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SubjectRe: [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).

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