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...

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