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

[Christoph Hellwig]

On Tue, Mar 22, 2022 at 04:16:07PM -0400, Mike Snitzer wrote:
> 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.

Note that the nvme io_uring passthrough patches also want a non-kiocb
cached alloc version.  That code isn't quite ready yet but shows we'll
need something there as well.

> 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;

Yes, an uring deals with making sure they are freed in the same place.

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

The name sounds good fine, and I just it would be best done by
lifting BIO_PERCPU_CACHE to the REQ_ namespace and ensure it is
cleared by the bio allocator if the percpu cache isn't actually
used.