The patch below does not apply to the 6.12-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y git checkout FETCH_HEAD git cherry-pick -x 595cf683519ab5a277d258a2251ee8cc7b838d6d # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025062000-pond-bobcat-914a@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 595cf683519ab5a277d258a2251ee8cc7b838d6d Mon Sep 17 00:00:00 2001 From: Shivank Garg shivankg@amd.com Date: Mon, 26 May 2025 18:28:18 +0000 Subject: [PATCH] mm/khugepaged: fix race with folio split/free using temporary reference

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

diff --git a/mm/khugepaged.c b/mm/khugepaged.c index cdf5a581368b..7731a162a1a7 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2293,6 +2293,17 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, 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 */ @@ -2303,23 +2314,27 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, * 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; }

@@ -2331,6 +2346,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, */

present += folio_nr_pages(folio); + folio_put(folio);

if (need_resched()) { xas_pause(&xas);