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

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Message-ID: <7680e775-d277-45ea-9b6c-1f16b8b55a3f@redhat.com>
Date: Mon, 19 May 2025 14:16:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 catalin.marinas@....com, will@...nel.org
Cc: 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 v2] arm64: Restrict pagetable teardown to avoid false
 warning

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@...r.kernel.org>
>> Reported-by: Ryan Roberts <ryan.roberts@....com>
>> Signed-off-by: Dev Jain <dev.jain@....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).

> 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))


:)

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

-- 
Cheers,

David / dhildenb