linux-ext4 - sb_dirt question

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Date:	Thu, 15 Mar 2012 17:00:40 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Ted Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: sb_dirt question

Hi Ted,

could you please commend on this commit of yours a bit:

commit a0375156ca1041574b5d47cc7e32f10b891151b0
Author: Theodore Ts'o <tytso@....edu>
Date:   Fri Jun 11 23:14:04 2010 -0400

    ext4: Clean up s_dirt handling
    
    We don't need to set s_dirt in most of the ext4 code when journaling
    is enabled.  In ext3/4 some of the summary statistics for # of free
    inodes, blocks, and directories are calculated from the per-block
    group statistics when the file system is mounted or unmounted.  As a
    result the superblock doesn't have to be updated, either via the
    journal or by setting s_dirt.  There are a few exceptions, most
    notably when resizing the file system, where the superblock needs to
    be modified --- and in that case it should be done as a journalled
    operation if possible, and s_dirt set only in no-journal mode.
    
    This patch will optimize out some unneeded disk writes when using ext4
    with a journal.
    
    Signed-off-by: "Theodore Ts'o" <tytso@....edu>

Let's take this hunk (I preserved more context than usual)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 5313ae4..bd411c1 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -110,33 +110,33 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
        if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED) &&
                     !(sb->s_flags & MS_RDONLY))) {
                sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;
                /*
                 * Sample where the filesystem has been mounted and
                 * store it in the superblock for sysadmin convenience
                 * when trying to sort through large numbers of block
                 * devices or filesystem images.
                 */
                memset(buf, 0, sizeof(buf));
                path.mnt = mnt;
                path.dentry = mnt->mnt_root;
                cp = d_path(&path, buf, sizeof(buf));
                if (!IS_ERR(cp)) {
                        memcpy(sbi->s_es->s_last_mounted, cp,
                               sizeof(sbi->s_es->s_last_mounted));
-                       sb->s_dirt = 1;
+                       ext4_mark_super_dirty(sb);
                }

As I read this, it means that if do have journal, do not bother writing
out the updated superblock at all. When the superblock will be written
out then?

Thanks!

P.S. The reason I look at this is because I am trying to make sure ext4
does not use s_dirty and does not rely on VFS calling 'write_super()' at
all. I guess I will try to use a delayed work instead and move 's_dirt'
to sbi. The reason of this exercise is that I want to eliminate
'sync_supers()' completely, because it wakes up every 5 seconds no
matter what, and Al wanted me to push SB handling down to file-systems.

-- 
Best Regards,
Artem Bityutskiy

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