Re: [PATCH v2] arm64: Restrict pagetable teardown to avoid false warning

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

On 19/05/2025 13:16, David Hildenbrand wrote:
> On 19.05.25 11:08, Ryan Roberts wrote:
>> On 18/05/2025 10:54, Dev Jain wrote:
>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller
>>
>> nit: please use the standard format for describing commits: Commit 9c006972c3fe
>> ("arm64: mmu: drop pXd_present() checks from pXd_free_pYd_table()")
>>
>>> 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. Thus it is possible to
>>> hit a warning in the latter, since pmd_none => !pmd_table(). Thus, add
>>> a pmd_present() check in pud_free_pmd_page().
>>>
>>> This problem was found by code inspection.
>>>
>>> This patch is based on 6.15-rc6.
>>
>> nit: please remove this to below the "---", its not part of the commit log.
>>
>>>
>>> Fixes: 9c006972c3fe (arm64: mmu: drop pXd_present() checks from
>>> pXd_free_pYd_table())
>>>
>>
>> nit: remove empty line; the tags should all be in a single block with no empty
>> lines.
>>
>>> Cc: <stable@vger.kernel.org>
>>> Reported-by: Ryan Roberts <ryan.roberts@arm.com>
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> ---
>>> v1->v2:
>>>   - Enforce check in caller
>>>
>>>   arch/arm64/mm/mmu.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index ea6695d53fb9..5b1f4cd238ca 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -1286,7 +1286,8 @@ 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() is using READ_ONCE() to access the *pmdp to ensure it can't
>> be torn. I suspect we don't technically need that in these functions because
>> there can be no race with a writer.
> 
> Yeah, if there is no proper locking in place the function would already
> seriously mess up (double freeing etc).

Indeed; there is no locking, but this portion of the vmalloc VA space has been
allocated to us exclusively, so we know there can be no one else racing.

> 
>> But the arm64 arch code always uses
>> READ_ONCE() for dereferencing pgtable entries for safely. Perhaps we should be
>> consistent here?
> 
> mm/vmalloc.c:   if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr))

Yes, I saw that. I know that we don't technically need READ_ONCE(). I'm just
proposng that for arm64 code we should be consistent with what it already does.
See Commit 20a004e7b017 ("arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing
page tables")

So I'll just use pmdp_get()? (Hopefully my reply comes fine, I am replying
from the terminal)

Thanks,
Ryan

> 
> 
> :)
> 
> Acked-by: David Hildenbrand <david@redhat.com>
>