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

[Vernon Yang <vernon2gm@gmail.com>]

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@kylinos.cn>
> >
> > 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@vger.kernel.org
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >  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.

If we just reset PFN to -1 at the __ending__ of the loop (and set it
initially), then the above situation won't occur. If so, I will do it in
the next version.

--
Cheers,
Vernon