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

[Mostafa Saleh <smostafa@google.com>]

On Mon, Jan 12, 2026 at 1:32 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Mon, Jan 12, 2026 at 10:20:14AM +0000, Mostafa Saleh wrote:
> > On Fri, Jan 9, 2026 at 7:51 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Fri, Jan 09, 2026 at 05:18:04PM +0000, Mostafa Saleh wrote:
> > > > +static struct page_ext *get_iommu_page_ext(phys_addr_t phys)
> > > > +{
> > > > +     struct page *page = phys_to_page(phys);
> > > > +     struct page_ext *page_ext = page_ext_get(page);
> > > > +
> > > > +     return page_ext;
> > > > +}
> > > > +
> > > > +static struct iommu_debug_metadata *get_iommu_data(struct page_ext *page_ext)
> > > > +{
> > > > +     return page_ext_data(page_ext, &page_iommu_debug_ops);
> > > > +}
> > > > +
> > > > +static void iommu_debug_inc_page(phys_addr_t phys)
> > > > +{
> > > > +     struct page_ext *page_ext = get_iommu_page_ext(phys);
> > > > +     struct iommu_debug_metadata *d = get_iommu_data(page_ext);
> > >
> > > You cannot do this - phys_to_page() can only be called if we already
> > > know that phys is a struct page backed item and by the time you get
> > > here that information is lost.
> > >
> > > Probably the only way to resolve this is to somehow pass in an iommu
> > > prot flag that can tell the difference between struct page and
> > > non-struct page addresses.
> > >
> > > But I have to NAK this approach of blindly calling phys_to_page().
> >
> > The callers to this, first will check "pfn_valid", which is the right
> > check AFAICT (looking at similar patterns in page_owner for example).
>
> I'm not sure pfn_valid really works in all cases, it has a number of
> exclusions that can be relevant when phys_addr_t can be a MMIO
> address..
>
> So far we haven't been using it in the DMA paths at all, I'm not so
> keen to see that start..
>

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.

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

Thanks,
Mostafa

> Jason