Re: [RFC v1] io_uring/rsrc: add fast path huge page handling in buffer registration

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jun 10, 2026 at 11:54:01AM +0200, David Hildenbrand (Arm) wrote:
> > Yes.  iov_iter_extract_bvecs and thus the block direct I/O fast path
> > would instantly benefit from that.
> The tricky bit for such an interface is that, soon, some pages won't be folios,
> but we could still end up with non-folio pages in the address space (e.g.,
> vm_insert_page()) and have to pin+return them. So using folios is not future-proof.

I'm still doubtful on the "soon" beause of all the issues like this
in the I/O path.

> There are some long-term plans on providing an interface that would abstract how
> you refcount something you GUP'ed. (because, some pages we GUP in the future
> might not even have a dedicated refcount, all still fairly unclear). But it's
> all not really finalized I think.
> 
> For now, we could expose a folio+page/offset+nr_pages interface, where we,
> long-term, would not be able to return non-folio pages (e.g., vm_insert_page())
> and would instead, in the future, fail the request if we stumble over a
> non-folio thing in the page tables. That sounds reasonable for now.

I think whatever we're going to use for direct I/O has to also support
non-folio pages, especially PCI P2P memory.  So coming up with an
interface that support this ASAP would be helpful.

> Another solution would be, exposing page-ranges (e.g., page + nr_pages), whereby
> we'd say, that all pages in a range belong to the same compound page, and that
> we took a single reference for all pages in the range. IOW, page_folio() would
> for now be the same for all pages in a range.

This does sound like a reasonable short-term improvement.  One annoying
issue with returning only order 0 page in the current interfaces is
that it fills up the pages array in the caller for no good reason.