[dm-devel] [PATCH 1/5] block: Make generic_make_request handle arbitrary sized bios

[Kent Overstreet]

On Mon, Nov 25, 2013 at 10:09:21PM -0800, Christoph Hellwig wrote:
> > +	q->bio_split = bioset_create(4, 0);
> > +	if (!q->bio_split)
> > +		goto fail_id;
> 
> How did we arrive at a mempool size of 4 to make sure we can always make
> progress with arbitrarily sized bios?  Shouldn't we document the design
> decision somewhere?

It just has to be nonzero to guarantee forward progress - the
bio_alloc_bioset() rescuer thing I did awhile back guarantees that.

> > +static struct bio *blk_bio_discard_split(struct request_queue *q,
> > +					 struct bio *bio,
> > +					 struct bio_set *bs)
> > +{
> 
> One fallout of this should be that it would allows us to impement
> a range UNMAP/TRIP much more easily, which would help dramatically
> with TRIM performance!

Cool!