[3.16.y-ckt stable] Patch "writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    writeback: add missing INITIAL_JIFFIES init in global_update_bandwidth()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 77390dd5530fb3d084dc63131c0a24c67f8ed413 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj at kernel.org>
Date: Wed, 4 Mar 2015 10:37:43 -0500
Subject: writeback: add missing INITIAL_JIFFIES init in
 global_update_bandwidth()

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 at kernel.org>
Acked-by: Jan Kara <jack at suse.cz>
Cc: Wu Fengguang <fengguang.wu at intel.com>
Cc: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.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 dfe512e911c8..001d86441639 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -925,7 +925,7 @@ 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