linux-ext4 - Re: [PATCH V10 08/19] btrfs: move bio_pages

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Date:   Mon, 19 Nov 2018 16:19:24 +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,
        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 08/19] btrfs: move bio_pages_all() to btrfs

On Fri, Nov 16, 2018 at 02:38:45PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 15, 2018 at 04:52:55PM +0800, Ming Lei wrote:
> > BTRFS is the only user of this helper, so move this helper into
> > BTRFS, and implement it via bio_for_each_segment_all(), since
> > bio->bi_vcnt may not equal to number of pages after multipage bvec
> > is enabled.
> 
> btrfs only uses the value to check if it is larger than 1.  No amount
> of multipage bio merging should ever make bi_vcnt go from 0 to 1 or
> vice versa.

Could you explain a bit why?

Suppose 2 physically continuous pages are added to this bio, .bi_vcnt
can be 1 in case of multi-page bvec, but it is 2 in case of single-page
bvec.

Thanks,
Ming