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

[David Hildenbrand (Arm)]

On 8/26/26 04:44, Vernon Yang wrote:
> On Mon, Aug 24, 2026 at 01:54:18PM +0200, David Hildenbrand (Arm) wrote:
>> 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)?
> 
> When the `xas_for_each()` iteration to terminate and the folio operation
> preceding is normal, but pfn will be incorrect.

The PFN is only relevant when a folio participated in the failure. Maybe the
following would be cleanest?

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 75639298efc27..371ee0b16d10c 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 problematic_pfn = -1;

        present = 0;
        swap = 0;
@@ -2714,6 +2715,7 @@ static enum scan_result collapse_scan_file(struct
mm_struct *mm,
                }

                if (is_pmd_order(folio_order(folio))) {
+                       problematic_pfn = folio_pfn(folio);
                        result = SCAN_PTE_MAPPED_HUGEPAGE;
                        /*
                         * PMD-sized THP implies that we can only try
@@ -2725,6 +2727,7 @@ static enum scan_result collapse_scan_file(struct
mm_struct *mm,

                node = folio_nid(folio);
                if (collapse_scan_abort(node, cc)) {
+                       problematic_pfn = folio_pfn(folio);
                        result = SCAN_SCAN_ABORT;
                        folio_put(folio);
                        break;
@@ -2732,12 +2735,14 @@ static enum scan_result collapse_scan_file(struct
mm_struct *mm,
                cc->node_load[node]++;

                if (!folio_test_lru(folio)) {
+                       problematic_pfn = folio_pfn(folio);
                        result = SCAN_PAGE_LRU;
                        folio_put(folio);
                        break;
                }

                if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
+                       problematic_pfn = folio_pfn(folio);
                        result = SCAN_PAGE_COUNT;
                        folio_put(folio);
                        break;
@@ -2773,7 +2778,7 @@ 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, problematic_pfn, file, present, swap,
result);
        return result;
 }



-- 
Cheers,

David