linux-kernel - Re: [PATCH V6 12/30] block: introduce bio

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Date:   Wed, 13 Jun 2018 10:57:22 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        David Sterba <dsterba@...e.cz>,
        Huang Ying <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Theodore Ts'o <tytso@....edu>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Coly Li <colyli@...e.de>, Filipe Manana <fdmanana@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH V6 12/30] block: introduce bio_chunks()

On Wed, Jun 13, 2018 at 07:47:41AM -0700, Christoph Hellwig wrote:
> > +static inline unsigned bio_chunks(struct bio *bio)
> > +{
> > +	unsigned chunks = 0;
> > +	struct bio_vec bv;
> > +	struct bvec_iter iter;
> >  
> > -	return segs;
> > +	/*
> > +	 * We special case discard/write same/write zeroes, because they
> > +	 * interpret bi_size differently:
> > +	 */
> > +	switch (bio_op(bio)) {
> > +	case REQ_OP_DISCARD:
> > +	case REQ_OP_SECURE_ERASE:
> > +	case REQ_OP_WRITE_ZEROES:
> > +		return 0;
> > +	case REQ_OP_WRITE_SAME:
> > +		return 1;
> > +	default:
> > +		bio_for_each_chunk(bv, bio, iter)
> > +			chunks++;
> > +		return chunks;
> 
> Shouldn't this just return bio->bi_vcnt?

No.

bio->bi_vcnt is only for the owner of a bio (the code that originally allocated
it and filled it out) to use, and really the only legit use is
bio_for_each_segment_all() (iterating over segments without using bi_iter
because it's already been iterated to the end), and as a convenience thing for
bio_add_page.

Code that has a bio submitted to it can _not_ use bio->bi_vcnt, it's perfectly
legal for it to be 0 (and it is for e.g. bio splits).