mm/shmem, swap: simplify swapin path and result handling

Slightly tidy up the different handling of swap in and error handling for
SWP_SYNCHRONOUS_IO and non-SWP_SYNCHRONOUS_IO devices.  Now swapin will
always use either shmem_swap_alloc_folio or shmem_swapin_cluster, then
check the result.

Simplify the control flow and avoid a redundant goto label.

Link: https://lkml.kernel.org/r/20250728075306.12704-7-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.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: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ryncsn

mm/shmem.c

@@ -2357,40 +2357,33 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			count_memcg_event_mm(fault_mm, PGMAJFAULT);
 		}
 
-		/* Skip swapcache for synchronous device. */
 		if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
+			/* Direct swapin skipping swap cache & readahead */
 			folio = shmem_swap_alloc_folio(inode, vma, index, swap, order, gfp);
-			if (!IS_ERR(folio)) {
-				skip_swapcache = true;
-				goto alloced;
+			if (IS_ERR(folio)) {
+				error = PTR_ERR(folio);
+				folio = NULL;
+				goto failed;
 			}
-
+			skip_swapcache = true;
+		} else {
 			/*
-			 * Direct swapin handled order 0 fallback already,
-			 * if it failed, abort.
+			 * Cached swapin only supports order 0 folio, it is
+			 * necessary to recalculate the new swap entry based on
+			 * the offset, as the swapin index might be unalgined.
 			 */
-			error = PTR_ERR(folio);
-			folio = NULL;
-			goto failed;
-		}
-
-		/*
-		 * Now swap device can only swap in order 0 folio, it is
-		 * necessary to recalculate the new swap entry based on
-		 * the offset, as the swapin index might be unalgined.
-		 */
-		if (order) {
-			offset = index - round_down(index, 1 << order);
-			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
-		}
+			if (order) {
+				offset = index - round_down(index, 1 << order);
+				swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
+			}
 
-		folio = shmem_swapin_cluster(swap, gfp, info, index);
-		if (!folio) {
-			error = -ENOMEM;
-			goto failed;
+			folio = shmem_swapin_cluster(swap, gfp, info, index);
+			if (!folio) {
+				error = -ENOMEM;
+				goto failed;
+			}
 		}
 	}
-alloced:
 	if (order > folio_order(folio)) {
 		/*
 		 * Swapin may get smaller folios due to various reasons: