Re: [PATCH v4 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

[David Hildenbrand (Arm)]

On 8/28/26 07:59, 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.
> 
> About calling the respective trace_xxx() functions separately on success
> and failure, refer to [1].
> 
> [1] https://lore.kernel.org/linux-mm/ao6jVbVHLUmuY2UA@gremlin/
> 
> 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                    | 11 ++++++++++-
>  2 files changed, 13 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 75639298efc2..b597a3e68606 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2683,6 +2683,7 @@ 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 failed_pfn = -1;
>  
>  	present = 0;
>  	swap = 0;
> @@ -2715,6 +2716,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  
>  		if (is_pmd_order(folio_order(folio))) {
>  			result = SCAN_PTE_MAPPED_HUGEPAGE;
> +			failed_pfn = folio_pfn(folio);
>  			/*
>  			 * PMD-sized THP implies that we can only try
>  			 * retracting the PTE table.
> @@ -2726,6 +2728,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		node = folio_nid(folio);
>  		if (collapse_scan_abort(node, cc)) {
>  			result = SCAN_SCAN_ABORT;
> +			failed_pfn = folio_pfn(folio);
>  			folio_put(folio);
>  			break;
>  		}
> @@ -2733,12 +2736,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  
>  		if (!folio_test_lru(folio)) {
>  			result = SCAN_PAGE_LRU;
> +			failed_pfn = folio_pfn(folio);
>  			folio_put(folio);
>  			break;
>  		}
>  
>  		if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
>  			result = SCAN_PAGE_COUNT;
> +			failed_pfn = folio_pfn(folio);
>  			folio_put(folio);
>  			break;
>  		}
> @@ -2771,9 +2776,13 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		} else {
>  			result = collapse_file(mm, addr, file, start, cc);
>  		}
> +		trace_mm_khugepaged_scan_file(mm, -1, file, present, swap,
> +					      SCAN_SUCCEED);
> +	} else {
> +		trace_mm_khugepaged_scan_file(mm, failed_pfn, file, present,
> +					      swap, result);
>  	}

Just curious (maybe Lorenzo suggested that?), why separate out the SCAN_SUCCEED
stuff?

When result == SCAN_SUCCEED, failed_pfn == -1 no?

We set failed_pfn now only when setting result ...

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David