[merged mm-hotfixes-stable] mm-swap-dont-free-a-hibernation-slot-that-is-in-the-swap-cache.patch removed from -mm tree

The quilt patch titled
     Subject: mm, swap: don't free a hibernation slot that is in the swap cache
has been removed from the -mm tree.  Its filename was
     mm-swap-dont-free-a-hibernation-slot-that-is-in-the-swap-cache.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Youngjun Park <[email protected]>
Subject: mm, swap: don't free a hibernation slot that is in the swap cache
Date: Tue, 11 Aug 2026 22:22:06 +0900

A slot with a folio in the swap cache is freed when the folio leaves the
cache, not when its count drops.  swap_put_entries_cluster() follows that
rule.  swap_free_hibernation_slot() does not, it calls
__swap_cluster_free_entries() whether or not a folio sits on the slot.

Cluster readahead can put one there.  It walks a raw page_cluster sized
window of offsets around the faulting entry, and a hibernation slot passes
__swap_cache_add_check() because it is not a folio and its count is not
zero.  Freeing the slot then clears the entry under that folio.

The folio is now unreachable from the swap table, and the offset goes back
to the allocator.  The folio is still on the LRU though, so reclaim can
pick it up later.  It then takes the old offset out of folio->swap and
overwrites the table entry there, which by then may belong to someone
else.

This bug can trigger silent memory corruption, process crashes, or data
instability across completely unrelated userspace applications - typically
occurring when uswsusp is preparing the hibernation image.

I found this while working on giving hibernation slots their own marker in
the swap table, which I had discussed with Kairui. 
(https://lore.kernel.org/linux-mm/abp7aDgYLrxF3Me8@KASONG-MC4/) As far as
I know there are no reports, so there is no Reported-by/Closes to add.

Check for a cached folio before freeing.  The slot is then left in the
ordinary state where only the swap cache holds it, and it is freed when
the folio leaves the cache, either through the reclaim below or through
normal reclaim later.

Link: https://lore.kernel.org/[email protected]
Fixes: 0d6af9bcf383 ("mm, swap: use the swap table to track the swap count")
Signed-off-by: Youngjun Park <[email protected]>
Acked-by: Kairui Song <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Kemeng Shi <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/swapfile.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/mm/swapfile.c~mm-swap-dont-free-a-hibernation-slot-that-is-in-the-swap-cache
+++ a/mm/swapfile.c
@@ -2196,7 +2196,14 @@ void swap_free_hibernation_slot(swp_entr
 
 	ci = swap_cluster_lock(si, offset);
 	__swap_cluster_put_entry(ci, offset % SWAPFILE_CLUSTER);
-	__swap_cluster_free_entries(si, ci, offset % SWAPFILE_CLUSTER, 1);
+	/*
+	 * A slot with a folio in the swap cache is freed when the folio
+	 * leaves the cache, the same rule swap_put_entries_cluster() follows.
+	 * Readahead can put a folio here, and freeing the slot now would
+	 * leave that folio with no entry behind it.
+	 */
+	if (!swp_tb_is_folio(__swap_table_get(ci, offset % SWAPFILE_CLUSTER)))
+		__swap_cluster_free_entries(si, ci, offset % SWAPFILE_CLUSTER, 1);
 	swap_cluster_unlock(ci);
 
 	/* In theory readahead might add it to the swap cache by accident */
_

Patches currently in -mm which might be from [email protected] are