Re: [PATCHv3 2/5] io_uring: add support for kernel registered bvecs

[Caleb Sander Mateos <csander@purestorage.com>]

On Tue, Feb 18, 2025 at 11:59 AM Keith Busch <kbusch@kernel.org> wrote:
>
> On Fri, Feb 14, 2025 at 12:38:54PM -0800, Caleb Sander Mateos wrote:
> > On Fri, Feb 14, 2025 at 7:45 AM Keith Busch <kbusch@meta.com> wrote:
> > > +
> > > +       nr_bvecs = blk_rq_nr_phys_segments(rq);
> >
> > Is this guaranteed to match the number of bvecs in the request?
>
> Yes.
>
> > Wouldn't the number of physical segments depend on how the block
> > device splits the bvecs?
>
> Also yes.
>
> >lo_rw_aio() uses rq_for_each_bvec() to count
> > the number of bvecs, for example.
>
> Hm, that seems unnecessary. The request's nr_phys_segments is
> initialized to the number of bvecs rather than page segments, so it can
> be used instead of recounting them from a given struct request.
>
> The initial number of physical segments for a request is set in
> bio_split_rw_at(), which uses bio_for_each_bvec(). That's what
> rq_for_each_bvec would use, too. The same is used for any bio's that get
> merged into the bio.

Okay, thanks for verifying!

Best,
Caleb