[PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely

[KOSAKI Motohiro]

all_unreclaimable check in direct reclaim has been introduced at 2.6.19
by following commit.

	2006 Sep 25; commit 408d8544; oom: use unreclaimable info

And it went through strange history. firstly, following commit broke
the logic unintentionally.

	2008 Apr 29; commit a41f24ea; page allocator: smarter retry of
				      costly-order allocations

Two years later, I've found obvious meaningless code fragment and
restored original intention by following commit.

	2010 Jun 04; commit bb21c7ce; vmscan: fix do_try_to_free_pages()
				      return value when priority==0

But, the logic didn't works when 32bit highmem system goes hibernation
and Minchan slightly changed the algorithm and fixed it .

	2010 Sep 22: commit d1908362: vmscan: check all_unreclaimable
				      in direct reclaim path

But, recently, Andrey Vagin found the new corner case. Look,

	struct zone {
	  ..
	        int                     all_unreclaimable;
	  ..
	        unsigned long           pages_scanned;
	  ..
	}

zone->all_unreclaimable and zone->pages_scanned are neigher atomic
variables nor protected by lock. Therefore a zone can become a state
of zone->page_scanned=0 and zone->all_unreclaimable=1. In this case,
current all_unreclaimable() return false even though
zone->all_unreclaimabe=1.

Is this ignorable minor issue? No. Unfortunatelly, x86 has very
small dma zone and it become zone->all_unreclamble=1 easily. and
if it becase all_unreclaimable, it never return all_unreclaimable=0
beucase it typicall don't have reclaimable pages.

Eventually, oom-killer never works on such systems.  Let's remove
this problematic logic completely.

Reported-by: Andrey Vagin <avagin@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Minchan Kim <minchan.kim@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
 mm/vmscan.c |   36 +-----------------------------------
 1 files changed, 1 insertions(+), 35 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 060e4c1..254aada 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1989,33 +1989,6 @@ static bool zone_reclaimable(struct zone *zone)
 }
 
 /*
- * As hibernation is going on, kswapd is freezed so that it can't mark
- * the zone into all_unreclaimable. It can't handle OOM during hibernation.
- * So let's check zone's unreclaimable in direct reclaim as well as kswapd.
- */
-static bool all_unreclaimable(struct zonelist *zonelist,
-		struct scan_control *sc)
-{
-	struct zoneref *z;
-	struct zone *zone;
-	bool all_unreclaimable = true;
-
-	for_each_zone_zonelist_nodemask(zone, z, zonelist,
-			gfp_zone(sc->gfp_mask), sc->nodemask) {
-		if (!populated_zone(zone))
-			continue;
-		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
-			continue;
-		if (zone_reclaimable(zone)) {
-			all_unreclaimable = false;
-			break;
-		}
-	}
-
-	return all_unreclaimable;
-}
-
-/*
  * This is the main entry point to direct page reclaim.
  *
  * If a full scan of the inactive list fails to free enough memory then we
@@ -2105,14 +2078,7 @@ out:
 	delayacct_freepages_end();
 	put_mems_allowed();
 
-	if (sc->nr_reclaimed)
-		return sc->nr_reclaimed;
-
-	/* top priority shrink_zones still had more to do? don't OOM, then */
-	if (scanning_global_lru(sc) && !all_unreclaimable(zonelist, sc))
-		return 1;
-
-	return 0;
+	return sc->nr_reclaimed;
 }
 
 unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
-- 
1.6.5.2



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