Re: [PATCH v3 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

[David Hildenbrand (Arm)]

On 8/24/26 11:29, Vernon Yang wrote:
> From: Vernon Yang <yanglincheng@xxxxxxxxxx>
> 
> When the swap entries found exceed max_ptes_swap, the loop is left via
> break with folio still holding the xarray value that encodes the swap
> entry, not valid folio pointer.
> 
> That value is passed to trace_mm_khugepaged_scan_file(), which feeds it
> to folio_pfn(). On FLATMEM and SPARSEMEM_VMEMMAP, the page_to_pfn() is
> plain pointer arithmetic, so the trace event merely prints bogus
> scan_pfn. On classic SPARSEMEM, the page_to_pfn() reads page->flags,
> dereferencing the tiny encoded integer and oopsing khugepaged whenever
> the trace event is enabled.
> 
> So when folio is the swap entry value, simply set pfn to -1, just like
> exhausted scan naturally.
> 
> And the folio_put() has maybe dropped the last reference of folio. The
> trace_mm_khugepaged_scan_file() is left with a dangling folio pointer.
> so using the folio_pfn() before dropping the reference, closing
> use-after-free window.
> 
> Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
> ---
>  include/trace/events/huge_memory.h | 6 +++---
>  mm/khugepaged.c                    | 5 ++++-
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 5a48c5406cce..7b526528f85b 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -178,10 +178,10 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
>  
>  TRACE_EVENT(mm_khugepaged_scan_file,
>  
> -	TP_PROTO(struct mm_struct *mm, struct folio *folio, struct file *file,
> +	TP_PROTO(struct mm_struct *mm, unsigned long pfn, struct file *file,
>  		 int present, int swap, int result),
>  
> -	TP_ARGS(mm, folio, file, present, swap, result),
> +	TP_ARGS(mm, pfn, file, present, swap, result),
>  
>  	TP_STRUCT__entry(
>  		__field(struct mm_struct *, mm)
> @@ -194,7 +194,7 @@ TRACE_EVENT(mm_khugepaged_scan_file,
>  
>  	TP_fast_assign(
>  		__entry->mm = mm;
> -		__entry->pfn = folio ? folio_pfn(folio) : -1;
> +		__entry->pfn = pfn;
>  		__assign_str(filename);
>  		__entry->present = present;
>  		__entry->swap = swap;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 79effd3f3da4..00337405c0e0 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2689,6 +2689,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  	int present, swap;
>  	int node = NUMA_NO_NODE;
>  	enum scan_result result = SCAN_SUCCEED;
> +	unsigned long pfn;
>  
>  	present = 0;
>  	swap = 0;
> @@ -2719,6 +2720,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  			continue;
>  		}
>  
> +		pfn = folio_pfn(folio);
>  		if (is_pmd_order(folio_order(folio))) {
>  			result = SCAN_PTE_MAPPED_HUGEPAGE;
>  			/*
> @@ -2779,7 +2781,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		}
>  	}
>  
> -	trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result);
> +	trace_mm_khugepaged_scan_file(mm, (!folio || xa_is_value(folio)) ? -1 : pfn,
> +				      file, present, swap, result);
>  	return result;
>  }
>  

Shouldn't we just reset PFN to -1 at the beginning of the loop (and set it
initially)?

-- 
Cheers,

David