Re: [PATCH V10 08/19] btrfs: move bio_pages_all() to btrfs

[Ming Lei]

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