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

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Message-ID: <aSSBdu-P-E75-_iY@google.com>
Date: Mon, 24 Nov 2025 16:01:58 +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 Mon, Nov 24, 2025 at 03:35:47PM +0000, Will Deacon wrote:
> 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@...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.
> 
> I was thinking specifically about calling dump_page_owner().

I see, that makes sense.

Thanks,
Mostafa

> 
> Will