[PATCH 2/3 v3] mm: bug fix free page check in zone_watermark_ok

[Minchan Kim <minchan@kernel.org>]

In __zone_watermark_ok, free and min are signed long type
while z->lowmem_reserve[classzone_idx] is unsigned long type.
So comparision of them could be wrong due to type conversion
to unsigned although free_pages is minus value.

It could return true instead of false in case of order-0 check
so that kswapd could sleep forever. It means livelock because
direct reclaimer loops forever until kswapd set
zone->all_unreclaimable.

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>
---
This patch isn't dependent with this series.
It seems to be candidate for -stable but I'm not sure because of this part.
So, pass the decision to akpm.

" - It must fix a real bug that bothers people (not a, "This could be a
   problem..." type thing)."

 mm/page_alloc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f17e6e4..627653c 100644
--- 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 */
-- 
1.7.9.5

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