Re: [PATCH] block: allow for_each_bvec to support zero len bvec

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 10, 2020 at 04:33:09AM +0100, Matthew Wilcox wrote:
> On Mon, Aug 10, 2020 at 11:19:15AM +0800, Ming Lei wrote:
> > +++ b/include/linux/bvec.h
> > @@ -117,11 +117,18 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
> >  	return true;
> >  }
> >  
> > +static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
> > +{
> > +	iter->bi_bvec_done = 0;
> > +	iter->bi_idx++;
> > +}
> > +
> >  #define for_each_bvec(bvl, bio_vec, iter, start)			\
> >  	for (iter = (start);						\
> >  	     (iter).bi_size &&						\
> >  		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
> > -	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
> > +	     (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter),	\
> > +		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
> >  
> 
> What if you have two zero-length bvecs in a row?  Won't this just skip
> the first one?

The 2nd one will be skipped too when it is observed in next loop.

> 
> It would seem better to me to put the bv_len test in bvec_iter_advance()
> instead of making the macro more complicated.

The reason is that block layer won't support zero length bvec, and I'd
not bother bvec_iter_advance() for adding this check.


Thanks,
Ming