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

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Message-ID: <20260109195111.GQ545276@ziepe.ca>
Date: Fri, 9 Jan 2026 15:51:11 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
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,
	will@...nel.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, xiaqinxin@...wei.com,
	baolu.lu@...ux.intel.com, rdunlap@...radead.org,
	Samiullah Khawaja <skhawaja@...gle.com>
Subject: Re: [PATCH v6 3/4] iommu: debug-pagealloc: Track IOMMU pages

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().

Jason