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

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Message-ID: <ab918a83-edb2-4a19-821d-a96de9e097eb@linux.dev>
Date: Fri, 17 Oct 2025 16:37:26 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>, 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 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.

Cheers!