[PATCH v3] mm: fix condition for throttle_direct_reclaim

[Shakeel Butt]

Since "mm: fix 100% CPU kswapd busyloop on unreclaimable nodes" kswapd
has been modified to give up after MAX_RECLAIM_RETRIES number of
unsucessful iterations. Before going to sleep, kswapd thread will
unconditionally wakeup all threads sleeping on pfmemalloc_wait.
However the awoken threads will recheck the watermarks and wake the
kswapd thread and sleep again on pfmemalloc_wait. There is a chance
that the system might end up in livelock between unsuccessful kswapd
and direct reclaimers because all direct reclaimer might end up in
throttle_direct_reclaim and there is nobody to make a forward
progress. So, add kswapd_failures check on the throttle_direct_reclaim
condition.

Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
v3:
Commit message updated.

v2:
Instead of separate helper function for checking kswapd_failures,
added the check into pfmemalloc_watermark_ok() and renamed that
function.

 mm/vmscan.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bae698484e8e..afa5b20ab6d8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2783,7 +2783,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 	return 0;
 }
 
-static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
+static bool allow_direct_reclaim(pg_data_t *pgdat)
 {
 	struct zone *zone;
 	unsigned long pfmemalloc_reserve = 0;
@@ -2791,6 +2791,9 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
 	int i;
 	bool wmark_ok;
 
+	if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+		return true;
+
 	for (i = 0; i <= ZONE_NORMAL; i++) {
 		zone = &pgdat->node_zones[i];
 		if (!managed_zone(zone))
@@ -2873,7 +2876,7 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
 
 		/* Throttle based on the first usable node */
 		pgdat = zone->zone_pgdat;
-		if (pfmemalloc_watermark_ok(pgdat))
+		if (allow_direct_reclaim(pgdat))
 			goto out;
 		break;
 	}
@@ -2895,14 +2898,14 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
 	 */
 	if (!(gfp_mask & __GFP_FS)) {
 		wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
-			pfmemalloc_watermark_ok(pgdat), HZ);
+			allow_direct_reclaim(pgdat), HZ);
 
 		goto check_pending;
 	}
 
 	/* Throttle until kswapd wakes the process */
 	wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
-		pfmemalloc_watermark_ok(pgdat));
+		allow_direct_reclaim(pgdat));
 
 check_pending:
 	if (fatal_signal_pending(current))
@@ -3102,7 +3105,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
 {
 	/*
 	 * The throttled processes are normally woken up in balance_pgdat() as
-	 * soon as pfmemalloc_watermark_ok() is true. But there is a potential
+	 * soon as allow_direct_reclaim() is true. But there is a potential
 	 * race between when kswapd checks the watermarks and a process gets
 	 * throttled. There is also a potential race if processes get
 	 * throttled, kswapd wakes, a large process exits thereby balancing the
@@ -3271,7 +3274,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 		 * able to safely make forward progress. Wake them
 		 */
 		if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
-				pfmemalloc_watermark_ok(pgdat))
+				allow_direct_reclaim(pgdat))
 			wake_up_all(&pgdat->pfmemalloc_wait);
 
 		/* Check if kswapd should be suspending */
-- 
2.12.0.367.g23dc2f6d3c-goog