[PATCH] mm: khugepaged: don't pass swap entry value to trace_mm_khugepaged_scan_file()

[Vernon Yang <vernon2gm@gmail.com>]

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 set folio to NULL before breaking out, the tracepoint maps NULL to
scan_pfn of -1, just like exhausted scan naturally.

Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
Cc: stable@vger.kernel.org
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 mm/khugepaged.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 617bca76db49..bc0d04c9162d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2696,6 +2696,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		if (xa_is_value(folio)) {
 			swap += 1 << xas_get_order(&xas);
 			if (swap > max_ptes_swap) {
+				folio = NULL;
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;

base-commit: 075b74841bd0065a3bda3440873c747938e69b68
-- 
2.53.0