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Subject[PATCH 3.5 30/64] writeback: fix negative bdi max pause
Date
3.5.7.24 -stable review patch.  If anyone has any objections, please let me know.

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

From: Fengguang Wu <fengguang.wu@intel.com>

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@intel.com>
Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Tested-by: Toralf Förster <toralf.foerster@gmx.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Richard Weinberger <richard@nod.at>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@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
--
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