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

[Will Deacon <will@kernel.org>]

On Mon, Nov 24, 2025 at 12:37:31PM +0000, Mostafa Saleh wrote:
> 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@google.com>
> > > Tested-by: Qinxin Xia <xiaqinxin@huawei.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.

I was thinking specifically about calling dump_page_owner().

Will