Re: [PATCH V2 1/3] block: respect queue limit of max discard segment

[Christoph Hellwig <hch@lst.de>]

On Wed, Aug 12, 2020 at 07:44:18AM +0800, Ming Lei wrote:
> When queue_max_discard_segments(q) is 1, blk_discard_mergable() will
> return false for discard request, then normal request merge is applied.
> However, only queue_max_segments() is checked, so max discard segment
> limit isn't respected.
> 
> Check max discard segment limit in the request merge code for fixing
> the issue.
> 
> Discard request failure of virtio_blk is fixed.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> Fixes: 69840466086d ("block: fix the DISCARD request merge")
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  block/blk-merge.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 5196dc145270..d18fb88ca8bd 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -533,10 +533,16 @@ int __blk_rq_map_sg(struct request_queue *q, struct request *rq,
>  }
>  EXPORT_SYMBOL(__blk_rq_map_sg);
>  
> +static inline unsigned int blk_rq_get_max_segments(struct request *rq)
> +{
> +	return req_op(rq) == REQ_OP_DISCARD ?
> +		queue_max_discard_segments(rq->q) : queue_max_segments(rq->q);

I'd go for a good old if statement here..

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>