Re: [PATCH v2 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

[Vernon Yang]

On Mon, Aug 17, 2026 at 05:23:30PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 17, 2026 at 05:10:39PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Sat, Aug 15, 2026 at 01:19:22PM +0800, 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.
> >
> > Hmm I seem to recall that we were no longer spporting !VMEMMAP SPARSEMEM, but
> > maybe I was imagining that :)
>
> OK seems some museum piece architectures are causing us issues again, fun times
> (parisc and some mips).
>
> >
> > But this needs fixing in general anyway.
> >
> > >
> > > 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                    | 14 +++++++++-----
> > >  2 files changed, 12 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > > index 291fae364c62..d3572d4ef453 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;
> >
> > I wonder how easy it is for people to interpret that this is a PFN of something
> > only in the case of an early exit.
> >
> > It's a bit of a mess that we're exposing internal implementation details like
> > this, quite honestly.
> >
> > I hope there is no expectation of this being there indefinitely.
> >
> > >  		__assign_str(filename);
> > >  		__entry->present = present;
> > >  		__entry->swap = swap;
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 617bca76db49..e7830761d3a2 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 pfn;
> >
> > See below I think we should drop it.
>
> OK we can't because of museum piece architectures that will actually deref the
> folio to get the PFN...
>
> I'd startr by initialising this to -1 then since I guess in theory it's possible
> xas_for_each() could just not do anything is it? (folio is being set to NULL so
> that suggests so).
>
> >
> > >
> > >  	present = 0;
> > >  	swap = 0;
> > > @@ -2720,27 +2721,23 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> > >  			 * PMD-sized THP implies that we can only try
> > >  			 * retracting the PTE table.
> > >  			 */
> > > -			folio_put(folio);
>
> I still hate the idea of moving these outside of the loop, it's just asking for trouble.
>
> So I guess instead:
>
> 	xas_for_each(&xas, folio, start + HPAGE_PMD_NR - 1) {
> 		pfn = -1;
> 		...
> 		if (xa_is_value(folio)) {
> 			...
> 		}
> 		pfn = folio_pfn(folio);
> 		...
> 	}
> 	...
> 	trace_mm_khugepaged_scan_file(mm, pfn, file, present, swap, result);
>
> That way you avoid the horror or moving the folio put somewhere deeply
> unintuitive and inconsistent, and maintain pfn lifetime for some arches nobody
> uses.

Thank you for the reference solution. After studying the overall
process, I found that the above solution still has two issues:

- When `!folio_try_get(folio)` or `folio != xas_reload(&xas)`, if the
  next `xas_for_each()` iteration happens to terminate, pfn will be
  incorrect, which is inconsistent with previous versions.
- when the `xas_for_each()` iteration to terminate and the folio
  operation preceding is normal. but pfn will be incorrect, also same
  issue.

To keep it simple, I modified it as follows. Is this OK?

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;
 }

--
Cheers,
Vernon

>
> > >  			break;
> > >  		}
> > >
> > >  		node = folio_nid(folio);
> > >  		if (collapse_scan_abort(node, cc)) {
> > >  			result = SCAN_SCAN_ABORT;
> > > -			folio_put(folio);
> > >  			break;
> > >  		}
> > >  		cc->node_load[node]++;
> > >
> > >  		if (!folio_test_lru(folio)) {
> > >  			result = SCAN_PAGE_LRU;
> > > -			folio_put(folio);
> > >  			break;
> > >  		}
> > >
> > >  		if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
> > >  			result = SCAN_PAGE_COUNT;
> > > -			folio_put(folio);
> > >  			break;
> > >  		}
> > >
> > > @@ -2759,7 +2756,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> > >  			cond_resched_rcu();
> > >  		}
> > >  	}
> > > +	if (!folio || xa_is_value(folio)) {
> > > +		pfn = -1;
> > > +	} else {
> > > +		pfn = folio_pfn(folio);
> > > +		folio_put(folio);
> > > +	}
> >
> > Hmm kinda hate this you're now mixing clean up logic with some trace logic.
> >
> > You have to go check out xas_for_each() and see that xas_next_entry() sets folio
> > to NULL at the end for the normal case to know this is OK and then figure out
> > that this is just for the break cases.
> >
> > That's not intuitive in already not intuitive code.
> >
> > And this is a patch you're wanting to backport to fix a bug too... :)
> >
> > I think you should keep the folio_put()'s where they are for now and limit this
> > fix to the trace code please.
> >
> > I think you should drop the pfn var altogether and do this far more simply, see
> > below.
> >
> > >  	rcu_read_unlock();
> > > +
> >
> > Stray newline added, let's not do that for a backported fix please :)
> >
> > >  	if (result == SCAN_PTE_MAPPED_HUGEPAGE)
> > >  		cc->progress++;
> > >  	else
> > > @@ -2774,7 +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, pfn, file, present, swap, result);
> >
> > I mean maybe better to just reduce this to something like:
> >
> > 	/* folio is NULL unless exited early. */
> > 	trace_mm_khugepaged_scan_file(mm,
> > 			(!folio || xa_is_value(folio)) ? -1 : folio_pfn(folio),
> > 			file, present, swap, result);
> >
> > As the least invasive way of fixing the trace + limiting the contageon?
>
> Yeah as above this isn't right.
>
> >
> > >  	return result;
> > >  }
> > >
> > > --
> > > 2.53.0
> > >
> >
> > --
> > Cheers, Lorenzo
>
> --
> Cheers, Lorenzo
>