Re: [dm-devel] block: eliminate potential for infinite loop in blkdev_issue_discard

[Mike Snitzer <snitzer redhat com>]

On Mon, Sep 27 2010 at  2:13pm -0400,
Mike Snitzer <snitzer redhat com> wrote:

> Due to the recently identified overflow in read_capacity_16() it was
> possible for max_discard_sectors to be zero but still have discards
> enabled on the associated device's queue.
> 
> Eliminate the possibility for blkdev_issue_discard to infinitely loop.
> 
> Interestingly this issue wasn't identified until a device, whose
> discard_granularity was 0 due to read_capacity_16 overflow, was consumed
> by blk_stack_limits() to construct limits for a higher-level DM
> multipath device.  The multipath device's resulting limits never had the
> discard limits stacked because blk_stack_limits() will only do so if
> the bottom device's discard_granularity != 0.  This resulted in the
> multipath device's limits.max_discard_sectors being 0.

Hi Jens,

This patch would only serve as a future safety-net now that the
elimination of the overflow in read_capacity_16() has been staged for
2.6.37.  Defensive programming and all...

What do you (and others) think about this patch?

Thanks,
Mike

> ---
>  block/blk-lib.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index c392029..186f249 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -56,7 +56,10 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  	 * granularity
>  	 */
>  	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
> -	if (q->limits.discard_granularity) {
> +	if (unlikely(!max_discard_sectors)) {
> +		/* Avoid infinite loop (below) */
> +		return -EOPNOTSUPP;
> +	} else if (q->limits.discard_granularity) {
>  		unsigned int disc_sects = q->limits.discard_granularity >> 9;
>  
>  		max_discard_sectors &= ~(disc_sects - 1);
> 
> --
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