mm/page_alloc.c: place zone_id check before VM_BUG_ON_PAGE check
mm/page_alloc.c: place zone_id check before VM_BUG_ON_PAGE check

If the freeing page and its buddy page are not at the same zone, the
current holding zone->lock for the freeing page cann't prevent buddy page
getting allocated, this could trigger VM_BUG_ON_PAGE in page_is_buddy() at
a very tiny chance, such as:

cpu 0:						cpu 1:
hold zone_1 lock
check page and it buddy
PageBuddy(buddy) is true			hold zone_2 lock
page_order(buddy) == order is true		alloc buddy
trigger VM_BUG_ON_PAGE(page_count(buddy) != 0)

zone_1->lock prevents the freeing page getting allocated
zone_2->lock prevents the buddy page getting allocated
they are not the same zone->lock.

If we can't remove the zone_id check statement, it's better handle this
rare race.  This patch fixes this by placing the zone_id check before the
VM_BUG_ON_PAGE check.

Signed-off-by: Weijie Yang <1efac8fcaddebe669cb35e498dd211e17e274493@samsung.com>
Acked-by: Mel Gorman <39ea6a09710987173e32e37c86918024a0f441c1@suse.de>
Cc: Johannes Weiner <331be22c6b63ca3e0a03d408c2d906b1b02cd5f2@cmpxchg.org>
Cc: Rik van Riel <a21938f5d463ddf41aa718934c205ca2cce8ebbc@redhat.com>
Signed-off-by: Andrew Morton <5c1e68b099950c134891f0b6e179498a8ebe9cf9@linux-foundation.org>
Signed-off-by: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@linux-foundation.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs