[PATCH 5/5] mm/huge_memory: turn the swapcache-with-mapping error case into an assert

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

A folio in the swapcache has no mapping.

__folio_freeze_and_split_unmapped() tests the combination after the freeze
and returns -EINVAL. It leaves the folio frozen. It is not a real
recovery.

Assert the condition with VM_WARN_ON_ONCE_FOLIO() upfront, and do not
pretend to recover from an impossible state.

PG_swapcache is protected by the folio lock and stable from the start of
the function.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/huge_memory.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 001d89c57fa2..8fc844ba67a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3936,6 +3936,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 	int ret = 0;
 
 	VM_WARN_ON_ONCE(!mapping && end);
+	VM_WARN_ON_ONCE_FOLIO(mapping && folio_test_swapcache(folio), folio);
 
 	if (!folio_ref_freeze(folio, folio_cache_ref_count(folio) + 1))
 		return -EAGAIN;
@@ -3965,14 +3966,8 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 			lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
 	}
 
-	if (folio_test_swapcache(folio)) {
-		if (mapping) {
-			VM_WARN_ON_ONCE_FOLIO(mapping, folio);
-			return -EINVAL;
-		}
-
+	if (folio_test_swapcache(folio))
 		ci = swap_cluster_get_and_lock(folio);
-	}
 
 	/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
 	if (do_lru)
-- 
2.54.0