linux-kernel - Re: [PATCH V3] bio: modify __bio_add_page() to accept pages that don't start a new segment

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Message-ID: <20140527142726.GB30458@dhcp-27-189.brq.redhat.com>
Date:	Tue, 27 May 2014 16:27:26 +0200
From:	Maurizio Lombardi <mlombard@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>, axboe@...nel.dk
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	Christoph Hellwig <hch@....de>, linux-scsi@...r.kernel.org,
	Kent Overstreet <kmo@...erainc.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maurizio Lombardi <m.lombardi85@...il.com>
Subject: Re: [PATCH V3] bio: modify __bio_add_page() to accept pages that
 don't start a new segment

On Tue, May 27, 2014 at 10:04:58PM +0800, Ming Lei wrote:
> 
> Looks your approach is simpler.
> 
> But looks there is one problem if I understand correctly:
> __blk_recalc_rq_segments() may not cover the last vector
> because bio->bi_iter.bi_size isn't updated until the end of
> __bio_add_page().
> 
> But it shouldn't have been related with current virtio-blk problem.
>

This is a valid point, bi_iter.bi_size influences the behaviour of
blk_recount_segments(). Maybe Jens can confirm your observation.

Anyway it doesn't explain the reason behind the regression
introduced by commit 3979ef4dcf

Maurizio Lombardi
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