[PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()

[Richard Weinberger]

While block oriented filesystems use buffer_migrate_page()
as page migration function other filesystems which don't
implement ->migratepage() will automatically get fallback_migrate_page()
assigned. fallback_migrate_page() is not as generic as is should
be. Page migration is filesystem specific and a one-fits-all function
is hard to achieve. UBIFS leaned this lection the hard way.
It uses various page flags and fallback_migrate_page() does not
handle these flags as UBIFS expected.

To make sure that no further filesystem will get confused by
fallback_migrate_page() disable the automatic assignment and
allow filesystems to use this function explicitly if it is
really suitable.

Signed-off-by: Richard Weinberger <richard@xxxxxx>
---
 include/linux/migrate.h |  9 +++++++++
 mm/migrate.c            | 16 ++++++++++++----
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 9b50325..aba86d4 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -47,6 +47,9 @@ extern int migrate_page_move_mapping(struct address_space *mapping,
 		struct page *newpage, struct page *page,
 		struct buffer_head *head, enum migrate_mode mode,
 		int extra_count);
+extern int generic_migrate_page(struct address_space *mapping,
+				struct page *newpage, struct page *page,
+				enum migrate_mode mode);
 #else
 
 static inline void putback_movable_pages(struct list_head *l) {}
@@ -67,6 +70,12 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
 	return -ENOSYS;
 }
 
+static inline int generic_migrate_page(struct address_space *mapping,
+				       struct page *newpage, struct page *page,
+				       enum migrate_mode mode)
+{
+	return -ENOSYS;
+}
 #endif /* CONFIG_MIGRATION */
 
 #ifdef CONFIG_NUMA_BALANCING
diff --git a/mm/migrate.c b/mm/migrate.c
index 9baf41c..5129143 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -719,8 +719,9 @@ static int writeout(struct address_space *mapping, struct page *page)
 /*
  * Default handling if a filesystem does not provide a migration function.
  */
-static int fallback_migrate_page(struct address_space *mapping,
-	struct page *newpage, struct page *page, enum migrate_mode mode)
+int generic_migrate_page(struct address_space *mapping,
+			 struct page *newpage, struct page *page,
+			 enum migrate_mode mode)
 {
 	if (PageDirty(page)) {
 		/* Only writeback pages in full synchronous migration */
@@ -771,8 +772,15 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 		 * is the most common path for page migration.
 		 */
 		rc = mapping->a_ops->migratepage(mapping, newpage, page, mode);
-	else
-		rc = fallback_migrate_page(mapping, newpage, page, mode);
+	else {
+		/*
+		 * Dear filesystem maintainer, please verify whether
+		 * generic_migrate_page() is suitable for your
+		 * filesystem, especially wrt. page flag handling.
+		 */
+		WARN_ON_ONCE(1);
+		rc = -EINVAL;
+	}
 
 	/*
 	 * When successful, old pagecache page->mapping must be cleared before
-- 
2.7.3