Re: [dm-devel] [PATCH v2 2/3] block: reorganize rounding of max_discard_sectors

Vivek Goyal
2012-07-03T02:49:28Z

On Mon, Jul 02, 2012 at 03:20:24PM +0200, Paolo Bonzini wrote:
> Mostly a preparation for the next patch.
>
> In principle this fixes an infinite loop if max_discard_sectors < granularity,
> but that really shouldn't happen.
>
> Cc: Jens Axboe <axboe@kernel.dk>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> block/blk-lib.c | 9 +++++----
> 1 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 2b461b4..16b06f6 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -44,6 +44,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> struct request_queue *q = bdev_get_queue(bdev);
> int type = REQ_WRITE | REQ_DISCARD;
> unsigned int max_discard_sectors;
> + unsigned int granularity;
> struct bio_batch bb;
> struct bio *bio;
> int ret = 0;
> @@ -54,18 +55,18 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> if (!blk_queue_discard(q))
> return -EOPNOTSUPP;
>
> + /* Zero-sector (unknown) and one-sector granularities are the same. */
> + granularity = max(q->limits.discard_granularity >> 9, 1U);
> +
> /*
> * Ensure that max_discard_sectors is of the proper
> * granularity
> */
> max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
> + max_discard_sectors = round_down(max_discard_sectors, granularity);
> if (unlikely(!max_discard_sectors)) {
> /* Avoid infinite loop below. Being cautious never hurts. */
> return -EOPNOTSUPP;
> - } else if (q->limits.discard_granularity) {
> - unsigned int disc_sects = q->limits.discard_granularity >> 9;
> -
> - max_discard_sectors &= ~(disc_sects - 1);

This is kind of odd. If discard_granularity is zero, we assume that
discards are supported and granularity is 1. But if max_discard_sectors
is zero, we assume discards are disabled. Not sure if we should treat
max_discard_sectors and discard_granularity in same way or not.

Thanks
Vivek