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

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Message-ID: <746f5adb-1d91-4ca2-8ae0-a2d171203b66@kernel.org>
Date: Mon, 12 Jan 2026 20:11:50 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Mostafa Saleh <smostafa@...gle.com>, 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, 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 1/12/26 15:58, Mostafa Saleh wrote:
> On Mon, Jan 12, 2026 at 1:52 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>>
>> On Mon, Jan 12, 2026 at 01:43:41PM +0000, Mostafa Saleh wrote:
>>> 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.
>>
>> A struct page that has never been initialize shouldn't ever be read. I
>> don't know how that relates to page_ext, but are you really sure that
>> is all you need?
>>
> 
> AFAIU, if pfn_valid() returns true, it means the struct page is valid,
> and lookup_page_ext() will check that a valid page_ext exists for this
> entry.

Not always. Offline memory blocks have a memory map but no page ext. We 
allocate the page ext at memory onlining time.

Also, I'm not sure about ZONE_DEVICE memory, very likely we never 
allocate a page_ext for them?

I'd assume both cases are not relevant for your use case, though.

-- 
Cheers

David