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

[Dev Jain <dev.jain@arm.com>]

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.

> 
>>
>>>
>>> If you were able to trigger this WARN, it's always a good idea to
>>> include the splat in the commit.
>>
>> I wasn't able to, it is just an observation from code inspection.
> 
> That better be included in the patch description :)
>