Hi Minchan,

Nick's original goal is to prevent OOM killing until all zone we're
interested in are unreclaimable and whether zone is reclaimable or not
depends on kswapd. And Nick's original solution is just peeking
zone->all_unreclaimable but I made it dirty when we are considering
kswapd freeze in hibernation. So I think we still need it to handle
kswapd freeze problem and we should add original behavior we missed at
that time like below.

static bool zone_reclaimable(struct zone *zone)
{
        if (zone->all_unreclaimable)
                return false;

        return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
}

If you remove the logic, the problem Nick addressed would be showed
up, again. How about addressing the problem in your patch? If you
remove the logic, __alloc_pages_direct_reclaim lose the chance calling
dran_all_pages. Of course, it was a side effect but we should handle
it.

Ok, you are successfull to persuade me. lost drain_all_pages() chance has
a risk.

And my last concern is we are going on right way?

I think fundamental cause of this problem is page_scanned and
all_unreclaimable is race so isn't the approach fixing the race right
way?

Hmm..
If we can avoid lock, we should. I think. that's performance reason.
therefore I'd like to cap the issue in do_try_to_free_pages(). it's
slow path.

Is the following patch acceptable to you? it is
 o rewrote the description
 o avoid mix to use zone->all_unreclaimable and zone->pages_scanned
 o avoid to reintroduce hibernation issue
 o don't touch fast path

If it is hard or very costly, your and my approach will be fallback.

-----------------------------------------------------------------
From f3d277057ad3a092aa1c94244f0ed0d3ebe5411c Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <redacted>
Date: Sat, 14 May 2011 05:07:48 +0900
Subject: [PATCH] vmscan: all_unreclaimable() use zone->all_unreclaimable as the name

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 zones 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 become all_unreclaimable=1, it never restore all_unreclaimable=0.
Why? if all_unreclaimable=1, vmscan only try DEF_PRIORITY reclaim and
a-few-lru-pages>>DEF_PRIORITY always makes 0. that mean no page scan
at all!

Eventually, oom-killer never works on such systems. That said, we
can't use zone->pages_scanned for this purpose. This patch restore
all_unreclaimable() use zone->all_unreclaimable as old. and in addition,
to add oom_killer_disabled check to avoid reintroduce the issue of
commit d1908362.

Reported-by: Andrey Vagin <redacted>
Cc: Nick Piggin <redacted>
Cc: Minchan Kim <redacted>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <redacted>
Cc: KAMEZAWA Hiroyuki <redacted>
Signed-off-by: KOSAKI Motohiro <redacted>
---
 mm/vmscan.c |   24 +++++++++++++-----------
 1 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 060e4c1..54ac548 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -41,6 +41,7 @@
 #include <linux/memcontrol.h>
 #include <linux/delayacct.h>
 #include <linux/sysctl.h>
+#include <linux/oom.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>

@@ -1988,17 +1989,12 @@ static bool zone_reclaimable(struct zone *zone)
 	return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
 }
 
-/*
- * 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.
- */
+/* All zones in zonelist are unreclaimable? */
 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) {

@@ -2006,13 +2002,11 @@ static bool all_unreclaimable(struct zonelist *zonelist,
 			continue;
 		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
 			continue;
-		if (zone_reclaimable(zone)) {
-			all_unreclaimable = false;
-			break;
-		}
+		if (!zone->all_unreclaimable)
+			return false;
 	}
 
-	return all_unreclaimable;
+	return true;
 }
 
 /*

@@ -2108,6 +2102,14 @@ out:
 	if (sc->nr_reclaimed)
 		return sc->nr_reclaimed;
 
+	/*
+	 * As hibernation is going on, kswapd is freezed so that it can't mark
+	 * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
+	 * check.
+	 */
+	if (oom_killer_disabled)
+		return 0;
+
 	/* top priority shrink_zones still had more to do? don't OOM, then */
 	if (scanning_global_lru(sc) && !all_unreclaimable(zonelist, sc))
 		return 1;

-- 
1.6.5.2



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