[PATCH v2] mm/page_alloc: don't wake kswapd from rmqueue() unless __GFP_KSWAPD_RECLAIM is specified

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock
held") moved wakeup_kswapd() from steal_suitable_fallback() to rmqueue()
using ZONE_BOOSTED_WATERMARK flag.

Only allocation contexts that include ALLOC_KSWAPD (which corresponds to
__GFP_KSWAPD_RECLAIM) should wake kswapd, for callers are supposed to
remove __GFP_KSWAPD_RECLAIM if trying to hold pgdat->kswapd_wait has a
risk of deadlock. But since zone->flags is a shared variable, a thread
doing !__GFP_KSWAPD_RECLAIM allocation request might observe this flag
being set immediately after another thread doing __GFP_KSWAPD_RECLAIM
allocation request set this flag, causing possibility of deadlock.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock held")
---
Changes in v2:
  Check ALLOC_KSWAPD before checking ZONE_BOOSTED_WATERMARK and update
  description, suggested by Mel Gorman <mgorman@techsingularity.net>.

 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 47421bedc12b..ecad680cec53 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3052,7 +3052,8 @@ struct page *rmqueue(struct zone *preferred_zone,
 
 out:
 	/* Separate test+clear to avoid unnecessary atomics */
-	if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
+	if ((alloc_flags & ALLOC_KSWAPD) &&
+	    unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
 		clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
 		wakeup_kswapd(zone, 0, 0, zone_idx(zone));
 	}
-- 
2.18.4