[PATCH] kswapd: assign new_order and new_classzone_idx after wakeup in sleeping

[PATCH] kswapd: assign new_order and new_classzone_idx after wakeup in sleeping

[Alex Shi]

There 2 place to read pgdat in kswapd. One is return from a successful
balance, another is waked up from sleeping. But the new_order and
new_classzone_idx are not assigned after kswapd_try_to_sleep(), that
will cause a bug in the following scenario.

After the last time successful balance, kswapd goes to sleep. So the
new_order and new_classzone_idx were assigned to 0 and MAX-1 since there
is no new wakeup during last time balancing. Now, a new wakeup came and
finish balancing successful with order > 0. But since new_order is still
0, this time successful balancing were judged as a failed balance. so,
if there is another new wakeup coming during balancing, kswapd cann't
read this and still want to try to sleep. And if the new wakeup is a
tighter request, kswapd may goes to sleep, not to do balancing. That is
incorrect.

So, to avoid above problem, the new_order and new_classzone_idx need to
be assigned for later successful comparison.

Paidrag Brady, Could like do a retry for your problem on this patch?

Signed-off-by: Alex Shi <alex.shi@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7ef6912..eb7bcce 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2850,6 +2850,8 @@ static int kswapd(void *p)
 			kswapd_try_to_sleep(pgdat, order, classzone_idx);
 			order = pgdat->kswapd_max_order;
 			classzone_idx = pgdat->classzone_idx;
+			new_order = order;
+			new_classzone_idx = classzone_idx;
 			pgdat->kswapd_max_order = 0;
 			pgdat->classzone_idx = pgdat->nr_zones - 1;
 		}
-- 
1.6.3.3

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RE: [PATCH] kswapd: assign new_order and new_classzone_idx after wakeup in sleeping

[Shi, Alex]

> -----Original Message-----
> From: Minchan Kim [mailto:minchan.kim@gmail.com]
> Sent: Friday, July 29, 2011 4:57 PM
> To: Shi, Alex
> Cc: majordomo@kvack.org; P@draigBrady.com; mgorman@suse.de;
> linux-kernel@vger.kernel.org; andrea@cpushare.com; Chen, Tim C; Li,
> Shaohua; akpm@linux-foundation.org; riel@redhat.com; luto@mit.edu
> Subject: Re: [PATCH] kswapd: assign new_order and new_classzone_idx after
> wakeup in sleeping
> 
> On Fri, Jul 29, 2011 at 09:34:42AM +0800, Alex Shi wrote:
> > There 2 place to read pgdat in kswapd. One is return from a successful
> > balance, another is waked up from sleeping. But the new_order and
> > new_classzone_idx are not assigned after kswapd_try_to_sleep(), that
> > will cause a bug in the following scenario.
> >
> > After the last time successful balance, kswapd goes to sleep. So the
> > new_order and new_classzone_idx were assigned to 0 and MAX-1 since there
> > is no new wakeup during last time balancing. Now, a new wakeup came and
> > finish balancing successful with order > 0. But since new_order is still
> > 0, this time successful balancing were judged as a failed balance. so,
> > if there is another new wakeup coming during balancing, kswapd cann't
> > read this and still want to try to sleep. And if the new wakeup is a
> > tighter request, kswapd may goes to sleep, not to do balancing. That is
> > incorrect.
> >
> > So, to avoid above problem, the new_order and new_classzone_idx need to
> > be assigned for later successful comparison.
> >
> > Paidrag Brady, Could like do a retry for your problem on this patch?
> >
> > Signed-off-by: Alex Shi <alex.shi@intel.com>
> > Acked-by: Mel Gorman <mgorman@suse.de>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Thanks for review. 
BTW, I remove the incorrect email address of linux-mm. sorry for this! 

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