Re: [PATCH v2] writeback: fix negative bdi max pause

[Jan Kara]

On Sat 12-10-13 12:45:17, Wu Fengguang wrote:
> 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.
> 
> Reported-by: Toralf Förster <toralf.foerster@xxxxxx>
> Tested-by: Toralf Förster <toralf.foerster@xxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Richard Weinberger <richard@xxxxxx>
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Signed-off-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
  The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  mm/page-writeback.c |   10 +++++-----
>  mm/readahead.c      |    2 +-
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
>  Changes since v1: Add CC list.
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3f0c895..241a746 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1104,11 +1104,11 @@ 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
> @@ -1120,7 +1120,7 @@ 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,
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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