On 07/20/2011 02:47 PM, Tim Gardner wrote:

From 64ba82a4b63a45fd0c102dd97ea81c59fc522b76 Mon Sep 17 00:00:00 2001
From: Shaohua Li<shaohua.li@intel.com>
Date: Tue, 19 Jul 2011 08:49:26 -0700
Subject: [PATCH] vmscan: fix a livelock in kswapd

BugLink: http://bugs.launchpad.net/bugs/813797

I'm running a workload which triggers a lot of swap in a machine with 4
nodes. After I kill the workload, I found a kswapd livelock. Sometimes
kswapd3 or kswapd2 are keeping running and I can't access filesystem,
but most memory is free.

This looks like a regression since commit 08951e545918c159 ("mm: vmscan:
correct check for kswapd sleeping in sleeping_prematurely").

Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0
for classzone_idx. The reason is end_zone in balance_pgdat() is 0 by
default, if all zones have watermark ok, end_zone will keep 0.

Later sleeping_prematurely() always returns true. Because this is an
order 3 wakeup, and if classzone_idx is 0, both balanced_pages and
present_pages in pgdat_balanced() are 0. We add a special case here.
If a zone has no page, we think it's balanced. This fixes the livelock.

Signed-off-by: Shaohua Li<shaohua.li@intel.com>
Acked-by: Mel Gorman<mgorman@suse.de>
Cc: Minchan Kim<minchan.kim@gmail.com>
Cc:<stable@kernel.org>
Signed-off-by: Andrew Morton<akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds<torvalds@linux-foundation.org>
(cherry picked from commit 4746efded84d7c5a9c8d64d4c6e814ff0cf9fb42)

Signed-off-by: Tim Gardner<tim.gardner@canonical.com>
---
mm/vmscan.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4b8b37c..1e0eefe 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2245,7 +2245,8 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
for (i = 0; i<= classzone_idx; i++)
present_pages += pgdat->node_zones[i].present_pages;

- return balanced_pages> (present_pages>> 2);
+ /* A special case here: if zone has no page, we think it's balanced */
+ return balanced_pages>= (present_pages>> 2);
}

/* is kswapd sleeping prematurely? */



--
Tim Gardner tim.gardner@canonical.com

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