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

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Message-ID: <CAFgf54r_au6isA10Nrve=MHL455X=tKhNsSwH1ej-TX08J3xLA@mail.gmail.com>
Date: Mon, 12 Jan 2026 10:20:14 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 9, 2026 at 7:51 PM Jason Gunthorpe <jgg@...pe.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).

Thanks,
Mostafa

> Jason
>