linux-kernel - Re: [PATCH] block: makes bio_split support bio without data

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Message-ID: <20120925093743.757f0a95@notabene.brown>
Date:	Tue, 25 Sep 2012 09:37:43 +1000
From:	NeilBrown <neilb@...e.de>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...ionio.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: makes bio_split support bio without data

On Mon, 24 Sep 2012 17:35:34 +0900 Namhyung Kim <namhyung@...nel.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@...ionio.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@...ionio.com>
> > Signed-off-by: NeilBrown <neilb@...e.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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