linux-kernel - Re: [PATCH v2 02/26] block: Add bio

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Date:	Thu, 20 Sep 2012 16:13:08 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, axboe@...nel.dk, neilb@...e.de
Subject: Re: [PATCH v2 02/26] block: Add bio_advance()

On Thu, Sep 20, 2012 at 02:58:27PM -0700, Tejun Heo wrote:
> On Mon, Sep 10, 2012 at 05:22:13PM -0700, Kent Overstreet wrote:
> > +/**
> > + * bio_advance - increment/complete a bio by some number of bytes
> > + * @bio:	bio to advance
> > + * @bytes:	number of bytes to complete
> > + *
> > + * This updates bi_sector, bi_size and bi_idx; if the number of bytes to
> > + * complete doesn't align with a bvec boundary, then bv_len and bv_offset will
> > + * be updated on the last bvec as well.
> > + *
> > + * @bio will then represent the remaining, uncompleted portion of the io.
> > + */
> > +void bio_advance(struct bio *bio, unsigned bytes)
> > +{
> > +	if (bio_integrity(bio))
> > +		bio_integrity_advance(bio, bytes);
> > +
> > +	bio->bi_sector += bytes >> 0;
> 
> Hmmm.... bytes >> 0?

Whoops...

> > +	bio->bi_size -= bytes;
> > +
> > +	if (!bio->bi_size)
> > +		return;
> > +
> > +	while (bytes) {
> > +		if (unlikely(bio->bi_idx >= bio->bi_vcnt)) {
> > +			printk(KERN_ERR "%s: bio idx %d >= vcnt %d\n",
> 
> pr_err() is preferred but maybe WARN_ON_ONCE() is better fit here?
> This happening would be a bug, right?

I just cut and pasted that from blk_update_request(), which is what the
next patch refactors...

But yes it would be a bug. It gets converted to a BUG_ON() in a later
patch (not in this series), as this gets further abstracted into a
wrapper around bvec_advance_iter() which doesn't know about struct bio
(as bio integrity gets its own iterator).

Might drop it entirely, depending on what exactly I end up doing with
bi_vcnt...
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