Re: [PATCH] mm: fix COW mapping handing in generic_access_phys

[David Hildenbrand <david@redhat.com>]

On 28.05.25 18:29, Jason Gunthorpe wrote:
> On Wed, May 28, 2025 at 12:06:07PM -0400, Peter Xu wrote:
>> #define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */
>>
>> I'm not confident to blame any driver yet to have those special cases for
>> VM_PFNMAP, because it only says "managed without struct page", it didn't
>> say "it must not contain struct page"..  Hence it hints the core mm "please
>> do not manage these mappings with struct page at all".  Still sounds fair
>> contract, even if not ideal.
> 
> I think it is pretty clear, if a VMA has VM_PFNMAP then nothing must
> ever try to obtain a struct page from any PTEs in it, for any reason,
> even if things in it might have a struct page. In practice it means
> nothing can call vm_normal_page() on a VM_PFNMAP.

No, not until we remove any COW mappings of VM_PFNMAP.

But maybe I misunderstood what you mean.

> 
> It would be nice to update the comment to make it clearer.
> 
> If the VMA owner wanted to permit access to the struct page then it
> should have used VM_MIXEDMAP.
> 
> The fundamental difference between PFNMAP and MIXEDMAP is that
> vm_normal_page() is allowed on MIXEDMAP. That comes with some extra
> rules and restrictions to support arches without the special pte bit.
 > > VM_IO | VM_PFNMAP further means that all the pfns in the VMA require
> the use of io accessors (writel/readl) to access them.
 > > No idea what VM_IO | VM_MIXEDMAP is supposed to mean. Only the special
> ptes need io accessors?
 > > In either case GUP doesn't really work on the VMA. PFNMAP is totally
> blocked, and for MIXEDMAP userspace has no way to discover which
> subset of the VMA is GUPable. I think that GUP is supported on
> MIXEDMAP at all is a bit of a weirdo thing.

IIRC (after recent discussions with Lorenzo) there are use cases for that.

And there is no way to block GUP-fast either way without pte_special(). 
And pte_special() ... is not for refcounted pages.

-- 
Cheers,

David / dhildenb