Re: [PATCH] arm64/mm: restrict transparent contpte mappings to valid PTEs

[Zhao Gongyi]

On 10/24/26 4:55 AM, David Hildenbrand (Arm) wrote:
> 
> Hi,
> 
> sorry for the late reply.
> 
> If we have the spare PTE bit, why shouldn't we just use it?
> 
> It will make batching over PTEs faster. And we care about that for present PTEs.
> 
> So I don't quite see why the stable fix couldn't just do it right away? Do you
> have a prototype that shows that it would require significant code changes?
> 

Hi David,

I tried a small prototype that keeps PTE_CONT on present-invalid PTEs.
It leaves contpte_set_ptes() unchanged and makes contpte_try_unfold()
recognize pte_present(pte) && pte_cont(pte), instead of checking only
pte_valid_cont(pte).

With this change, the guard-marker reproducer passed 100 iterations
in an arm64 QEMU/KVM guest running a KASAN-enabled kernel. The fork
and PMD-split probes passed five runs each, with no warnings or KASAN
reports. In the earlier A/B test, the unmodified kernel failed on the
first guard-marker iteration.

The prototype makes the reported reproducer pass with a small change
while retaining PTE_CONT. I therefore cannot point to evidence that
fixing this failure requires significant code changes.

Thanks,
Gongyi