ext4: remove unnecessary superblock dirtying

This patch changes the 'ext4_handle_dirty_super()' function which
submits the superblock for I/O in the following cases:

1. When creating the first large file on a file system without
   EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature.
2. When re-sizing the file-system.
3. When creating an xattr on a file-system without the
   EXT4_FEATURE_COMPAT_EXT_ATTR feature.

If the file-system has journal enabled, the superblock is written via
the journal. We do not modify this path.

If the file-system has no journal, this function, falls back to just
marking the superblock as dirty using the 's_dirt' superblock
flag. This means that it delays the actual superblock I/O submission
by 5 seconds (default setting).  Namely, the 'sync_supers()' kernel
thread will call 'ext4_write_super()' later and will actually submit
the superblock for I/O.

And this is the behavior this patch modifies: we stop using 's_dirt'
and just mark the superblock buffer as dirty right away. Indeed, all 3
cases above are extremely rare and it does not add any value to delay
the I/O submission for them.

Note: 'ext4_handle_dirty_super()' executes
'__ext4_handle_dirty_super()' with 'now = 0'. This patch basically
makes the 'now' argument unneeded and it will be deleted in one of the
next patches.

This patch also removes 's_dirt' condition on the unmount path because
we never set it anymore, so we should not test it.

Tested using xfstests for both journalled and non-journalled ext4.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/ext4_jbd2.c

@@ -151,11 +151,10 @@ int __ext4_handle_dirty_super(const char *where, unsigned int line,
 		if (err)
 			ext4_journal_abort_handle(where, line, __func__,
 						  bh, handle, err);
-	} else if (now) {
+	} else {
 		ext4_superblock_csum_set(sb,
 				(struct ext4_super_block *)bh->b_data);
 		mark_buffer_dirty(bh);
-	} else
-		sb->s_dirt = 1;
+	}
 	return err;
 }

fs/ext4/super.c

@@ -896,7 +896,7 @@ static void ext4_put_super(struct super_block *sb)
 		EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER);
 		es->s_state = cpu_to_le16(sbi->s_mount_state);
 	}
-	if (sb->s_dirt || !(sb->s_flags & MS_RDONLY))
+	if (!(sb->s_flags & MS_RDONLY))
 		ext4_commit_super(sb, 1);
 
 	if (sbi->s_proc) {