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Date: Tue, 18 Feb 2025 12:59:40 -0700
From: Keith Busch <kbusch@kernel.org>
To: Caleb Sander Mateos <csander@purestorage.com>
Cc: Keith Busch <kbusch@meta.com>, ming.lei@redhat.com,
	asml.silence@gmail.com, axboe@kernel.dk,
	linux-block@vger.kernel.org, io-uring@vger.kernel.org,
	bernd@bsbernd.com
Subject: Re: [PATCHv3 2/5] io_uring: add support for kernel registered bvecs
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 <20250214154348.2952692-3-kbusch@meta.com>
 <CADUfDZpbb0mtGSRSqcepXnM9sijP6-3WAZnzUJrDGbC0AuXTrg@mail.gmail.com>
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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.