Re: [PATCH] block: Add bvec_folio()

On Mon, May 25, 2026 at 11:43:22PM -0700, Christoph Hellwig wrote:
> On Mon, May 25, 2026 at 02:29:27PM +0100, Matthew Wilcox wrote:
> > > 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.
> 
> Well, that is a very good thing to put into the comment.  We can also
> put them into high-level bvecs, so framing this as 'only use if you
> know the memory is folios, which you can't unless you are the entity
> who filled the bio' might be a good choice.

How about:

/**
 * bvec_folio - Return the first folio referenced by this bvec
 * @bv: bvec to access
 *
 * bvecs can contain non-folio memory, so this should only be called by
 * the creator of the bvec; drivers have no business looking at the owner
 * of the memory.  It may not even be the right interface for the caller
 * to use as bvecs can span multiple folios.  You may be better off using
 * something like bio_for_each_folio_all() which iterates over all folios.
 */