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

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Date:	Tue, 2 Oct 2012 16:22:01 +1000
From:	NeilBrown <neilb@...e.de>
To:	Kent Overstreet <koverstreet@...gle.com>
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 Fri, 28 Sep 2012 09:23:43 -0700 Kent Overstreet <koverstreet@...gle.com>
wrote:

> On Mon, Sep 24, 2012 at 02:56:39PM +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'.
> 
> Thing is, at some point in the stack a discard bio is going to have data
> - see blk_add_rquest_payload(), and it used to be the single page was
> added to discard bios above generic_make_request(), in
> blkdev_issue_discard() or whatever it's called.
> 
> So while I'm sure your code works, it's just a fragile way of doing it.
> 
> There's also other types of bios where bi_size has nothing to do with
> the amount of data in the bi_io_vec - actually I think this is a new
> thing, since Martin Petersen just added REQ_WRITE_SAME and I don't think
> there were any other instances besides REQ_DISCARD before.
> 
> So my preference would be defining a mask (REQ_DISCARD|REQ_WRITE_SAME),
> and if bio->bi_rw & that mask is true, just duplicate the bvec or
> whatever.

Hi Kent,
 I'm afraid I don't see the relevance of your comments to the patch.

The current bio_split code can successfully split a bio with zero or one
bi_vec entry.  If there are more than that, we cannot split.

How does it matter whether the bio is a DISCARD or a WRITE_SAME or a DATA or
whatever?

NeilBrown


> 
> That way it's much more explicit and less likely to trip someone else
> up later.
> 
> (I've actually got a patch in my tree that does just that, but it's
> special cased in bio_advance() which makes things work out really
> nicely).
> 
> >  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);
> >  	BUG_ON(bi->bi_idx != 0);
> >  	atomic_set(&bp->cnt, 3);
> >  	bp->error = 0;
> > @@ -1511,17 +1511,19 @@ struct bio_pair *bio_split(struct bio *bi, int first_sectors)
> >  	bp->bio2.bi_size -= first_sectors << 9;
> >  	bp->bio1.bi_size = first_sectors << 9;
> >  
> > -	bp->bv1 = bi->bi_io_vec[0];
> > -	bp->bv2 = bi->bi_io_vec[0];
> > -	bp->bv2.bv_offset += first_sectors << 9;
> > -	bp->bv2.bv_len -= first_sectors << 9;
> > -	bp->bv1.bv_len = first_sectors << 9;
> > +	if (bi->bi_vcnt != 0) {
> > +		bp->bv1 = bi->bi_io_vec[0];
> > +		bp->bv2 = bi->bi_io_vec[0];
> > +		bp->bv2.bv_offset += first_sectors << 9;
> > +		bp->bv2.bv_len -= first_sectors << 9;
> > +		bp->bv1.bv_len = first_sectors << 9;
> >  
> > -	bp->bio1.bi_io_vec = &bp->bv1;
> > -	bp->bio2.bi_io_vec = &bp->bv2;
> > +		bp->bio1.bi_io_vec = &bp->bv1;
> > +		bp->bio2.bi_io_vec = &bp->bv2;
> >  
> > -	bp->bio1.bi_max_vecs = 1;
> > -	bp->bio2.bi_max_vecs = 1;
> > +		bp->bio1.bi_max_vecs = 1;
> > +		bp->bio2.bi_max_vecs = 1;
> > +	}
> >  
> >  	bp->bio1.bi_end_io = bio_pair_end_1;
> >  	bp->bio2.bi_end_io = bio_pair_end_2;
> 
> 
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