vfs: kill write_super and sync_supers

Finally we can kill the 'sync_supers' kernel thread along with the
'->write_super()' superblock operation because all the users are gone.
Now every file-system is supposed to self-manage own superblock and
its dirty state.

The nice thing about killing this thread is that it improves power management.
Indeed, 'sync_supers' is a source of monotonic system wake-ups - it woke up
every 5 seconds no matter what - even if there were no dirty superblocks and
even if there were no file-systems using this service (e.g., btrfs and
journalled ext4 do not need it). So it was wasting power most of the time. And
because the thread was in the core of the kernel, all systems had to have it.
So I am quite happy to make it go away.

Interestingly, this thread is a left-over from the pdflush kernel thread which
was a self-forking kernel thread responsible for all the write-back in old
Linux kernels. It was turned into per-block device BDI threads, and
'sync_supers' was a left-over. Thus, R.I.P, pdflush as well.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 0 insertions, 40 deletions

diff --git a/fs/super.c b/fs/super.c
index b05cf47463d0..0902cfa6a12e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -537,46 +537,6 @@ void drop_super(struct super_block *sb)
 EXPORT_SYMBOL(drop_super);
 
 /**
- * sync_supers - helper for periodic superblock writeback
- *
- * Call the write_super method if present on all dirty superblocks in
- * the system.  This is for the periodic writeback used by most older
- * filesystems.  For data integrity superblock writeback use
- * sync_filesystems() instead.
- *
- * Note: check the dirty flag before waiting, so we don't
- * hold up the sync while mounting a device. (The newly
- * mounted device won't need syncing.)
- */
-void sync_supers(void)
-{
-        struct super_block *sb, *p = NULL;
-
-        spin_lock(&sb_lock);
-        list_for_each_entry(sb, &super_blocks, s_list) {
-                if (hlist_unhashed(&sb->s_instances))
-                        continue;
-                if (sb->s_op->write_super && sb->s_dirt) {
-                        sb->s_count++;
-                        spin_unlock(&sb_lock);
-
-                        down_read(&sb->s_umount);
-                        if (sb->s_root && sb->s_dirt && (sb->s_flags & MS_BORN))
-                                sb->s_op->write_super(sb);
-                        up_read(&sb->s_umount);
-
-                        spin_lock(&sb_lock);
-                        if (p)
-                                __put_super(p);
-                        p = sb;
-                }
-        }
-        if (p)
-                __put_super(p);
-        spin_unlock(&sb_lock);
-}
-
-/**
  *      iterate_supers - call function for all active superblocks
  *      @f: function to call
  *      @arg: argument to pass to it