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

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Message-ID: <aRW6LWh_1lTce7kU@willie-the-truck>
Date: Thu, 13 Nov 2025 10:59:57 +0000
From: Will Deacon <will@...nel.org>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: linux-mm@...ck.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, corbet@....net, joro@...tes.org,
	robin.murphy@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
	surenb@...gle.com, mhocko@...e.com, jackmanb@...gle.com,
	hannes@...xchg.org, ziy@...dia.com, david@...hat.com,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
	rppt@...nel.org, Qinxin Xia <xiaqinxin@...wei.com>
Subject: Re: [PATCH v2 4/4] drivers/iommu-debug-pagealloc: Check
 mapped/unmapped kernel memory

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@...gle.com>
> Tested-by: Qinxin Xia <xiaqinxin@...wei.com>
> ---
>  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