[dm-devel] [PATCH 1/3] block: allow BIOSET_PERCPU_CACHE use from bio_alloc_clone

[Mike Snitzer]

On Tue, Mar 22 2022 at  3:54P -0400,
Christoph Hellwig <hch at lst.de> wrote:

> On Tue, Mar 22, 2022 at 03:49:25PM -0400, Mike Snitzer wrote:
> > -	bio = bio_alloc_bioset(bdev, 0, bio_src->bi_opf, gfp, bs);
> > +	if (bs->cache && bio_src->bi_opf & REQ_POLLED)
> > +		bio = bio_alloc_percpu_cache(bdev, 0, bio_src->bi_opf, gfp, bs);
> > +	else
> 
> I don't think we can just unconditionally do this based on REQ_POLLED.
> We'd either need a flag for bio_alloc_bioset or (probably better)
> a separate interface.
> 

I did initially think it worthwhile to push the use of
bio_alloc_percpu_cache() down to bio_alloc_bioset() rather than
bio_alloc_clone() -- but I started slower with more targetted change
for DM's needs.

And yeah, since there isn't a REQ_ flag equivalent for IOCB_ALLOC_CACHE
(other than just allowing all REQ_POLLED access) there isn't a
meaningful way to limit access to the bioset's percpu cache.

Curious: how do bio_alloc_kiocb() callers know when it appropriate to
set IOCB_ALLOC_CACHE or not?  Seems io_uring is only one and it
unilaterally does:
kiocb->ki_flags |= IOCB_HIPRI | IOCB_ALLOC_CACHE;

So like IOCB_HIPRI maps to REQ_POLL, should IOCB_ALLOC_CACHE map to
REQ_ALLOC_CACHE? Or better name?

Open to further suggestions on which way to go with these details.

Thanks,
Mike