Message-ID: <aSRRizzS39jI3BjH@google.com>
Date: Mon, 24 Nov 2025 12:37:31 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Will Deacon <will@...nel.org>
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 13, 2025 at 11:00:29AM +0000, Will Deacon wrote:
> 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.

These WARN_ON are for overflows, which should never happen.
I initially thought about using the refcount_t, but it didn’t seem
suitable as refcount_add() expects that the refcount is already “1”
indicating that an object was already created which doesn’t fit
in the semantics of what this is. Similar for refcount_dec().

In the next patch there is a WARN_ON for the refcount check
to capture the mis-behaving context, I will add a debug print with
the leaked physical address in that case as this is the important one.

> 
> > +	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.

Will do.

> 
> > +	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?
> 

Makes sense, I will add some more checks.

> > +	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?
> 

Yes, that's a bug, I will fix it.

> > +		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 do.

Thank,
Mostafa

> 
> Will

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