[patch] fs: add a remount,ro coherency point

[Nick Piggin <npiggin@suse.de>]

fs: invalidate sb->s_bdev on remount,ro

Fixes a problem reported by "Jorge Boncompte [DTI2]" <jorge@dti2.net>
who is trying to snapshot a minix filesystem image.

Signed-off-by: Nick Piggin <npiggin@suse.de>

---
 fs/super.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c
+++ linux-2.6/fs/super.c
@@ -647,6 +647,14 @@ int do_remount_sb(struct super_block *sb
 		acct_auto_close(sb);
 	shrink_dcache_sb(sb);
 	fsync_super(sb);
+	 /* Some filesystems modify their metadata via some other path
+	    than the bdev buffer cache (eg. use a private mapping, or
+	    directories in pagecache, etc). Also file data modifications
+	    go via their own mappings. So If we try to mount readonly
+	    then copy the filesystem from bdev, we could get stale data,
+	    so invalidate it to give a best effort at coherency. */
+	if (flags & MS_RDONLY && sb->s_bdev)
+		invalidate_bdev(sb->s_bdev);
 
 	/* If we are remounting RDONLY and current sb is read/write,
 	   make sure there are no rw files opened */