Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables

[David Hildenbrand <david@redhat.com>]

On 15.05.25 14:56, Will Deacon wrote:
> On Thu, May 15, 2025 at 11:32:22AM +0200, David Hildenbrand wrote:
>> On 15.05.25 11:27, Dev Jain wrote:
>>>
>>>
>>> On 15/05/25 2:23 pm, David Hildenbrand wrote:
>>>> On 15.05.25 10:47, Dev Jain wrote:
>>>>>
>>>>>
>>>>> On 15/05/25 2:06 pm, David Hildenbrand wrote:
>>>>>> On 15.05.25 10:22, Dev Jain wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 15/05/25 1:43 pm, David Hildenbrand wrote:
>>>>>>>> On 15.05.25 08:34, Dev Jain wrote:
>>>>>>>>> Commit 9c006972c3fe removes the pxd_present() checks because the
>>>>>>>>> caller
>>>>>>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller
>>>>>>>>> only
>>>>>>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd
>>>>>>>>> through
>>>>>>>>> pmd_free_pte_page(), wherein the pmd may be none.
>>>>>>>> The commit states: "The core code already has a check for pXd_none()",
>>>>>>>> so I assume that assumption was not true in all cases?
>>>>>>>>
>>>>>>>> Should that one problematic caller then check for pmd_none() instead?
>>>>>>>
>>>>>>>      From what I could gather of Will's commit message, my
>>>>>>> interpretation is
>>>>>>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd.
>>>>>>> These individually check for pxd_present():
>>>>>>>
>>>>>>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr))
>>>>>>>        return 0;
>>>>>>>
>>>>>>> The problem is that vmap_try_huge_pud will also iterate on pte entries.
>>>>>>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page
>>>>>>> may encounter a none pmd and trigger a WARN.
>>>>>>
>>>>>> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky.
>>>>>>
>>>>>> I assume we should either have an explicit pmd_none() check in
>>>>>> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in
>>>>>> pmd_free_pte_page().
>>>>>>
>>>>>> With your patch, we'd be calling pte_free_kernel() on a NULL pointer,
>>>>>> which sounds wrong -- unless I am missing something important.
>>>>>
>>>>> Ah thanks, you seem to be right. We will be extracting table from a none
>>>>> pmd. Perhaps we should still bail out for !pxd_present() but without the
>>>>> warning, which the fix commit used to do.
>>>>
>>>> Right. We just make sure that all callers of pmd_free_pte_page() already
>>>> check for it.
>>>>
>>>> I'd just do something like:
>>>>
>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>>> index 8fcf59ba39db7..e98dd7af147d5 100644
>>>> --- a/arch/arm64/mm/mmu.c
>>>> +++ b/arch/arm64/mm/mmu.c
>>>> @@ -1274,10 +1274,8 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long
>>>> addr)
>>>>
>>>>            pmd = READ_ONCE(*pmdp);
>>>>
>>>> -       if (!pmd_table(pmd)) {
>>>> -               VM_WARN_ON(1);
>>>> -               return 1;
>>>> -       }
>>>> +       VM_WARN_ON(!pmd_present(pmd));
>>>> +       VM_WARN_ON(!pmd_table(pmd));
>>>
>>> And also return 1?
>>
>> I'll leave that to Catalin + Will.
>>
>> I'm not a friend for adding runtime-overhead for soemthing that should not
>> happen and be caught early during testing -> VM_WARN_ON_ONCE().
> 
> I definitely think we should return early if the pmd isn't a table.
> Otherwise, we could end up descending into God-knows-what!

The question is: how did something that is not a table end up here, and 
why is it valid to check exactly that at runtime. Not strong opinion, it 
just feels a bit arbitrary to test for exactly that at runtime if it is 
completely unexpected.

-- 
Cheers,

David / dhildenb