linux-kernel - Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd

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Message-ID: <c06930f0-f98c-4089-aa33-6789b95fd08f@redhat.com>
Date: Thu, 15 May 2025 10:53:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, catalin.marinas@....com, will@...nel.org
Cc: ryan.roberts@....com, anshuman.khandual@....com, mark.rutland@....com,
 yang@...amperecomputing.com, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while
 tearing down page tables

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));
  
         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);
         } while (pmdp++, next += PMD_SIZE, next != end);
  
         pud_clear(pudp);


-- 
Cheers,

David / dhildenb