Re: [PATCH 3/4] fsx: fix infinite/too long loops when generating ranges for copy_file_range

[Brian Foster <bfoster@redhat.com>]

On Wed, Apr 08, 2020 at 11:36:16AM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> While running generic/521 I've had fsx taking a lot of CPU time and not
> making any progress for several hours. Attaching gdb to the fsx process
> revealed that fsx was in the loop that generates the ranges for a
> copy_file_range operation, in particular the loop seemed to never end
> because the range defined by 'offset2' kept overlapping with the range
> defined by 'offset'.
> So far this happened one time only in one of my test VMs with generic/521.
> 
> Fix this by breaking out of the loop after trying 30 times, like we
> currently do for dedupe operations, which results in logging the operation
> as skipped.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  ltp/fsx.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/ltp/fsx.c b/ltp/fsx.c
> index 5949ebf0..89a5f60e 100644
> --- a/ltp/fsx.c
> +++ b/ltp/fsx.c
> @@ -2042,17 +2042,25 @@ test(void)
>  			break;
>  		}
>  	case OP_COPY_RANGE:
> -		TRIM_OFF_LEN(offset, size, file_size);
> -		offset -= offset % readbdy;
> -		if (o_direct)
> -			size -= size % readbdy;
> -		do {
> -			offset2 = random();
> -			TRIM_OFF(offset2, maxfilelen);
> -			offset2 -= offset2 % writebdy;
> -		} while (range_overlaps(offset, offset2, size) ||
> -			 offset2 + size > maxfilelen);
> -		break;
> +		{
> +			int tries = 0;
> +
> +			TRIM_OFF_LEN(offset, size, file_size);
> +			offset -= offset % readbdy;
> +			if (o_direct)
> +				size -= size % readbdy;
> +			do {
> +				if (tries++ >= 30) {
> +					size = 0;
> +					break;
> +				}
> +				offset2 = random();
> +				TRIM_OFF(offset2, maxfilelen);
> +				offset2 -= offset2 % writebdy;
> +			} while (range_overlaps(offset, offset2, size) ||
> +				 offset2 + size > maxfilelen);
> +			break;
> +		}
>  	}
>  
>  have_op:
> -- 
> 2.11.0
>