mm: migrate: split folio_migrate_mapping()

The folio refcount check is moved out for both !mapping and mapping folio,
also update comment from page to folio for folio_migrate_mapping().

No functional change intended.

Link: https://lkml.kernel.org/r/20240626085328.608006-4-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -393,28 +393,23 @@ static int folio_expected_refs(struct address_space *mapping,
 }
 
 /*
- * Replace the page in the mapping.
+ * Replace the folio in the mapping.
  *
  * The number of remaining references must be:
- * 1 for anonymous pages without a mapping
- * 2 for pages with a mapping
- * 3 for pages with a mapping and PagePrivate/PagePrivate2 set.
+ * 1 for anonymous folios without a mapping
+ * 2 for folios with a mapping
+ * 3 for folios with a mapping and PagePrivate/PagePrivate2 set.
  */
-int folio_migrate_mapping(struct address_space *mapping,
-		struct folio *newfolio, struct folio *folio, int extra_count)
+static int __folio_migrate_mapping(struct address_space *mapping,
+		struct folio *newfolio, struct folio *folio, int expected_count)
 {
 	XA_STATE(xas, &mapping->i_pages, folio_index(folio));
 	struct zone *oldzone, *newzone;
 	int dirty;
-	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
 	long nr = folio_nr_pages(folio);
 	long entries, i;
 
 	if (!mapping) {
-		/* Anonymous page without mapping */
-		if (folio_ref_count(folio) != expected_count)
-			return -EAGAIN;
-
 		/* Take off deferred split queue while frozen and memcg set */
 		if (folio_test_large(folio) &&
 		    folio_test_large_rmappable(folio)) {
@@ -465,7 +460,7 @@ int folio_migrate_mapping(struct address_space *mapping,
 		entries = 1;
 	}
 
-	/* Move dirty while page refs frozen and newpage not yet exposed */
+	/* Move dirty while folio refs frozen and newfolio not yet exposed */
 	dirty = folio_test_dirty(folio);
 	if (dirty) {
 		folio_clear_dirty(folio);
@@ -479,7 +474,7 @@ int folio_migrate_mapping(struct address_space *mapping,
 	}
 
 	/*
-	 * Drop cache reference from old page by unfreezing
+	 * Drop cache reference from old folio by unfreezing
 	 * to one less reference.
 	 * We know this isn't the last reference.
 	 */
@@ -490,11 +485,11 @@ int folio_migrate_mapping(struct address_space *mapping,
 
 	/*
 	 * If moved to a different zone then also account
-	 * the page for that zone. Other VM counters will be
+	 * the folio for that zone. Other VM counters will be
 	 * taken care of when we establish references to the
-	 * new page and drop references to the old page.
+	 * new folio and drop references to the old folio.
 	 *
-	 * Note that anonymous pages are accounted for
+	 * Note that anonymous folios are accounted for
 	 * via NR_FILE_PAGES and NR_ANON_MAPPED if they
 	 * are mapped to swap space.
 	 */
@@ -534,6 +529,17 @@ int folio_migrate_mapping(struct address_space *mapping,
 
 	return MIGRATEPAGE_SUCCESS;
 }
+
+int folio_migrate_mapping(struct address_space *mapping,
+		struct folio *newfolio, struct folio *folio, int extra_count)
+{
+	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
+
+	if (folio_ref_count(folio) != expected_count)
+		return -EAGAIN;
+
+	return __folio_migrate_mapping(mapping, newfolio, folio, expected_count);
+}
 EXPORT_SYMBOL(folio_migrate_mapping);
 
 /*