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>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 80bb4265625d190bb401532b7d79eb37e3a00b3b..2209a91425739e9188c86ddfadf58aabad999591 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1594,6 +1594,7 @@ 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;
@@ -1602,7 +1603,7 @@ 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 */