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

[David Hildenbrand <david@redhat.com>]

On 15.05.25 12:07, Ryan Roberts wrote:
> On 15/05/2025 09:53, 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:
> 
> I just reviewed the patch and had the same feedback as David. I agree with the
> patch below, with some small mods...
> 
>>
>> 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));
> 
> You don't need both of these warnings; pmd_table() is only true if the pmd is
> present (well actually only if it's _valid_ which is more strict than present),
> so the second one is sufficient on its own.

Ah, right.

> 
>>   
>>          table = pte_offset_kernel(pmdp, addr);
>>          pmd_clear(pmdp);
>> @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
> 
> Given you are removing the runtime check and early return in
> pmd_free_pte_page(), I think you should modify this function to use the same
> style too.

BTW, the "return 1" is weird. But looking at x86, we seem to be making a 
private copy of the page table first, to defer freeing the page tables 
after the TLB flush.

I wonder if there isn't a better way (e.g., clear PUDP + flush tlb, then 
walk over the effectively-disconnected page table). But I'm sure there 
is a good reason for that.

> 
>>          next = addr;
>>          end = addr + PUD_SIZE;
>>          do {
>> -               pmd_free_pte_page(pmdp, next);
>> +               if (pmd_present(*pmdp))
> 
> question: I wonder if it is better to use !pmd_none() as the condition here? It
> should either be none or a table at this point, so this allows the warning in
> pmd_free_pte_page() to catch more error conditions. No strong opinion though.

Same here. The existing callers check pmd_present().

-- 
Cheers,

David / dhildenb