[PATCH block/for-4.0-fixes] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

From: Tejun Heo
Date: Wed Mar 04 2015 - 10:37:52 EST


Subject: writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

global_update_bandwidth() uses static variable update_time as the
timestamp for the last update but forgets to initialize it to
INITIALIZE_JIFFIES.

This means that global_dirty_limit will be 5 mins into the future on
32bit and some large amount jiffies into the past on 64bit. This
isn't critical as the only effect is that global_dirty_limit won't be
updated for the first 5 mins after booting on 32bit machines,
especially given the auxiliary nature of global_dirty_limit's role -
protecting against global dirty threshold's sudden dips; however, it
does lead to unintended suboptimal behavior. Fix it.

Fixes: c42843f2f0bb ("writeback: introduce smoothed global dirty limit")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Jan Kara <jack@xxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Added the "fixes" tag. Jens, can you please route this one?

Thanks.

mm/page-writeback.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -922,7 +922,7 @@ static void global_update_bandwidth(unsi
unsigned long now)
{
static DEFINE_SPINLOCK(dirty_lock);
- static unsigned long update_time;
+ static unsigned long update_time = INITIAL_JIFFIES;

/*
* check locklessly first to optimize away locking for the most time
--
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