[3.5.y.z extended stable] Patch "writeback: fix negative bdi max pause" 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: fix negative bdi max pause

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

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

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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>From cfe9216b70e2d567635c0504bdfddae529e26186 Mon Sep 17 00:00:00 2001
From: Fengguang Wu <fengguang.wu at intel.com>
Date: Wed, 16 Oct 2013 13:47:03 -0700
Subject: [PATCH] writeback: fix negative bdi max pause
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit e3b6c655b91e01a1dade056cfa358581b47a5351 upstream.

Toralf runs trinity on UML/i386.  After some time it hangs and the last
message line is

	BUG: soft lockup - CPU#0 stuck for 22s! [trinity-child0:1521]

It's found that pages_dirtied becomes very large.  More than 1000000000
pages in this case:

	period = HZ * pages_dirtied / task_ratelimit;
	BUG_ON(pages_dirtied > 2000000000);
	BUG_ON(pages_dirtied > 1000000000);      <---------

UML debug printf shows that we got negative pause here:

	ick: pause : -984
	ick: pages_dirtied : 0
	ick: task_ratelimit: 0

	 pause:
	+       if (pause < 0)  {
	+               extern int printf(char *, ...);
	+               printf("ick : pause : %li\n", pause);
	+               printf("ick: pages_dirtied : %lu\n", pages_dirtied);
	+               printf("ick: task_ratelimit: %lu\n", task_ratelimit);
	+               BUG_ON(1);
	+       }
	        trace_balance_dirty_pages(bdi,

Since pause is bounded by [min_pause, max_pause] where min_pause is also
bounded by max_pause.  It's suspected and demonstrated that the
max_pause calculation goes wrong:

	ick: pause : -717
	ick: min_pause : -177
	ick: max_pause : -717
	ick: pages_dirtied : 14
	ick: task_ratelimit: 0

The problem lies in the two "long = unsigned long" assignments in
bdi_max_pause() which might go negative if the highest bit is 1, and the
min_t(long, ...) check failed to protect it falling under 0.  Fix all of
them by using "unsigned long" throughout the function.

Signed-off-by: Fengguang Wu <fengguang.wu at intel.com>
Reported-by: Toralf Förster <toralf.foerster at gmx.de>
Tested-by: Toralf Förster <toralf.foerster at gmx.de>
Reviewed-by: Jan Kara <jack at suse.cz>
Cc: Richard Weinberger <richard at nod.at>
Cc: Geert Uytterhoeven <geert at linux-m68k.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 mm/page-writeback.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e252db8..194cc02 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1072,11 +1072,11 @@ static unsigned long dirty_poll_interval(unsigned long dirty,
 	return 1;
 }

-static long bdi_max_pause(struct backing_dev_info *bdi,
-			  unsigned long bdi_dirty)
+static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
+				   unsigned long bdi_dirty)
 {
-	long bw = bdi->avg_write_bandwidth;
-	long t;
+	unsigned long bw = bdi->avg_write_bandwidth;
+	unsigned long t;

 	/*
 	 * Limit pause time for small memory systems. If sleeping for too long
@@ -1088,7 +1088,7 @@ static long bdi_max_pause(struct backing_dev_info *bdi,
 	t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
 	t++;

-	return min_t(long, t, MAX_PAUSE);
+	return min_t(unsigned long, t, MAX_PAUSE);
 }

 static long bdi_min_pause(struct backing_dev_info *bdi,
--
1.8.3.2