[PATCH] mm, swap: fix false warning for large allocation with !THP_SWAP

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

The !CONFIG_THP_SWAP check existed before just fine because slot
cache would reject high order allocation and let the caller split
all folios and try again.

But slot cache is gone, so large allocation will directly go
to the allocator, and the allocator should just fail silently
to inform caller to do the folio split, this is totally fine
and expected.

Remove this meaning less warning.

Fixes: 0ff67f990bd4 ("mm, swap: remove swap slot cache")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Closes: https://lore.kernel.org/linux-mm/20250428135252.25453B17-hca@linux.ibm.com/
Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/swapfile.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index e727021b8e2c..b86637cfb17a 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1272,13 +1272,22 @@ int folio_alloc_swap(struct folio *folio, gfp_t gfp)
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(!folio_test_uptodate(folio), folio);
 
-	/*
-	 * Should not even be attempting large allocations when huge
-	 * page swap is disabled. Warn and fail the allocation.
-	 */
-	if (order && (!IS_ENABLED(CONFIG_THP_SWAP) || size > SWAPFILE_CLUSTER)) {
-		VM_WARN_ON_ONCE(1);
-		return -EINVAL;
+	if (order) {
+		/*
+		 * Reject large allocation when THP_SWAP is disabled,
+		 * the caller should split the folio and try again.
+		 */
+		if (!IS_ENABLED(CONFIG_THP_SWAP))
+			return -EAGAIN;
+
+		/*
+		 * Allocation size should never exceed cluster size
+		 * (HPAGE_PMD_SIZE).
+		 */
+		if (size > SWAPFILE_CLUSTER) {
+			VM_WARN_ON_ONCE(1);
+			return -EINVAL;
+		}
 	}
 
 	local_lock(&percpu_swap_cluster.lock);
-- 
2.49.0