[PATCH 2/2] mm: vmscan: correct use of pgdat_balanced in sleeping_prematurely

[Colin King]

From: Johannes Weiner <hannes at cmpxchg.org>

There are a few reports of people experiencing hangs when copying large
amounts of data with kswapd using a large amount of CPU which appear to be
due to recent reclaim changes.  SLUB using high orders is the trigger but
not the root cause as SLUB has been using high orders for a while.  The
root cause was bugs introduced into reclaim which are addressed by the
following two patches.

Patch 1 corrects logic introduced by commit 1741c877 ("mm: kswapd:
        keep kswapd awake for high-order allocations until a percentage of
        the node is balanced") to allow kswapd to go to sleep when
        balanced for high orders.

Patch 2 notes that it is possible for kswapd to miss every
        cond_resched() and updates shrink_slab() so it'll at least reach
        that scheduling point.

Chris Wood reports that these two patches in isolation are sufficient to
prevent the system hanging.  AFAIK, they should also resolve similar hangs
experienced by James Bottomley.

This patch:

Johannes Weiner poined out that the logic in commit 1741c877 ("mm: kswapd:
keep kswapd awake for high-order allocations until a percentage of the
node is balanced") is backwards.  Instead of allowing kswapd to go to
sleep when balancing for high order allocations, it keeps it kswapd
running uselessly.

Signed-off-by: Mel Gorman <mgorman at suse.de>
Reviewed-by: Rik van Riel <riel at redhat.com>
Signed-off-by: Johannes Weiner <hannes at cmpxchg.org>
Reviewed-by: Wu Fengguang <fengguang.wu at intel.com>
Cc: James Bottomley <James.Bottomley at HansenPartnership.com>
Tested-by: Colin King <colin.king at canonical.com>
Cc: Raghavendra D Prabhu <raghu.prabhu13 at gmail.com>
Cc: Jan Kara <jack at suse.cz>
Cc: Chris Mason <chris.mason at oracle.com>
Cc: Christoph Lameter <cl at linux.com>
Cc: Pekka Enberg <penberg at kernel.org>
Cc: Rik van Riel <riel at redhat.com>
Reviewed-by: Minchan Kim <minchan.kim at gmail.com>
Reviewed-by: Wu Fengguang <fengguang.wu at intel.com>
Cc: <stable at kernel.org>		[2.6.38+]
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
---
 mm/vmscan.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 648aab8..a74bf72 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2291,7 +2291,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 	 * must be balanced
 	 */
 	if (order)
-		return pgdat_balanced(pgdat, balanced, classzone_idx);
+		return !pgdat_balanced(pgdat, balanced, classzone_idx);
 	else
 		return !all_zones_ok;
 }
-- 
1.7.0.4