linux-kernel - Re: [PATCH V11 03/19] block: introduce bio_for_each

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Date:   Thu, 22 Nov 2018 09:17:06 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Theodore Ts'o <tytso@....edu>, Omar Sandoval <osandov@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        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, 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>,
        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 V11 03/19] block: introduce bio_for_each_bvec()

On Wed, Nov 21, 2018 at 05:10:25PM +0100, Christoph Hellwig wrote:
> On Wed, Nov 21, 2018 at 11:31:36PM +0800, Ming Lei wrote:
> > > But while looking over this I wonder why we even need the max_seg_len
> > > here.  The only thing __bvec_iter_advance does it to move bi_bvec_done
> > > and bi_idx forward, with corresponding decrements of bi_size.  As far
> > > as I can tell the only thing that max_seg_len does is that we need
> > > to more iterations of the while loop to archive the same thing.
> > > 
> > > And actual bvec used by the caller will be obtained using
> > > bvec_iter_bvec or segment_iter_bvec depending on if they want multi-page
> > > or single-page variants.
> > 
> > Right, we let __bvec_iter_advance() serve for both multi-page and single-page
> > case, then we have to tell it via one way or another, now we use the constant
> > of 'max_seg_len'.
> > 
> > Or you suggest to implement two versions of __bvec_iter_advance()?
> 
> No - I think we can always use the code without any segment in
> bvec_iter_advance.  Because bvec_iter_advance only operates on the
> iteractor, the generation of an actual single-page or multi-page
> bvec is left to the caller using the bvec_iter_bvec or segment_iter_bvec
> helpers.  The only difference is how many bytes you can move the
> iterator forward in a single loop iteration - so if you pass in
> PAGE_SIZE as the max_seg_len you just will have to loop more often
> for a large enough bytes, but not actually do anything different.

Yeah, I see that.

The difference is made by bio_iter_iovec()/bio_iter_mp_iovec() in
__bio_for_each_segment()/__bio_for_each_bvec().


Thanks,
Ming