[PATCH rfcv2 06/11] mm: migrate: split folio_migrate_mapping()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

The folio_migrate_mapping() function is splitted into two parts,
folio_refs_check_and_freeze() and folio_replace_mapping_and_unfreeze(),
also update comment from page to folio.

Note, the folio_ref_freeze() is moved out of xas_lock_irq(), since the
folio is already isolated and locked during migration, so suppose that
there is no functional change.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 mm/migrate.c | 74 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 42 insertions(+), 32 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 2dcd0d422056..1db93b5eb819 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -390,50 +390,49 @@ static int folio_expected_refs(struct address_space *mapping,
 }
 
 /*
- * Replace the page 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_refs_check_and_freeze(struct address_space *mapping,
+				       struct folio *folio, int expected_cnt)
+{
+	if (!mapping) {
+		if (folio_ref_count(folio) != expected_cnt)
+			return -EAGAIN;
+	} else {
+		if (!folio_ref_freeze(folio, expected_cnt))
+			return -EAGAIN;
+	}
+
+	return 0;
+}
+
+/* The folio refcount must be freezed if folio with a mapping */
+static void folio_replace_mapping_and_unfreeze(struct address_space *mapping,
+		struct folio *newfolio, struct folio *folio, int expected_cnt)
 {
 	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;
+	int dirty;
 
 	if (!mapping) {
-		/* Anonymous page without mapping */
-		if (folio_ref_count(folio) != expected_count)
-			return -EAGAIN;
-
-		/* No turning back from here */
+		/* Anonymous folio without mapping */
 		newfolio->index = folio->index;
 		newfolio->mapping = folio->mapping;
 		if (folio_test_swapbacked(folio))
 			__folio_set_swapbacked(newfolio);
-
-		return MIGRATEPAGE_SUCCESS;
+		return;
 	}
 
 	oldzone = folio_zone(folio);
 	newzone = folio_zone(newfolio);
 
+	/* Now we know that no one else is looking at the folio */
 	xas_lock_irq(&xas);
-	if (!folio_ref_freeze(folio, expected_count)) {
-		xas_unlock_irq(&xas);
-		return -EAGAIN;
-	}
-
-	/*
-	 * Now we know that no one else is looking at the folio:
-	 * no turning back from here.
-	 */
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
 	folio_ref_add(newfolio, nr); /* add cache reference */
@@ -449,7 +448,7 @@ 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);
@@ -463,22 +462,22 @@ int folio_migrate_mapping(struct address_space *mapping,
 	}
 
 	/*
-	 * Drop cache reference from old page by unfreezing
-	 * to one less reference.
+	 * Since old folio's refcount freezed, now drop cache reference from
+	 * old folio by unfreezing to one less reference.
 	 * We know this isn't the last reference.
 	 */
-	folio_ref_unfreeze(folio, expected_count - nr);
+	folio_ref_unfreeze(folio, expected_cnt - nr);
 
 	xas_unlock(&xas);
 	/* Leave irq disabled to prevent preemption while updating stats */
 
 	/*
 	 * 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.
 	 */
@@ -515,7 +514,18 @@ int folio_migrate_mapping(struct address_space *mapping,
 		}
 	}
 	local_irq_enable();
+}
+
+int folio_migrate_mapping(struct address_space *mapping, struct folio *newfolio,
+			  struct folio *folio, int extra_count)
+{
+	int ret, expected = folio_expected_refs(mapping, folio) + extra_count;
+
+	ret = folio_refs_check_and_freeze(mapping, folio, expected);
+	if (ret)
+		return ret;
 
+	folio_replace_mapping_and_unfreeze(mapping, newfolio, folio, expected);
 	return MIGRATEPAGE_SUCCESS;
 }
 EXPORT_SYMBOL(folio_migrate_mapping);
-- 
2.27.0