Re: [PATCH v2 4/4] drivers/iommu-debug-pagealloc: Check mapped/unmapped kernel memory

[Will Deacon]

On Thu, Nov 06, 2025 at 04:39:53PM +0000, Mostafa Saleh wrote:
> Now, as the page_ext holds count of IOMMU mappings, we can use it to
> assert that any page allocated/freed is indeed not in the IOMMU.
> 
> The sanitizer doesn’t protect against mapping/unmapping during this
> period. However, that’s less harmful as the page is not used by the
> kernel.
> 
> Signed-off-by: Mostafa Saleh <smostafa@xxxxxxxxxx>
> Tested-by: Qinxin Xia <xiaqinxin@xxxxxxxxxx>
> ---
>  drivers/iommu/iommu-debug-pagealloc.c | 19 +++++++++++++++++++
>  include/linux/iommu-debug-pagealloc.h | 12 ++++++++++++
>  include/linux/mm.h                    |  5 +++++
>  3 files changed, 36 insertions(+)
> 
> diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c
> index 0e14104b971c..5b26c84d3a0e 100644
> --- a/drivers/iommu/iommu-debug-pagealloc.c
> +++ b/drivers/iommu/iommu-debug-pagealloc.c
> @@ -71,6 +71,25 @@ static size_t iommu_debug_page_size(struct iommu_domain *domain)
>  	return 1UL << __ffs(domain->pgsize_bitmap);
>  }
>  
> +static unsigned int iommu_debug_page_count(unsigned long phys)

'phys_addr_t phys' ?

But having said that, wouldn't you be better off taking the
'struct page *' here rather than converting it to a physical address
only for get_iommu_page_ext() to convert it straight back again?

> +{
> +	unsigned int ref;
> +	struct page_ext *page_ext = get_iommu_page_ext(phys);
> +	struct iommu_debug_metadate *d = get_iommu_data(page_ext);
> +
> +	ref = atomic_read(&d->ref);
> +	page_ext_put(page_ext);
> +	return ref;
> +}
> +
> +void __iommu_debug_check_unmapped(const struct page *page, int numpages)
> +{
> +	while (numpages--) {
> +		WARN_ON(iommu_debug_page_count(page_to_phys(page)));

Since you only care about the count being non-zero, perhaps tweak
iommu_debug_page_count() to be something like:

	bool iommu_debug_page_referenced(struct page *);

Will