mm/huge_memory: refactor after-split (page) cache code

Smatch/coverity checkers report NULL mapping referencing issues[1][2][3]
every time the code is modified, because they do not understand that
mapping cannot be NULL when a folio is in page cache in the code. 
Refactor the code to make it explicit.

Remove "end = -1" for anonymous folios, since after code refactoring, end
is no longer used by anonymous folio handling code.

No functional change is intended.

Link: https://lkml.kernel.org/r/20250718023000.4044406-7-ziy@nvidia.com
Link: https://lore.kernel.org/linux-mm/2afe3d59-aca5-40f7-82a3-a6d976fb0f4f@stanley.mountain/ [1]
Link: https://lore.kernel.org/oe-kbuild/64b54034-f311-4e7d-b935-c16775dbb642@suswa.mountain/ [2]
Link: https://lore.kernel.org/linux-mm/20250716145804.4836-1-antonio@mandelbit.com/ [3]
Link: https://lkml.kernel.org/r/20250718183720.4054515-7-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <k.shutemov@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: x-y-z

mm/huge_memory.c

@@ -3640,7 +3640,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 			ret = -EBUSY;
 			goto out;
 		}
-		end = -1;
 		mapping = NULL;
 		anon_vma_lock_write(anon_vma);
 	} else {
@@ -3793,32 +3792,45 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 */
 		for (new_folio = folio_next(folio); new_folio != end_folio;
 		     new_folio = next) {
+			unsigned long nr_pages = folio_nr_pages(new_folio);
+
 			next = folio_next(new_folio);
 
 			expected_refs = folio_expected_ref_count(new_folio) + 1;
 			folio_ref_unfreeze(new_folio, expected_refs);
 
 			lru_add_split_folio(folio, new_folio, lruvec, list);
 
-			/* Some pages can be beyond EOF: drop them from cache */
-			if (new_folio->index >= end) {
-				if (shmem_mapping(mapping))
-					nr_shmem_dropped += folio_nr_pages(new_folio);
-				else if (folio_test_clear_dirty(new_folio))
-					folio_account_cleaned(
-						new_folio,
-						inode_to_wb(mapping->host));
-				__filemap_remove_folio(new_folio, NULL);
-				folio_put_refs(new_folio,
-					       folio_nr_pages(new_folio));
-			} else if (mapping) {
-				__xa_store(&mapping->i_pages, new_folio->index,
-					   new_folio, 0);
-			} else if (swap_cache) {
+			/*
+			 * Anonymous folio with swap cache.
+			 * NOTE: shmem in swap cache is not supported yet.
+			 */
+			if (swap_cache) {
 				__xa_store(&swap_cache->i_pages,
 					   swap_cache_index(new_folio->swap),
 					   new_folio, 0);
+				continue;
+			}
+
+			/* Anonymous folio without swap cache */
+			if (!mapping)
+				continue;
+
+			/* Add the new folio to the page cache. */
+			if (new_folio->index < end) {
+				__xa_store(&mapping->i_pages, new_folio->index,
+					   new_folio, 0);
+				continue;
 			}
+
+			/* Drop folio beyond EOF: ->index >= end */
+			if (shmem_mapping(mapping))
+				nr_shmem_dropped += nr_pages;
+			else if (folio_test_clear_dirty(new_folio))
+				folio_account_cleaned(
+					new_folio, inode_to_wb(mapping->host));
+			__filemap_remove_folio(new_folio, NULL);
+			folio_put_refs(new_folio, nr_pages);
 		}
 		/*
 		 * Unfreeze @folio only after all page cache entries, which