Re: [PATCH] ext4: do not try to write superblock on journal-less readonly remount

[Theodore Ts'o <tytso@mit.edu>]

OK, I'll take this; I'm going to take your suggestion and only call
ext4_commit_super() when we are going from a read-write to read-only
mount, since that's the only time when we need to force that the
superblock be written out.

					- Ted

commit 86fffe43a74af16e306896eba8210bebd33bfd1f
Author: Michael Tokarev <mjt@tls.msk.ru>
Date:   Tue Dec 25 14:08:16 2012 -0500

    ext4: do not try to write superblock on ro remount w/o journal
    
    When a journal-less ext4 filesystem is mounted on a read-only block
    device (blockdev --setro will do), each remount (for other, unrelated,
    flags, like suid=>nosuid etc) results in a series of scary messages
    from kernel telling about I/O errors on the device.
    
    This is becauese of the following code ext4_remount():
    
           if (sbi->s_journal == NULL)
                    ext4_commit_super(sb, 1);
    
    at the end of remount procedure, which forces writing (flushing) of
    a superblock regardless whenever it is dirty or not, if the filesystem
    is readonly or not, and whenever the device itself is readonly or not.
    
    We only need call ext4_commit_super when the file system had been
    previously mounted read/write.
    
    Thanks to Eric Sandeen for help in diagnosing this issue.
    
    Cc: stable@vger.kernel.org
    Signed-off-By: Michael Tokarev <mjt@tls.msk.ru>
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 4969167..183ae34 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4729,7 +4729,7 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 	}
 
 	ext4_setup_system_zone(sb);
-	if (sbi->s_journal == NULL)
+	if (sbi->s_journal == NULL && !(old_sb_flags & MS_RDONLY))
 		ext4_commit_super(sb, 1);
 
 #ifdef CONFIG_QUOTA