linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <1937040d-5e70-4d9a-b77a-261bf0f4994e@redhat.com>
Date: Fri, 17 Oct 2025 16:51:00 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
 baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: guard is_zero_pfn() calls
 with pte_present()

On 17.10.25 11:38, 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>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
> Applies against commit 0f22abd9096e in mm-new.
> 
> v1 -> v2:
>   - Collect Reviewed-by from Dev, Wei and Baolin - thanks!
>   - Reduce a level of indentation (per Dev)
>   - https://lore.kernel.org/linux-mm/20251016033643.10848-1-lance.yang@linux.dev/
> 
>   mm/khugepaged.c | 29 ++++++++++++++++-------------
>   1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d635d821f611..648d9335de00 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;


Isn't it rather that if we would ever get a !pte_none() && 
!pte_present() here, something would be deeply flawed?

I'd much rather spell that out and do here

VM_WARN_ON_ONCE(!pte_present(pteval));

keeping the original check.


>   		pfn = pte_pfn(pteval);
>   		if (is_zero_pfn(pfn))
> @@ -690,17 +690,18 @@ 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)))) {

This now seems to be a common pattern now :)


Should we have a simple helper

static inline void pte_none_or_zero(pte_t pte)
{
	if (pte_none(pte))
		return true;
	return pte_present(pte) && is_zero_pfn(pte_pfn(pte)
}

initially maybe local to this file?


-- 
Cheers

David / dhildenb