[PATCH] Add a sync_fs super_block operation

This is infrastructure for fixing the journalled-data ext3 unmount data
loss problem. It was sent for comment to linux-fsdevel a week ago; there
was none.

Add a `sync_fs' superblock operation whose mandate is to perform
filesystem-specific operations to ensure a successful sync.

It is called in two places:

1: fsync_super() - for umount.

2: sys_sync() - for global sync.

In the sys_sync() case we call all the ->write_super() methods first.
write_super() is an async flushing operation.  It should not block.

After that, we call all the ->sync_fs functions.  This is independent
of the state of s_dirt!  That was all confused up before, and in this
patch ->write_super() and ->sync_fs() are quite separate.

With ext3 as an example, the initial ->write_super() will start a
transaction, but will not wait on it.  (But only if s_dirt was set!)

The first ->sync_fs() call will get the IO underway.

The second ->sync_fs() call will wait on the IO.

And we really do need to be this elaborate, because all the testing of
s_dirt in there makes ->write_super() an unreliable way of detecting
when the VFS is trying to sync the filesystem.

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f39d21e5bcd9..afbb9474f25b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -631,6 +631,7 @@ struct super_block {
 	struct semaphore	s_lock;
 	int			s_count;
 	int			s_syncing;
+	int			s_need_sync_fs;
 	atomic_t		s_active;
 	void                    *s_security;
 
@@ -810,6 +811,7 @@ struct super_operations {
 	void (*delete_inode) (struct inode *);
 	void (*put_super) (struct super_block *);
 	void (*write_super) (struct super_block *);
+	int (*sync_fs)(struct super_block *sb, int wait);
 	void (*write_super_lockfs) (struct super_block *);
 	void (*unlockfs) (struct super_block *);
 	int (*statfs) (struct super_block *, struct statfs *);
@@ -1143,6 +1145,7 @@ extern void write_inode_now(struct inode *, int);
 extern int filemap_fdatawrite(struct address_space *);
 extern int filemap_fdatawait(struct address_space *);
 extern void sync_supers(void);
+extern void sync_filesystems(int wait);
 extern sector_t bmap(struct inode *, sector_t);
 extern int setattr_mask(unsigned int);
 extern int notify_change(struct dentry *, struct iattr *);