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mm: fix free page check in zone_watermark_ok()

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__zone_watermark_ok currently compares free_pages which is a signed type
with z->lowmem_reserve[classzone_idx] which is unsigned which might lead
to sign overflow if free_pages doesn't satisfy the given order (or it came
as negative already) and then we rely on the following order loop to fix
it (which doesn't work for order-0).  Let's fix the type conversion and do
not rely on the given value of free_pages or follow up fixups.

This patch fixes it because "memory-hotplug: fix kswapd looping forever
problem" depends on this.

As benefit of this patch, it doesn't rely on the loop to exit
__zone_watermark_ok in case of high order check and make the first test
effective.(ie, if (free_pages <= min + lowmem_reserve))

Aaditya reported this problem when he test my hotplug patch.

Reported-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
Tested-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
Signed-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Minchan Kim 2012-07-31 16:43:53 -07:00 committed by Linus Torvalds
parent ee6f509c32
commit 2cfed07528
1 changed files with 2 additions and 1 deletions
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3
mm/page_alloc.c
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@ -1595,6 +1595,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
{
/* free_pages my go negative - that's OK */
long min = mark;
long lowmem_reserve = z->lowmem_reserve[classzone_idx];
int o;
free_pages -= (1 << order) - 1;
@ -1603,7 +1604,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
if (alloc_flags & ALLOC_HARDER)
min -= min / 4;
if (free_pages <= min + z->lowmem_reserve[classzone_idx])
if (free_pages <= min + lowmem_reserve)
return false;
for (o = 0; o < order; o++) {
/* At the next order, this order's pages become unavailable */