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Date: Wed, 21 Nov 2018 08:59:03 +0800 From: Ming Lei <ming.lei@...hat.com> To: Sagi Grimberg <sagi@...mberg.me> Cc: Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, Dave Chinner <dchinner@...hat.com>, Kent Overstreet <kent.overstreet@...il.com>, Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org, linux-erofs@...ts.ozlabs.org, David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org, "Darrick J . Wong" <darrick.wong@...cle.com>, linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>, Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org, Coly Li <colyli@...e.de>, linux-bcache@...r.kernel.org, Boaz Harrosh <ooo@...ctrozaur.com>, Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com Subject: Re: [PATCH V10 09/19] block: introduce bio_bvecs() 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
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