[PATCH] mm, vmscan: Make zone_reclaimable less fragile

[Michal Hocko]

zone_reclaimable considers a zone unreclaimable if we have scanned all
the reclaimable pages sufficient times since the last page has been
freed and that still hasn't led to an allocation success. This can,
however, lead to a livelock/trashing when a single freed page resets
PAGES_SCANNED while memory consumers are looping over small LRUs without
making any progress (e.g. remaining pages on the LRU are dirty and all
the flushers are blocked) and failing to invoke the OOM killer beause
zone_reclaimable would consider the zone reclaimable.

Tetsuo Handa has reported the following:
: [   66.821450] zone_reclaimable returned 1 at line 2646
: [   66.823020] (ACTIVE_FILE=26+INACTIVE_FILE=10) * 6 > PAGES_SCANNED=32
: [   66.824935] shrink_zones returned 1 at line 2706
: [   66.826392] zones_reclaimable=1 at line 2765
: [   66.827865] do_try_to_free_pages returned 1 at line 2938
: [   67.102322] __perform_reclaim returned 1 at line 2854
: [   67.103968] did_some_progress=1 at line 3301
: (...snipped...)
: [  281.439977] zone_reclaimable returned 1 at line 2646
: [  281.439977] (ACTIVE_FILE=26+INACTIVE_FILE=10) * 6 > PAGES_SCANNED=32
: [  281.439978] shrink_zones returned 1 at line 2706
: [  281.439978] zones_reclaimable=1 at line 2765
: [  281.439979] do_try_to_free_pages returned 1 at line 2938
: [  281.439979] __perform_reclaim returned 1 at line 2854
: [  281.439980] did_some_progress=1 at line 3301

In his case anon LRUs are not reclaimable because there is no swap enabled.

It is not clear who frees a page that regularly but it is clear that no
progress can be made but zone_reclaimable still consider the zone
reclaimable.

This patch makes zone_reclaimable less fragile by checking the number of
reclaimable pages against low watermark. It doesn't make much sense to
rely on a PAGES_SCANNED heuristic if there are not enough reclaimable
pages to get us over min watermark.

Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/vmscan.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c88d74ad9304..f16266e0af70 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -209,8 +209,14 @@ static unsigned long zone_reclaimable_pages(struct zone *zone)
 
 bool zone_reclaimable(struct zone *zone)
 {
+	unsigned long reclaimable = zone_reclaimable_pages(zone);
+	unsigned long free = zone_page_state(zone, NR_FREE_PAGES);
+
+	if (reclaimable + free < min_wmark_pages(zone))
+		return false;
+
 	return zone_page_state(zone, NR_PAGES_SCANNED) <
-		zone_reclaimable_pages(zone) * 6;
+		reclaimable * 6;
 }
 
 static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
-- 
2.5.1

-- 
Michal Hocko
SUSE Labs
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