mm, swap: wrap swap cache replacement with a helper

There are currently three swap cache users that are trying to replace an
existing folio with a new one: huge memory splitting, migration, and shmem
replacement.  What they are doing is quite similar.

Introduce a common helper for this.  In later commits, this can be easily
switched to use the swap table by updating this helper.

The newly added helper also makes the swap cache API better defined, and
make debugging easier by adding a few more debug checks.

Migration and shmem replace are meant to clone the folio, including
content, swap entry value, and flags.  And splitting will adjust each sub
folio's swap entry according to order, which could be non-uniform in the
future.  So document it clearly that it's the caller's responsibility to
set up the new folio's swap entries and flags before calling the helper. 
The helper will just follow the new folio's entry value.

This also prepares for replacing high-order folios in the swap cache. 
Currently, only splitting to order 0 is allowed for swap cache folios. 
Using the new helper, we can handle high-order folio splitting better.

Link: https://lkml.kernel.org/r/20250916160100.31545-11-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Chris Li <chrisl@kernel.org>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index f3a32a06a950..d1f5b8fa52fc 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -235,6 +235,39 @@ void swap_cache_del_folio(struct folio *folio)
 }
 
 /**
+ * __swap_cache_replace_folio - Replace a folio in the swap cache.
+ * @old: The old folio to be replaced.
+ * @new: The new folio.
+ *
+ * Replace an existing folio in the swap cache with a new folio. The
+ * caller is responsible for setting up the new folio's flag and swap
+ * entries. Replacement will take the new folio's swap entry value as
+ * the starting offset to override all slots covered by the new folio.
+ *
+ * Context: Caller must ensure both folios are locked, also lock the
+ * swap address_space that holds the old folio to avoid races.
+ */
+void __swap_cache_replace_folio(struct folio *old, struct folio *new)
+{
+	swp_entry_t entry = new->swap;
+	unsigned long nr_pages = folio_nr_pages(new);
+	unsigned long offset = swap_cache_index(entry);
+	unsigned long end = offset + nr_pages;
+
+	XA_STATE(xas, &swap_address_space(entry)->i_pages, offset);
+
+	VM_WARN_ON_ONCE(!folio_test_swapcache(old) || !folio_test_swapcache(new));
+	VM_WARN_ON_ONCE(!folio_test_locked(old) || !folio_test_locked(new));
+	VM_WARN_ON_ONCE(!entry.val);
+
+	/* Swap cache still stores N entries instead of a high-order entry */
+	do {
+		WARN_ON_ONCE(xas_store(&xas, new) != old);
+		xas_next(&xas);
+	} while (++offset < end);
+}
+
+/**
  * swap_cache_clear_shadow - Clears a set of shadows in the swap cache.
  * @type: Indicates the swap device.
  * @begin: Beginning offset of the range.