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SubjectRe: [PATCH] block: makes bio_split support bio without data
On Mon, 24 Sep 2012 17:35:34 +0900 Namhyung Kim <namhyung@kernel.org> wrote:

> Hi,
>
> On Mon, 24 Sep 2012 14:56:39 +1000, NeilBrown wrote:
> > Hi Jens,
> > this patch has been sitting in my -next tree for a little while and I was
> > hoping for it to go in for the next merge window.
> > It simply allows bio_split() to be used on bios without a payload, such as
> > 'discard'.
> > Are you happy with it going in though my 'md' tree, or would you rather take
> > it though your 'block' tree?
> >
> > Thanks,
> > NeilBrown
> >
> >
> > From: Shaohua Li <shli@fusionio.com>
> > Date: Thu, 20 Sep 2012 09:36:03 +1000
> > Subject: [PATCH] block: makes bio_split support bio without data
> >
> > discard bio hasn't data attached. We hit a BUG_ON with such bio. This makes
> > bio_split works for such bio.
> >
> > Signed-off-by: Shaohua Li <shli@fusionio.com>
> > Signed-off-by: NeilBrown <neilb@suse.de>
> >
> > diff --git a/fs/bio.c b/fs/bio.c
> > index 71072ab..dbb7a6c 100644
> > --- a/fs/bio.c
> > +++ b/fs/bio.c
> > @@ -1501,7 +1501,7 @@ struct bio_pair *bio_split(struct bio *bi, int first_sectors)
> > trace_block_split(bdev_get_queue(bi->bi_bdev), bi,
> > bi->bi_sector + first_sectors);
> >
> > - BUG_ON(bi->bi_vcnt != 1);
> > + BUG_ON(bi->bi_vcnt != 1 && bi->bi_vcnt != 0);
>
> Why not
> BUG_ON(bi->bi_vcnt > 1);
> ?

Either is fine with me.
'1' and '0' are the cases that bio_split explicitly supports.
'>1' are the cases which will cause problems.

As bi_vnt is unsigned, both conditions should produce exactly the same
machine code.

As I see no reason to prefer one over the other, I'm happy to go with what the
original author wrote.

Thanks,
NeilBrown
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