[patch] fs: improve remount,ro coherency

[Nick Piggin <npiggin@suse.de>]

No feedback on this, and no merge, so resending.
--
fs: improve remount,ro coherency

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. Added comments
explain the problem.

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

---
 fs/super.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c
+++ linux-2.6/fs/super.c
@@ -637,7 +637,7 @@ retry:
 int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 {
 	int retval;
-	int remount_rw;
+	int remount_rw, remount_ro;
 	
 #ifdef CONFIG_BLOCK
 	if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
@@ -648,9 +648,12 @@ int do_remount_sb(struct super_block *sb
 	shrink_dcache_sb(sb);
 	fsync_super(sb);
 
+	remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);
+	remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
+
 	/* If we are remounting RDONLY and current sb is read/write,
 	   make sure there are no rw files opened */
-	if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {
+	if (remount_ro) {
 		if (force)
 			mark_files_ro(sb);
 		else if (!fs_may_remount_ro(sb))
@@ -659,7 +662,6 @@ int do_remount_sb(struct super_block *sb
 		if (retval < 0 && retval != -ENOSYS)
 			return -EBUSY;
 	}
-	remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
 
 	if (sb->s_op->remount_fs) {
 		lock_super(sb);
@@ -671,6 +673,14 @@ int do_remount_sb(struct super_block *sb
 	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
 	if (remount_rw)
 		DQUOT_ON_REMOUNT(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 (remount_ro && sb->s_bdev)
+		invalidate_bdev(sb->s_bdev);
 	return 0;
 }