Re: [PATCH] block: Add bvec_folio()

[Matthew Wilcox <willy@infradead.org>]

On Sun, May 24, 2026 at 11:06:26PM -0700, Christoph Hellwig wrote:
> > +/**
> > + * bvec_folio - Return the first folio referenced by this bvec
> > + * @bv: bvec to access
> > + *
> > + * bvecs can span multiple folios.  Unless you know that this
> > + * bvec does not, you may be better off using something like
> > + * bio_for_each_folio_all() which iterates over all folios.
> > + */
> > +static inline struct folio *bvec_folio(const struct bio_vec *bv)
> > +{
> > +	return page_folio(bv->bv_page);
> > +}
> 
> The comment here is confusing.  bio_for_each_folio_all is a helper that
> only works in the submitter side, and not for anything using the
> bvec_iter required for drivers or anything else sitting below a
> potential bio clone/split or using bvecs from an upper layer (like
> ITER_BVEC direct I/O).  Additionally bv_page can be a different
> page than the fist page due to large bv_offset on split bios.
> 
> So I'm not against the function per se, but the documentation must
> explain the minefields it is stepping into a bit better.

Lower level drivers shouldn't be concerning themselves with folios.
For a start, we can put non-folios (eg slab memory) into bvecs.
I'm happy to clarify this comment further, but I don't understand
who's going to look at this function and need to have more explanation.