linux-kernel - Re: [PATCH v6 4/4] iommu: debug-pagealloc: Check mapped/unmapped kernel memory

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Message-ID: <aWFT-Oh9bQsbqrXj@google.com>
Date: Fri, 9 Jan 2026 19:16:08 +0000
From: Pranjal Shrivastava <praan@...gle.com>
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 4/4] iommu: debug-pagealloc: Check mapped/unmapped
 kernel memory

On Fri, Jan 09, 2026 at 05:18:05PM +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.
> 
> Reviewed-by: Samiullah Khawaja <skhawaja@...gle.com>
> Reviewed-by: Lu Baolu <baolu.lu@...ux.intel.com>
> Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> ---
>  drivers/iommu/iommu-debug-pagealloc.c | 23 +++++++++++++++++++++++
>  include/linux/iommu-debug-pagealloc.h | 14 ++++++++++++++
>  include/linux/mm.h                    |  5 +++++
>  3 files changed, 42 insertions(+)
> 

Reviewed-by: Pranjal Shrivastava <praan@...gle.com>

Thanks,
Praan

> diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c
> index 9eb49e1230ee..c080a38f45a4 100644
> --- a/drivers/iommu/iommu-debug-pagealloc.c
> +++ b/drivers/iommu/iommu-debug-pagealloc.c
> @@ -9,6 +9,7 @@
>  #include <linux/iommu-debug-pagealloc.h>
>  #include <linux/kernel.h>
>  #include <linux/page_ext.h>
> +#include <linux/page_owner.h>
>  
>  #include "iommu-priv.h"
>  
> @@ -73,6 +74,28 @@ static size_t iommu_debug_page_size(struct iommu_domain *domain)
>  	return 1UL << __ffs(domain->pgsize_bitmap);
>  }
>  
> +static bool iommu_debug_page_count(const struct page *page)
> +{
> +	unsigned int ref;
> +	struct page_ext *page_ext = page_ext_get(page);
> +	struct iommu_debug_metadata *d = get_iommu_data(page_ext);
> +
> +	ref = atomic_read(&d->ref);
> +	page_ext_put(page_ext);
> +	return ref != 0;
> +}
> +
> +void __iommu_debug_check_unmapped(const struct page *page, int numpages)
> +{
> +	while (numpages--) {
> +		if (WARN_ON(iommu_debug_page_count(page))) {
> +			pr_warn("iommu: Detected page leak!\n");
> +			dump_page_owner(page);
> +		}
> +		page++;
> +	}
> +}
> +
>  void __iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys, size_t size)
>  {
>  	size_t off, end;
> diff --git a/include/linux/iommu-debug-pagealloc.h b/include/linux/iommu-debug-pagealloc.h
> index a439d6815ca1..46c3c1f70150 100644
> --- a/include/linux/iommu-debug-pagealloc.h
> +++ b/include/linux/iommu-debug-pagealloc.h
> @@ -13,6 +13,20 @@ DECLARE_STATIC_KEY_FALSE(iommu_debug_initialized);
>  
>  extern struct page_ext_operations page_iommu_debug_ops;
>  
> +void __iommu_debug_check_unmapped(const struct page *page, int numpages);
> +
> +static inline void iommu_debug_check_unmapped(const struct page *page, int numpages)
> +{
> +	if (static_branch_unlikely(&iommu_debug_initialized))
> +		__iommu_debug_check_unmapped(page, numpages);
> +}
> +
> +#else
> +static inline void iommu_debug_check_unmapped(const struct page *page,
> +					      int numpages)
> +{
> +}
> +
>  #endif /* CONFIG_IOMMU_DEBUG_PAGEALLOC */
>  
>  #endif /* __LINUX_IOMMU_DEBUG_PAGEALLOC_H */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f959d8ca4b4..32205d2a24b2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -36,6 +36,7 @@
>  #include <linux/rcuwait.h>
>  #include <linux/bitmap.h>
>  #include <linux/bitops.h>
> +#include <linux/iommu-debug-pagealloc.h>
>  
>  struct mempolicy;
>  struct anon_vma;
> @@ -4133,12 +4134,16 @@ extern void __kernel_map_pages(struct page *page, int numpages, int enable);
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  static inline void debug_pagealloc_map_pages(struct page *page, int numpages)
>  {
> +	iommu_debug_check_unmapped(page, numpages);
> +
>  	if (debug_pagealloc_enabled_static())
>  		__kernel_map_pages(page, numpages, 1);
>  }
>  
>  static inline void debug_pagealloc_unmap_pages(struct page *page, int numpages)
>  {
> +	iommu_debug_check_unmapped(page, numpages);
> +
>  	if (debug_pagealloc_enabled_static())
>  		__kernel_map_pages(page, numpages, 0);
>  }
> -- 
> 2.52.0.457.g6b5491de43-goog
> 
>