linux-kernel - Re: [PATCH v5 3/4] iommu: debug-pagealloc: Track IOMMU pages

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Message-ID: <aWCviSCK7IBddLTn@google.com>
Date: Fri, 9 Jan 2026 07:34:33 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Mostafa Saleh <smostafa@...gle.com>, 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,
	rdunlap@...radead.org
Subject: Re: [PATCH v5 3/4] iommu: debug-pagealloc: Track IOMMU pages

On Fri, Jan 09, 2026 at 11:28:32AM +0800, Baolu Lu wrote:
> On 1/8/26 19:33, Mostafa Saleh wrote:
> > I have this, it should have the same effect + a WARN, I will include
> > it in the new version
> > 
> > diff --git a/drivers/iommu/iommu-debug-pagealloc.c
> > b/drivers/iommu/iommu-debug-pagealloc.c
> > index 5353417e64f9..64ec0795fe4c 100644
> > --- a/drivers/iommu/iommu-debug-pagealloc.c
> > +++ b/drivers/iommu/iommu-debug-pagealloc.c
> > @@ -146,16 +146,12 @@ void __iommu_debug_unmap_end(struct iommu_domain *domain,
> >          if (unmapped == size)
> >                  return;
> > 
> > -       /*
> > -        * If unmap failed, re-increment the refcount, but if it unmapped
> > -        * larger size, decrement the extra part.
> > -        */
> > +       /* If unmap failed, re-increment the refcount. */
> >          if (unmapped < size)
> >                  __iommu_debug_update_iova(domain, iova + unmapped,
> >                                            size - unmapped, true);
> >          else
> > -               __iommu_debug_update_iova(domain, iova + size,
> > -                                         unmapped - size, false);
> > +               WARN_ONCE(1, "iommu: unmap larger than requested is
> > not supported in debug_pagealloc\n");
> >   }
> > 
> >   void iommu_debug_init(void)
> 
> How aobut
> 
> 	if ((unmapped == size) || WARN_ON_ONCE(unmapped > size))
> 		return;
> 
> 	/* If unmap failed, re-increment the refcount. */
> 	__iommu_debug_update_iova(domain, iova + unmapped, size - unmapped, true);
> 
> ?
> 

That's nice, We could also print the message as Mostafa mentioned, like:

	if ((unmapped == size) || WARN_ON_ONCE(unmapped > size, 
	"iommu:	unmap larger than requested is not supported in
	debug_pagealloc\n"))

An explicit "This is unsupported" warning makes it clear IMHO.
If a driver triggers this, at least we know the sanitizer state isn't
supported.

Thanks,
Praan