From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ming Lei <ming.lei@redhat.com>
Date: Mon, 19 Nov 2018 16:19:24 +0800
Subject: [Cluster-devel] [PATCH V10 08/19] btrfs: move bio_pages_all()
	to btrfs
In-Reply-To: <20181116133845.GG3165@lst.de>
References: <20181115085306.9910-1-ming.lei@redhat.com>
	<20181115085306.9910-9-ming.lei@redhat.com>
	<20181116133845.GG3165@lst.de>
Message-ID: <20181119081922.GB16736@ming.t460p>
List-Id: <cluster-devel.redhat.com>
To: cluster-devel.redhat.com
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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