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Subject[PATCH 1/3] mm: Don't blindly assign fallback_migrate_page()
Date
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@nod.at>
---
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
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