Btrfs: fix recursive auto-defrag

Follow those steps:

  # mount -o autodefrag /dev/sda7 /mnt
  # dd if=/dev/urandom of=/mnt/tmp bs=200K count=1
  # sync
  # dd if=/dev/urandom of=/mnt/tmp bs=8K count=1 conv=notrunc

and then it'll go into a loop: writeback -> defrag -> writeback ...

It's because writeback writes [8K, 200K] and then writes [0, 8K].

I tried to make writeback know if the pages are dirtied by defrag,
but the patch was a bit intrusive. Here I simply set writeback_index
when we defrag a file.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 6f89bcc4e555..df40b7c5f06b 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1055,6 +1055,13 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
         if (!max_to_defrag)
                 max_to_defrag = last_index - 1;
 
+        /*
+         * make writeback starts from i, so the defrag range can be
+         * written sequentially.
+         */
+        if (i < inode->i_mapping->writeback_index)
+                inode->i_mapping->writeback_index = i;
+
         while (i <= last_index && defrag_count < max_to_defrag) {
                 /*
                  * make sure we stop running if someone unmounts