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 <[email protected]>

---
 fs/btrfs/ioctl.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

fs/btrfs/ioctl.c +7 -2

index 970977a..7a10f94 100644
@@ -1047,6 +1047,13 @@ 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
-- 1.7.3.1 
--

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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.