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

[David Hildenbrand (Arm)]

On 6/10/26 08:16, Christoph Hellwig wrote:
> On Tue, Jun 09, 2026 at 08:36:43PM +0200, David Hildenbrand (Arm) wrote:
>> I really don't like arbitrary GUP users to starting to special case hugetlb
>> folios, and making assumptions of how other pages they pinned look like (IOW,
>> how the page table mappings actually looked like).
> 
> Me neither, but the current interfaces are kind forcing them :P

Yeah :)

But general rule: if you're outside of MM core and test for hugetlb folios, you
are doing something very wrong.

> 
>>
>> Ideally, we'd have a pin_user_pages_fast() variant that would give you a list of
>> folio ranges instead of individual pages.
> 
> 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.

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.

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.

As soon as some mapped pages are no longer folios, we'll likely have to modify
plenty of drivers either way, that blindly cast pages to folios ...

So maybe a folio-range based interface is good enough for now.

-- 
Cheers,

David