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Date:   Mon, 19 Nov 2018 16:19:24 +0800
From:   Ming Lei <>
To:     Christoph Hellwig <>
Cc:     Jens Axboe <>,,,,
        Dave Chinner <>,
        Kent Overstreet <>,
        Mike Snitzer <>,,
        Alexander Viro <>,, Shaohua Li <>,,,
        David Sterba <>,,
        "Darrick J . Wong" <>,, Gao Xiang <>,
        Theodore Ts'o <>,,
        Coly Li <>,,
        Boaz Harrosh <>,
        Bob Peterson <>,
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


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