Re: [PATCH v3 01/11] block: make generic_make_request handle arbitrarily sized bios

[Ming Lin]

On Wed, May 20, 2015 at 5:44 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> This needs the patch below for the blk-mq case with multiple queues.

Will add it.

>
> Also I wonder why we really want to push this below ->make_request.
> Moving it just outside ->make_request would make the callchain more
> obvious and avoid the recursion into the driver.

Are you saying move the split to generic_make_request()?
Below is an off-list discussion.

Ming Lei asked:
> > I am wondering why the bio isn't splitted just before q->make_request_fn
> > is called in generic_make_request()?  By this way, drivers won't need
> > to call blk_queue_split() at all.  Is it because performance reason? or
> > others?

Kent replied:
> > The reasoning was because blk_queue_split() was only an interim solution -
> > there's actually no inherent need to split the bios at all, but not splitting
> > them takes (a small amount of) driver modification. You just have the driver
> > consume the bios incrementally, advancing the iterator and using bi_remaining
> > when they have multiple tags pointing to the same bio.

>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e9bbc04..e7fae76 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1256,6 +1256,8 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
>                 return;
>         }
>
> +       blk_queue_split(q, &bio, q->bio_split);
> +
>         rq = blk_mq_map_request(q, bio, &data);
>         if (unlikely(!rq))
>                 return;
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