mm: page_alloc: fix watermark check in __zone_watermark_ok()
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.
Change-Id: I899919476cfca4bfa48bb5877ed6644784a43ce0
Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Patch-mainline: linux-mm @ 05/09/2013, 12:50AM
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
1 file changed