Re: [PATCH 1/9] block: Make generic_make_request handle arbitrary sized bios

[Kent Overstreet]

On Thu, Feb 27, 2014 at 12:22:54PM -0500, Matthew Wilcox wrote:
> On Wed, Feb 26, 2014 at 03:39:49PM -0800, Kent Overstreet wrote:
> > We do this by adding calls to blk_queue_split() to the various
> > make_request functions that need it - a few can already handle arbitrary
> > size bios. Note that we add the call _after_ any call to blk_queue_bounce();
> > this means that blk_queue_split() and blk_recalc_rq_segments() don't need to
> > be concerned with bouncing affecting segment merging.
> 
> > diff --git a/drivers/block/nvme-core.c b/drivers/block/nvme-core.c
> > index 51824d1f23..e4376b9613 100644
> > --- a/drivers/block/nvme-core.c
> > +++ b/drivers/block/nvme-core.c
> > @@ -737,6 +737,8 @@ static void nvme_make_request(struct request_queue *q, struct bio *bio)
> >  	struct nvme_queue *nvmeq = get_nvmeq(ns->dev);
> >  	int result = -EBUSY;
> >  
> > +	blk_queue_split(q, &bio, q->bio_split);
> > +
> >  	if (!nvmeq) {
> >  		put_nvmeq(NULL);
> >  		bio_endio(bio, -EIO);
> 
> I'd suggest that we do:
> 
> -	struct nvme_queue *nvmeq = get_nvmeq(ns->dev);
> +	struct nvme_queue *nvmeq;
>  	int result = -EBUSY;
> 
> +	blk_queue_split(q, &bio, q->bio_split);
> +
> +	nvmeq = get_nvmeq(ns->dev);
>  	if (!nvmeq) {
> 
> so that we're running the blk_queue_split() code outside the get_cpu()
> call.
> 
> Now, the NVMe driver has its own rules about when BIOs have to be split.
> Right now, that's way down inside the nvme_map_bio() call when we're
> walking the bio to compose the scatterlist.  Should we instead have an
> nvme_bio_split() routine that is called instead of blk_queue_split(),
> and we can simplify nvme_map_bio() since it'll know that it's working
> with bios that don't have to be split.
> 
> In fact, I think it would have little NVMe-specific in it at that point,
> so we could name __blk_bios_map_sg() better, export it to drivers and
> call it from nvme_map_bio(), which I think would make everybody happier.

Actually, reading nvme_map_bio() (it's different since last I looked at
it) it looks like nvme should already be able to handle arbitrary size
bios?

I do intend to rework the blk_bio_map_sg() (or add a new one?) to
incrementally map as much of a bio as will fit in the provided
scatterlist, but it looks like nvme has some odd restrictions where it's
using BIOVEC_PHYS_MERGABLE()/BIOVEC_NOT_VIRT_MERGABLE() so I dunno if
it's worth bothering to try and have it use generic code.

However we don't need an explicit split here: if the sg fills up (i.e.
the places nvme_split_and_submit() is called), we can just mark the bio
as partially completed (set bio->bi_iter = iter, i.e. use the iterator
you passed to bio_for_each_segment), then increment bi_remaining (which
just counts completions, i.e. bio_endio() calls before the bio is really
completed) and resubmit the original bio. No need to allocate a split
bio, or loop over the bio again in bio_split().
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