linux-kernel - [PATCH 3.4 123/172] writeback: add missing INITIAL_JIFFIES init in global_update

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Date:	Tue, 16 Jun 2015 16:36:17 +0800
From:	lizf@...nel.org
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Jens Axboe <axboe@...nel.dk>, Jens Axboe <axboe@...com>,
	Zefan Li <lizefan@...wei.com>
Subject: [PATCH 3.4 123/172] writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

From: Tejun Heo <tj@...nel.org>

3.4.108-rc1 review patch.  If anyone has any objections, please let me know.

------------------


commit 7d70e15480c0450d2bfafaad338a32e884fc215e upstream.

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@...nel.org>
Acked-by: Jan Kara <jack@...e.cz>
Cc: Wu Fengguang <fengguang.wu@...el.com>
Cc: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Jens Axboe <axboe@...com>
Signed-off-by: Zefan Li <lizefan@...wei.com>
---
 mm/page-writeback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 057884c..b747dcd 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -831,7 +831,7 @@ static void global_update_bandwidth(unsigned long thresh,
 				    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
-- 
1.9.1

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