+ mm-khugepaged-fix-race-with-folio-split-free-using-temporary-reference.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/khugepaged: fix race with folio split/free using temporary reference
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-khugepaged-fix-race-with-folio-split-free-using-temporary-reference.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-khugepaged-fix-race-with-folio-split-free-using-temporary-reference.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Shivank Garg <shivankg@amd.com>
Subject: mm/khugepaged: fix race with folio split/free using temporary reference
Date: Mon, 26 May 2025 18:28:18 +0000

hpage_collapse_scan_file() calls is_refcount_suitable(), which in turn
calls folio_mapcount().  folio_mapcount() checks folio_test_large() before
proceeding to folio_large_mapcount(), but there is a race window where the
folio may get split/freed between these checks, triggering:

  VM_WARN_ON_FOLIO(!folio_test_large(folio), folio)

Take a temporary reference to the folio in hpage_collapse_scan_file(). 
This stabilizes the folio during refcount check and prevents incorrect
large folio detection due to concurrent split/free.  Use helper
folio_expected_ref_count() + 1 to compare with folio_ref_count() instead
of using is_refcount_suitable().

Link: https://lkml.kernel.org/r/20250526182818.37978-1-shivankg@amd.com
Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value")
Signed-off-by: Shivank Garg <shivankg@amd.com>
Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Fengwei Yin <fengwei.yin@intel.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- a/mm/khugepaged.c~mm-khugepaged-fix-race-with-folio-split-free-using-temporary-reference
+++ a/mm/khugepaged.c
@@ -2295,6 +2295,17 @@ static int hpage_collapse_scan_file(stru
 			continue;
 		}
 
+		if (!folio_try_get(folio)) {
+			xas_reset(&xas);
+			continue;
+		}
+
+		if (unlikely(folio != xas_reload(&xas))) {
+			folio_put(folio);
+			xas_reset(&xas);
+			continue;
+		}
+
 		if (folio_order(folio) == HPAGE_PMD_ORDER &&
 		    folio->index == start) {
 			/* Maybe PMD-mapped */
@@ -2305,23 +2316,27 @@ static int hpage_collapse_scan_file(stru
 			 * it's safe to skip LRU and refcount checks before
 			 * returning.
 			 */
+			folio_put(folio);
 			break;
 		}
 
 		node = folio_nid(folio);
 		if (hpage_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 (!is_refcount_suitable(folio)) {
+		if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
 			result = SCAN_PAGE_COUNT;
+			folio_put(folio);
 			break;
 		}
 
@@ -2333,6 +2348,7 @@ static int hpage_collapse_scan_file(stru
 		 */
 
 		present += folio_nr_pages(folio);
+		folio_put(folio);
 
 		if (need_resched()) {
 			xas_pause(&xas);
_

Patches currently in -mm which might be from shivankg@amd.com are

mm-khugepaged-fix-race-with-folio-split-free-using-temporary-reference.patch
mm-khugepaged-clean-up-refcount-check-using-folio_expected_ref_count.patch