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writeback: fix negative bdi max pause

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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 <[email protected]>
Reported-by: Toralf Förster <[email protected]>
Tested-by: Toralf Förster <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Fengguang Wu 2013-10-16 13:47:03 -07:00 • committed by Linus Torvalds
commit e3b6c655b9
1 changed files with 5 additions and 5 deletions
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10
mm/page-writeback.c
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@ -1210,11 +1210,11 @@ 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
@ -1226,7 +1226,7 @@ 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,