[merged mm-hotfixes-stable] mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split.patch removed from -mm tree

[merged mm-hotfixes-stable] mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm/shmem, swap: fix race of truncate and swap entry split
has been removed from the -mm tree.  Its filename was
     mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split.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: Kairui Song <kasong@tencent.com>
Subject: mm/shmem, swap: fix race of truncate and swap entry split
Date: Tue, 20 Jan 2026 00:11:21 +0800

The helper for shmem swap freeing is not handling the order of swap
entries correctly.  It uses xa_cmpxchg_irq to erase the swap entry, but it
gets the entry order before that using xa_get_order without lock
protection, and it may get an outdated order value if the entry is split
or changed in other ways after the xa_get_order and before the
xa_cmpxchg_irq.

And besides, the order could grow and be larger than expected, and cause
truncation to erase data beyond the end border.  For example, if the
target entry and following entries are swapped in or freed, then a large
folio was added in place and swapped out, using the same entry, the
xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.

To fix that, open code the Xarray cmpxchg and put the order retrieval and
value checking in the same critical section.  Also, ensure the order won't
exceed the end border, skip it if the entry goes across the border.

Skipping large swap entries crosses the end border is safe here.  Shmem
truncate iterates the range twice, in the first iteration,
find_lock_entries already filtered such entries, and shmem will swapin the
entries that cross the end border and partially truncate the folio (split
the folio or at least zero part of it).  So in the second loop here, if we
see a swap entry that crosses the end order, it must at least have its
content erased already.

I observed random swapoff hangs and kernel panics when stress testing
ZSWAP with shmem.  After applying this patch, all problems are gone.

Link: https://lkml.kernel.org/r/20260120-shmem-swap-fix-v3-1-3d33ebfbc057@tencent.com
Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/shmem.c |   45 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 11 deletions(-)

--- a/mm/shmem.c~mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split
+++ a/mm/shmem.c
@@ -962,17 +962,29 @@ static void shmem_delete_from_page_cache
  * being freed).
  */
 static long shmem_free_swap(struct address_space *mapping,
-			    pgoff_t index, void *radswap)
+			    pgoff_t index, pgoff_t end, void *radswap)
 {
-	int order = xa_get_order(&mapping->i_pages, index);
-	void *old;
+	XA_STATE(xas, &mapping->i_pages, index);
+	unsigned int nr_pages = 0;
+	pgoff_t base;
+	void *entry;
+
+	xas_lock_irq(&xas);
+	entry = xas_load(&xas);
+	if (entry == radswap) {
+		nr_pages = 1 << xas_get_order(&xas);
+		base = round_down(xas.xa_index, nr_pages);
+		if (base < index || base + nr_pages - 1 > end)
+			nr_pages = 0;
+		else
+			xas_store(&xas, NULL);
+	}
+	xas_unlock_irq(&xas);
 
-	old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
-	if (old != radswap)
-		return 0;
-	free_swap_and_cache_nr(radix_to_swp_entry(radswap), 1 << order);
+	if (nr_pages)
+		free_swap_and_cache_nr(radix_to_swp_entry(radswap), nr_pages);
 
-	return 1 << order;
+	return nr_pages;
 }
 
 /*
@@ -1124,8 +1136,8 @@ static void shmem_undo_range(struct inod
 			if (xa_is_value(folio)) {
 				if (unfalloc)
 					continue;
-				nr_swaps_freed += shmem_free_swap(mapping,
-							indices[i], folio);
+				nr_swaps_freed += shmem_free_swap(mapping, indices[i],
+								  end - 1, folio);
 				continue;
 			}
 
@@ -1191,12 +1203,23 @@ whole_folios:
 			folio = fbatch.folios[i];
 
 			if (xa_is_value(folio)) {
+				int order;
 				long swaps_freed;
 
 				if (unfalloc)
 					continue;
-				swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+				swaps_freed = shmem_free_swap(mapping, indices[i],
+							      end - 1, folio);
 				if (!swaps_freed) {
+					/*
+					 * If found a large swap entry cross the end border,
+					 * skip it as the truncate_inode_partial_folio above
+					 * should have at least zerod its content once.
+					 */
+					order = shmem_confirm_swap(mapping, indices[i],
+								   radix_to_swp_entry(folio));
+					if (order > 0 && indices[i] + (1 << order) > end)
+						continue;
 					/* Swap was replaced by page: retry */
 					index = indices[i];
 					break;
_

Patches currently in -mm which might be from kasong@tencent.com are

mm-swap-rename-__read_swap_cache_async-to-swap_cache_alloc_folio.patch
mm-swap-split-swap-cache-preparation-loop-into-a-standalone-helper.patch
mm-swap-never-bypass-the-swap-cache-even-for-swp_synchronous_io.patch
mm-swap-always-try-to-free-swap-cache-for-swp_synchronous_io-devices.patch
mm-swap-simplify-the-code-and-reduce-indention.patch
mm-swap-free-the-swap-cache-after-folio-is-mapped.patch
mm-shmem-never-bypass-the-swap-cache-for-swp_synchronous_io.patch
mm-swap-swap-entry-of-a-bad-slot-should-not-be-considered-as-swapped-out.patch
mm-swap-consolidate-cluster-reclaim-and-usability-check.patch
mm-swap-split-locked-entry-duplicating-into-a-standalone-helper.patch
mm-swap-use-swap-cache-as-the-swap-in-synchronize-layer.patch
mm-swap-use-swap-cache-as-the-swap-in-synchronize-layer-fix.patch
mm-swap-remove-workaround-for-unsynchronized-swap-map-cache-state.patch
mm-swap-cleanup-swap-entry-management-workflow.patch
mm-swap-cleanup-swap-entry-management-workflow-fix.patch
mm-swap-add-folio-to-swap-cache-directly-on-allocation.patch
mm-swap-check-swap-table-directly-for-checking-cache.patch
mm-swap-clean-up-and-improve-swap-entries-freeing.patch
mm-swap-drop-the-swap_has_cache-flag.patch
mm-swap-remove-no-longer-needed-_swap_info_get.patch

[merged mm-hotfixes-stable] mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm, shmem: prevent infinite loop on truncate race
has been removed from the -mm tree.  Its filename was
     mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split.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: Kairui Song <kasong@tencent.com>
Subject: mm, shmem: prevent infinite loop on truncate race
Date: Thu, 29 Jan 2026 00:19:23 +0800

When truncating a large swap entry, shmem_free_swap() returns 0 when the
entry's index doesn't match the given index due to lookup alignment.  The
failure fallback path checks if the entry crosses the end border and
aborts when it happens, so truncate won't erase an unexpected entry or
range.  But one scenario was ignored.

When `index` points to the middle of a large swap entry, and the large
swap entry doesn't go across the end border, find_get_entries() will
return that large swap entry as the first item in the batch with
`indices[0]` equal to `index`.  The entry's base index will be smaller
than `indices[0]`, so shmem_free_swap() will fail and return 0 due to the
"base < index" check.  The code will then call shmem_confirm_swap(), get
the order, check if it crosses the END boundary (which it doesn't), and
retry with the same index.

The next iteration will find the same entry again at the same index with
same indices, leading to an infinite loop.

Fix this by retrying with a round-down index, and abort if the index is
smaller than the truncate range.

Link: https://lkml.kernel.org/r/aXo6ltB5iqAKJzY8@KASONG-MC4
Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
Fixes: 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reported-by: Chris Mason <clm@meta.com>
Closes: https://lore.kernel.org/linux-mm/20260128130336.727049-1-clm@meta.com/
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/shmem.c |   23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

--- a/mm/shmem.c~mm-shmem-swap-fix-race-of-truncate-and-swap-entry-split
+++ a/mm/shmem.c
@@ -1211,17 +1211,22 @@ whole_folios:
 				swaps_freed = shmem_free_swap(mapping, indices[i],
 							      end - 1, folio);
 				if (!swaps_freed) {
-					/*
-					 * If found a large swap entry cross the end border,
-					 * skip it as the truncate_inode_partial_folio above
-					 * should have at least zerod its content once.
-					 */
+					pgoff_t base = indices[i];
+
 					order = shmem_confirm_swap(mapping, indices[i],
 								   radix_to_swp_entry(folio));
-					if (order > 0 && indices[i] + (1 << order) > end)
-						continue;
-					/* Swap was replaced by page: retry */
-					index = indices[i];
+					/*
+					 * If found a large swap entry cross the end or start
+					 * border, skip it as the truncate_inode_partial_folio
+					 * above should have at least zerod its content once.
+					 */
+					if (order > 0) {
+						base = round_down(base, 1 << order);
+						if (base < start || base + (1 << order) > end)
+							continue;
+					}
+					/* Swap was replaced by page or extended, retry */
+					index = base;
 					break;
 				}
 				nr_swaps_freed += swaps_freed;
_

Patches currently in -mm which might be from kasong@tencent.com are