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

[Jason Gunthorpe <jgg@nvidia.com>]

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.

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.

The main value of MIXEDMAP having access to the struct page is to
allow the page table itself to manage the life-cycle of memory that
was allocated specifically and only for the VMA. It will do fork and
unmap properly and free the memory at the right time.

If a driver is managing the memory lifecylce on its own and using zap
to clear the PTEs (because it must to manage the non-struct page PTE
life-cycle), then there isn't much point in using MIXEDMAP, IMHO. I
wonder if some of the GPU drivers are confused like this.

Jason