mm: fix free page check in zone_watermark_ok()

__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>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 228194728ccd..2e6635993558 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -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 */