[PATCH V10 09/19] block: introduce bio_bvecs()

[Ming Lei]

On Tue, Nov 20, 2018 at 12:11:35PM -0800, Sagi Grimberg wrote:
> 
> > > > The only user in your final tree seems to be the loop driver, and
> > > > even that one only uses the helper for read/write bios.
> > > > 
> > > > I think something like this would be much simpler in the end:
> > > 
> > > The recently submitted nvme-tcp host driver should also be a user
> > > of this. Does it make sense to keep it as a helper then?
> > 
> > I did take a brief look at the code, and I really don't understand
> > why the heck it even deals with bios to start with.  Like all the
> > other nvme transports it is a blk-mq driver and should iterate
> > over segments in a request and more or less ignore bios.  Something
> > is horribly wrong in the design.
> 
> Can you explain a little more? I'm more than happy to change that but
> I'm not completely clear how...
> 
> Before we begin a data transfer, we need to set our own iterator that
> will advance with the progression of the data transfer. We also need to
> keep in mind that all the data transfer (both send and recv) are
> completely non blocking (and zero-copy when we send).
> 
> That means that every data movement needs to be able to suspend
> and resume asynchronously. i.e. we cannot use the following pattern:
> rq_for_each_segment(bvec, rq, rq_iter) {
> 	iov_iter_bvec(&iov_iter, WRITE, &bvec, 1, bvec.bv_len);
> 	send(sock, iov_iter);
> }

Not sure I understand the 'blocking' problem in this case.

We can build a bvec table from this req, and send them all
in send(), can this way avoid your blocking issue? You may see this
example in branch 'rq->bio != rq->biotail' of lo_rw_aio().

If this way is what you need, I think you are right, even we may
introduce the following helpers:

	rq_for_each_bvec()
	rq_bvecs()

So looks nvme-tcp host driver might be the 2nd driver which benefits
from multi-page bvec directly.

The multi-page bvec V11 has passed my tests and addressed almost
all the comments during review on V10. I removed bio_vecs() in V11,
but it won't be big deal, we can introduce them anytime when there
is the requirement.

Thanks,
Ming