Re: [PATCH v6 3/4] iommu: debug-pagealloc: Track IOMMU pages

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Jan 12, 2026 at 01:43:41PM +0000, Mostafa Saleh wrote:
> But I don’t see why not. from the documentation:
> /**
>  * pfn_valid - check if there is a valid memory map entry for a PFN
>  * @pfn: the page frame number to check
>  *
>  * Check if there is a valid memory map entry aka struct page for the @pfn.
>  * Note, that availability of the memory map entry does not imply that
>  * there is actual usable memory at that @pfn. The struct page may
>  * represent a hole or an unusable page frame.
> …
> 
> That means that struct page exists, which is all what we need here.

A struct page that has never been initialize shouldn't ever be read. I
don't know how that relates to page_ext, but are you really sure that
is all you need?

> I can see many places have the same pattern in the kernel already, for example:
> - vfio_iommu_type1.c, is_invalid_reserved_pfn() which does the same
> check which can include MMIO and then get the page struct.

This whole flow is nonsensical and wrong though, I wouldn't point to
it as something reliable.

> - kvm_main.c: in __kvm_vcpu_map(), it distinguishes MMIO from memory
> and then accesses the page struct.

That's sure looks sketchy to me.. Eg if CONFIG_WANT_PAGE_VIRTUAL is
set and you try to feed a MMIO through through that kmap() it will
explode.

KVM can argue that it doesn't work with CONFIG_WANT_PAGE_VIRTUAL but
iommu cannot.

So, again, IDK, we are trying not to use pfn_valid() in the DMA code.

Jason