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

[Vernon Yang <vernon2gm@gmail.com>]

On Mon, Sep 07, 2026 at 02:25:57PM +0200, David Hildenbrand (Arm) wrote:
> On 8/28/26 07:59, 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.
> >
> > 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@vger.kernel.org
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >  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?

Yes, this is the approach Lorenzo[1] suggested in the previous version.

[1] https://lore.kernel.org/linux-mm/ao6jVbVHLUmuY2UA@gremlin/

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

Your understanding is completely correct, and I explained at the time
that this was redundant.

> We set failed_pfn now only when setting result ...

If you prefer one trace_xxx(), please let me know, and I'd be happy to
send V5.

--
Cheers,
Vernon