linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <4a7a7fbb-e33e-4033-91e7-efce7915cf7f@redhat.com>
Date: Fri, 17 Oct 2025 10:43:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, Wei Yang <richard.weiyang@...il.com>,
 Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, ioworker0@...il.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()

On 17.10.25 10:37, Lance Yang wrote:
> 
> 
> On 2025/10/17 16:11, David Hildenbrand wrote:
>> On 17.10.25 03:27, Wei Yang wrote:
>>> On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote:
>>>>
>>>> On 16/10/25 9:06 am, Lance Yang wrote:
>>>>> From: Lance Yang <lance.yang@...ux.dev>
>>>>>
>>>>> A non-present entry, like a swap PTE, contains completely different
>>>>> data
>>>>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
>>>>> non-present entry, it will spit out a junk PFN.
>>>>>
>>>>> What if that junk PFN happens to match the zeropage's PFN by sheer
>>>>> chance? While really unlikely, this would be really bad if it did.
>>>>>
>>>>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
>>>>> in khugepaged.c are properly guarded by a pte_present() check.
>>>>>
>>>>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>>>> ---
>>>>>     mm/khugepaged.c | 13 ++++++++-----
>>>>>     1 file changed, 8 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>>> index d635d821f611..0341c3d13e9e 100644
>>>>> --- a/mm/khugepaged.c
>>>>> +++ b/mm/khugepaged.c
>>>>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t
>>>>> *_pte,
>>>>>             pte_t pteval = ptep_get(_pte);
>>>>>             unsigned long pfn;
>>>>> -        if (pte_none(pteval))
>>>>> +        if (!pte_present(pteval))
>>>>>                 continue;
>>>>>             pfn = pte_pfn(pteval);
>>>>>             if (is_zero_pfn(pfn))
>>>>> @@ -690,9 +690,10 @@ static void
>>>>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>>>>              address += nr_ptes * PAGE_SIZE) {
>>>>>             nr_ptes = 1;
>>>>>             pteval = ptep_get(_pte);
>>>>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>>>>> +        if (pte_none(pteval) ||
>>>>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>>>>                 add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>>>>> -            if (is_zero_pfn(pte_pfn(pteval))) {
>>>>> +            if (!pte_none(pteval)) {
>>>>
>>>> Could save a level of indentation by saying
>>>> if (pte_none(pteval))
>>>>      continue;
>>>>
>>>
>>> Vote for this :-)
>>
>> I suspect there will be a v2, correct?
> 
> I was hoping a v2 wouldn't be necessary for this ;p
> 
> Of course, if we'd prefer a v2, I'm happy to send one out.

I lost track of what the result will be, so a v2 would be nice at least 
for me :)

-- 
Cheers

David / dhildenb