Patch "writeback: fix negative bdi max pause" has been added to the 3.4-stable tree

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This is a note to let you know that I've just added the patch titled

    writeback: fix negative bdi max pause

to the 3.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     writeback-fix-negative-bdi-max-pause.patch
and it can be found in the queue-3.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From e3b6c655b91e01a1dade056cfa358581b47a5351 Mon Sep 17 00:00:00 2001
From: Fengguang Wu <fengguang.wu@xxxxxxxxx>
Date: Wed, 16 Oct 2013 13:47:03 -0700
Subject: writeback: fix negative bdi max pause

From: Fengguang Wu <fengguang.wu@xxxxxxxxx>

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@xxxxxxxxx>
Reported-by: Toralf Förster <toralf.foerster@xxxxxx>
Tested-by: Toralf Förster <toralf.foerster@xxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Richard Weinberger <richard@xxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/page-writeback.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1072,11 +1072,11 @@ static unsigned long dirty_poll_interval
 	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
 	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,


Patches currently in stable-queue which might be from fengguang.wu@xxxxxxxxx are

queue-3.4/writeback-fix-negative-bdi-max-pause.patch
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