[PATCH 4/6] mm: migrate: Provide buffer_migrate_page_norefs()

Jan Kara
2018-12-11T17:21:51Z

Provide a variant of buffer_migrate_page() that also checks whether
there are no unexpected references to buffer heads. This function will
then be safe to use for block device pages.

Signed-off-by: Jan Kara <jack@suse.cz>
---
include/linux/fs.h | 4 ++++
mm/migrate.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++-------
2 files changed, 58 insertions(+), 7 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c95c0807471f..4bb1a8b65474 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3264,8 +3264,12 @@ extern int generic_check_addressable(unsigned, u64);
extern int buffer_migrate_page(struct address_space *,
struct page *, struct page *,
enum migrate_mode);
+extern int buffer_migrate_page_norefs(struct address_space *,
+ struct page *, struct page *,
+ enum migrate_mode);
#else
#define buffer_migrate_page NULL
+#define buffer_migrate_page_norefs NULL
#endif

extern int setattr_prepare(struct dentry *, struct iattr *);
diff --git a/mm/migrate.c b/mm/migrate.c
index f8df1ad6e7cf..c4075d5ec073 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -747,13 +747,9 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
return true;
}

-/*
- * Migration function for pages with buffers. This function can only be used
- * if the underlying filesystem guarantees that no other references to "page"
- * exist.
- */
-int buffer_migrate_page(struct address_space *mapping,
- struct page *newpage, struct page *page, enum migrate_mode mode)
+static int __buffer_migrate_page(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode,
+ bool check_refs)
{
struct buffer_head *bh, *head;
int rc;
@@ -771,6 +767,33 @@ int buffer_migrate_page(struct address_space *mapping,
if (!buffer_migrate_lock_buffers(head, mode))
return -EAGAIN;

+ if (check_refs) {
+ bool busy;
+ bool invalidated = false;
+
+recheck_buffers:
+ busy = false;
+ spin_lock(&mapping->private_lock);
+ bh = head;
+ do {
+ if (atomic_read(&bh->b_count)) {
+ busy = true;
+ break;
+ }
+ bh = bh->b_this_page;
+ } while (bh != head);
+ spin_unlock(&mapping->private_lock);
+ if (busy) {
+ if (invalidated) {
+ rc = -EAGAIN;
+ goto unlock_buffers;
+ }
+ invalidate_bh_lrus();
+ invalidated = true;
+ goto recheck_buffers;
+ }
+ }
+
rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
if (rc != MIGRATEPAGE_SUCCESS)
goto unlock_buffers;
@@ -807,7 +830,31 @@ int buffer_migrate_page(struct address_space *mapping,

return rc;
}
+
+/*
+ * Migration function for pages with buffers. This function can only be used
+ * if the underlying filesystem guarantees that no other references to "page"
+ * exist. For example attached buffer heads are accessed only under page lock.
+ */
+int buffer_migrate_page(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+ return __buffer_migrate_page(mapping, newpage, page, mode, false);
+}
EXPORT_SYMBOL(buffer_migrate_page);
+
+/*
+ * Same as above except that this variant is more careful and checks that there
+ * are also no buffer head references. This function is the right one for
+ * mappings where buffer heads are directly looked up and referenced (such as
+ * block device mappings).
+ */
+int buffer_migrate_page_norefs(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+ return __buffer_migrate_page(mapping, newpage, page, mode, true);
+}
+EXPORT_SYMBOL(buffer_migrate_page_norefs);
#endif

/*
--
2.16.4