Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP_FIXED mappings

[Peter Xu]

On Mon, Jun 16, 2025 at 08:00:11PM -0300, Jason Gunthorpe wrote:
> On Mon, Jun 16, 2025 at 06:06:23PM -0400, Peter Xu wrote:
> 
> > Can I understand it as a suggestion to pass in a bitmask into the core mm
> > API (e.g. keep the name of mm_get_unmapped_area_aligned()), instead of a
> > constant "align", so that core mm would try to allocate from the largest
> > size to smaller until it finds some working VA to use?
> 
> I don't think you need a bitmask.
> 
> Split the concerns, the caller knows what is inside it's FD. It only
> needs to provide the highest pgoff aligned folio/pfn within the FD.

Ultimately I even dropped this hint.  I found that it's not really
get_unmapped_area()'s job to detect over-sized pgoffs.  It's mmap()'s job.
So I decided to avoid this parameter as of now.

> 
> The mm knows what leaf page tables options exist. It should try to
> align to the closest leaf page table size that is <= the FD's max
> aligned folio.

So again IMHO this is also not per-FD information, but needs to be passed
over from the driver for each call.

Likely the "order" parameter appeared in other discussions to imply a
maximum supported size from the driver side (or, for a folio, but that is
definitely another user after this series can land).

So far I didn't yet add the "order", because currently VFIO definitely
supports all max orders the system supports.  Maybe we can add the order
when there's a real need, but maybe it won't happen in the near future?

In summary, I came up with below changes, would below look reasonable?

I also added Liam and Lorenzo in this reply.

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