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

[David Hildenbrand <david@redhat.com>]

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().

> Also we should BUG_ON(!pmd_present(pmd)) to avoid the null dereference?

Definitely no BUG_ON :)

> 
>>
>>           table = pte_offset_kernel(pmdp, addr);
>>           pmd_clear(pmdp);
>> @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long
>> addr)
>>           next = addr;
>>           end = addr + PUD_SIZE;
>>           do {
>> -               pmd_free_pte_page(pmdp, next);
>> +               if (pmd_present(*pmdp))
>> +                       pmd_free_pte_page(pmdp, next);
> 
> Ah yes, the "caller" of pmd_free_pte_page() is not only
> vmap_try_huge_pmd but this also...my mind has been foggy lately...
> need to solve a math problem or two to sharpen it :)

Haha.

-- 
Cheers,

David / dhildenb