Re: [RFC 1/1] mm/pagewalk: don't split device-backed huge pfnmaps

[Boone, Max]

> On Mar 11, 2026, at 11:45 AM, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> The code in follow_fault_pfn() should likely be updated to handle more
> than one attempt. That's also what GUP does.
> 
> Likely, follow_fault_pfn() was never taught about PFNMAP mappings that
> can be faulted+zapped (in the past they were always static).
> 
> If you turn that into a (possibly) endless loop, does the problem go away?

Yep, was just trying that - with this change the problem goes away:

--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -532,7 +532,7 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,

                ret = follow_pfnmap_start(&args);
                if (ret)
-                       return ret;
+                       return -EAGAIN;
        }

        if (write_fault && !args.writable)

—

I’ll propose that with the VFIO folks when I get the patch for mm/pagewalk.c ready and will refer
to that patch, or would it be better to propose two commits under the same cover letter?

I can have a look at follow_fault_pfn but this problem is my first time diving into linux mm
so that will probably take a while (and some reading up on my end).

> 
> -- 
> Cheers,
> 
> David

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