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: <CACVXFVNwn+_6qk29HtqLLsyiJVHLgJ9XgP=owHJ_B0z_Y0_DuA@mail.gmail.com>
Date:	Tue, 27 May 2014 22:04:58 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Maurizio Lombardi <mlombard@...hat.com>
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 List <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 7:58 PM, Maurizio Lombardi <mlombard@...hat.com> wrote:
> On Tue, May 27, 2014 at 06:15:45PM +0800, Ming Lei wrote:
>>
>> If the page can be merged to last segment, it should have been
>> covered by code in branch of 'if (bio->bi_vcnt > 0) ...', shouldn't it?
>>
>> Or maybe it is better to make that code cover your case since
>> looks your case is similar with that one according to your commit
>> log.
>>
>
> I realized that maybe you mean this branch:
>
> if (bio->bi_vcnt > 1 && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec)))
>
> right?
> In this case the branch is not even reached in the original code because
> the function returned an error way before executing it (line 760)
>
> There was already a patchset trying to modify the code in a different way than mine:
>
> https://groups.google.com/forum/#!msg/linux.kernel/3IanUpBVhFQ/3Xbg3yLRFp4J
>
> but it has been ignored and in my opinion it takes a more
> complicated approach.

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.

Thanks,
-- 
Ming Lei
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