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

From: Vernon Yang <redacted>

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/ (local)

Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
Cc: stable@vger.kernel.org
Signed-off-by: Vernon Yang <redacted>
---
 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);
 	}