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

[David Hildenbrand (Arm)]

On 10/26/26 13:30, Catalin Marinas wrote:
> On Sun, Oct 25, 2026 at 12:00:33AM +0800, Zhao Gongyi wrote:
>> On 10/24/26 4:55 AM, David Hildenbrand (Arm) wrote:
>>> 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?
>>
>> 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).
> 
> I think it's worth doing but currently not sure it buys us much unless
> we also change pte_batch_hint() to cover such entries. However, if we do
> this, we need to audit the other pte_valid_cont() callers (e.g.
> ptep_get() needs updating as well).
> 
> So, if we aren't going to change pte_batch_hint() as a backport fix (I
> don't think we should, it's an optimisation), I'd rather take v2 now and
> continue with a proper pte_present_cont() etc. patchset on top.

Your call, the version actually looks quite clean:

https://lore.kernel.org/r/52e7e1f6-3535-474d-9b46-6bb62f96eefc@xxxxxxxxxxxxx

But yeah, audit needed (e.g., no need to flush when shattering such entries that
are not valid)

-- 
Cheers,

David