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

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 meaningless warning.

Link: https://lkml.kernel.org/r/20250429094803.85518-1-ryncsn@gmail.com
Fixes: 0ff67f9 ("mm, swap: remove swap slot cache")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Closes: https://lore.kernel.org/linux-mm/20250428135252.25453B17-hca@linux.ibm.com/
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ryncsn

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);