ext3: Speedup WB_SYNC_ALL pass

When doing filesystem wide sync, there's no need to force transaction
commit separately for each inode because ext3_sync_fs() takes care of
forcing commit at the end. Most of the time this slowness doesn't
manifest because previous WB_SYNC_NONE writeback doesn't leave much to
write but when there are processes aggressively creating new files and
several filesystems to sync, the sync slowness can be noticeable. In the
following test script sync(1) takes around 6 minutes when there are two
ext3 filesystems mounted on a standard SATA drive. After this patch sync
is about twice as fast in the default data=ordered mode. For
data=writeback mode we have even bigger speedup.

function run_writers
{
  for (( i = 0; i < 10; i++ )); do
    mkdir $1/dir$i
    for (( j = 0; j < 40000; j++ )); do
      dd if=/dev/zero of=$1/dir$i/$j bs=4k count=4 &>/dev/null
    done &
  done
}

for dir in "$@"; do
  run_writers $dir
done

sleep 40
time sync

Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext3/inode.c

@@ -3210,7 +3210,12 @@ int ext3_write_inode(struct inode *inode, struct writeback_control *wbc)
 		return -EIO;
 	}
 
-	if (wbc->sync_mode != WB_SYNC_ALL)
+	/*
+	 * No need to force transaction in WB_SYNC_NONE mode. Also
+	 * ext3_sync_fs() will force the commit after everything is
+	 * written.
+	 */
+	if (wbc->sync_mode != WB_SYNC_ALL || wbc->for_sync)
 		return 0;
 
 	return ext3_force_commit(inode->i_sb);