linux-kernel - Re: [PATCH v2 3/4] drivers/iommu-debug-pagealloc: Track IOMMU pages

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Message-ID: <aRW6Tf_G2ObR__vE@willie-the-truck>
Date: Thu, 13 Nov 2025 11:00:29 +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 3/4] drivers/iommu-debug-pagealloc: Track IOMMU pages

On Thu, Nov 06, 2025 at 04:39:52PM +0000, Mostafa Saleh wrote:
> Using the new calls, use an atomic refcount to track how many times
> a page is mapped in any of the IOMMUs.
> 
> For unmap we need to use iova_to_phys() to get the physical address
> of the pages.
> 
> We use the smallest supported page size as the granularity of tracking
> per domain.
> This is important as it possible to map pages and unmap them with
> larger sizes (as in map_sg()) cases.
> 
> Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> Tested-by: Qinxin Xia <xiaqinxin@...wei.com>
> ---
>  drivers/iommu/iommu-debug-pagealloc.c | 74 +++++++++++++++++++++++++++
>  1 file changed, 74 insertions(+)
> 
> diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c
> index a6a2f844b09d..0e14104b971c 100644
> --- a/drivers/iommu/iommu-debug-pagealloc.c
> +++ b/drivers/iommu/iommu-debug-pagealloc.c
> @@ -27,16 +27,90 @@ struct page_ext_operations page_iommu_debug_ops = {
>  	.need = need_iommu_debug,
>  };
>  
> +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_metadate *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_metadate *d = get_iommu_data(page_ext);
> +
> +	WARN_ON(atomic_inc_return(&d->ref) <= 0);

Is it worth dumping some information about the page in addition to the
WARN_ON()? That way, you might be able to benefit from other debug
options (e.g. PAGE_OWNER) if they are enabled.

> +	page_ext_put(page_ext);
> +}
> +
> +static void iommu_debug_dec_page(phys_addr_t phys)
> +{
> +	struct page_ext *page_ext = get_iommu_page_ext(phys);
> +	struct iommu_debug_metadate *d = get_iommu_data(page_ext);
> +
> +	WARN_ON(atomic_dec_return(&d->ref) < 0);

nit: I can't see why you need memory ordering guarantees for the refcount,
so you could use the relaxed variants for the inc/dec operations.

> +	page_ext_put(page_ext);
> +}
> +
> +/*
> + * IOMMU page size might not match the CPU page size, in that case, we use
> + * the smallest IOMMU page size to refcount the pages in the vmemmap.
> + * That is important as both map and unmap has to use the same page size
> + * to update the refcount to avoid double counting the same page.
> + * And as we can't know from iommu_unmap() what was the original page size
> + * used for map, we just use the minimum supported one for both.
> + */
> +static size_t iommu_debug_page_size(struct iommu_domain *domain)
> +{
> +	return 1UL << __ffs(domain->pgsize_bitmap);
> +}
> +
>  void __iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys, size_t size)
>  {
> +	size_t off;
> +	size_t page_size = iommu_debug_page_size(domain);

Since this is a debug feature, is it worth checking other properties of
the arguments too? For example, that phys is non-zero and that phys +
size doesn't overflow?

> +	for (off = 0 ; off < size ; off += page_size) {
> +		if (!pfn_valid(__phys_to_pfn(phys + off)))
> +			continue;
> +		iommu_debug_inc_page(phys + off);
> +	}
>  }
>  
>  void __iommu_debug_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
>  {
> +	size_t off;
> +	size_t page_size = iommu_debug_page_size(domain);
> +
> +	for (off = 0 ; off < size ; off += page_size) {
> +		phys_addr_t phys = iommu_iova_to_phys(domain, iova + off);
> +
> +		if (!phys || !pfn_valid(__phys_to_pfn(phys + off)))
> +			continue;

Hmm, it looks weird to add 'off' to both 'iova' _and_ the resulting
physical address. Is that correct?

> +		iommu_debug_dec_page(phys);
> +	}
>  }
>  
>  void __iommu_debug_remap(struct iommu_domain *domain, unsigned long iova, size_t size)
>  {
> +	size_t off;
> +	size_t page_size = iommu_debug_page_size(domain);
> +
> +	for (off = 0 ; off < size ; off += page_size) {
> +		phys_addr_t phys = iommu_iova_to_phys(domain, iova + off);
> +
> +		if (!phys || !pfn_valid(__phys_to_pfn(phys + off)))
> +			continue;
> +
> +		iommu_debug_inc_page(phys);
> +	}

You can make the bulk of this code common with the unmap function.

Will