Date: Wed, 21 Nov 2018 17:10:25 +0100
From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Theodore Ts'o <tytso@mit.edu>,
        Omar Sandoval <osandov@fb.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Dave Chinner <dchinner@redhat.com>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, Shaohua Li <shli@kernel.org>,
        linux-raid@vger.kernel.org, David Sterba <dsterba@suse.com>,
        linux-btrfs@vger.kernel.org,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        linux-xfs@vger.kernel.org, Gao Xiang <gaoxiang25@huawei.com>,
        linux-ext4@vger.kernel.org, Coly Li <colyli@suse.de>,
        linux-bcache@vger.kernel.org, Boaz Harrosh <ooo@electrozaur.com>,
        Bob Peterson <rpeterso@redhat.com>, cluster-devel@redhat.com
Subject: Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec()
Message-ID: <20181121161025.GB4977@lst.de>
References: <20181121032327.8434-1-ming.lei@redhat.com> <20181121032327.8434-4-ming.lei@redhat.com> <20181121133244.GB1640@lst.de> <20181121153135.GB19111@ming.t460p>
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Sender: linux-ext4-owner@vger.kernel.org

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.