if (!(alloc_flags & ALLOC_CMA))
free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
This prevents the zone from being drained from pages available for non-movable
allocations.
The second check prevents the zone from getting too fragmented.
for (o = 0; o < order; o++) {
free_pages -= z->free_area[o].nr_free << o;
min >>= 1;
if (free_pages <= min)
return false;
}
The field z->free_area[o].nr_free is equal to the number of free pages
including free CMA pages. Therefore the CMA pages are subtracted twice. This
may cause a false positive fail of __zone_watermark_ok() if the CMA area gets
strongly fragmented. In such a case there are many 0-order free pages located
in CMA. Those pages are subtracted twice therefore they will quickly drain
free_pages during the check against fragmentation. The test fails even though
there are many free non-cma pages in the zone.
This patch fixes this issue by subtracting CMA pages only for a purpose of
(free_pages <= min + lowmem_reserve) check.
Signed-off-by: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
---
mm/page_alloc.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8fcced7..0d4fef2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1626,6 +1626,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
long min = mark;
long lowmem_reserve = z->lowmem_reserve[classzone_idx];
int o;
+ long free_cma = 0;
free_pages -= (1 << order) - 1;
if (alloc_flags & ALLOC_HIGH)
@@ -1635,9 +1636,10 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
#ifdef CONFIG_CMA
/* If allocation can't use CMA areas don't use free CMA pages */
if (!(alloc_flags & ALLOC_CMA))
- free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
+ free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif
- if (free_pages <= min + lowmem_reserve)
+
+ if (free_pages - free_cma <= min + lowmem_reserve)
return false;
for (o = 0; o < order; o++) {
/* At the next order, this order's pages become unavailable */
--
1.7.9.5
--
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