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

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Message-ID: <CAFgf54quzFPtfXN0Tw5qO2HU0YExN3FBHK3uG2yiLxkNybfHfA@mail.gmail.com>
Date: Fri, 9 Jan 2026 11:02:33 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Pranjal Shrivastava <praan@...gle.com>
Cc: Baolu Lu <baolu.lu@...ux.intel.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 9, 2026 at 7:34 AM Pranjal Shrivastava <praan@...gle.com> wrote:
>
> 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.
>

I guess both are fine, the WARN points to the line which should be
simple to deduce the cause anyway.

Thanks,
Mostafa

> Thanks,
> Praan