Btrfs: fix loop device on top of btrfs

We cannot use the loop device which has been connected to a file in the btrf

The reproduce steps is following:
 # dd if=/dev/zero of=vdev0 bs=1M count=1024
 # losetup /dev/loop0 vdev0
 # mkfs.btrfs /dev/loop0
 ...
 failed to zero device start -5

The reason is that the btrfs don't implement either ->write_begin or ->write
the VFS API, so we fix it by setting ->write to do_sync_write().

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 79437c5eeb1e..abcb91867b56 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1197,6 +1197,7 @@ static int btrfs_file_mmap(struct file	*filp, struct vm_area_struct *vma)
 const struct file_operations btrfs_file_operations = {
         .llseek         = generic_file_llseek,
         .read           = do_sync_read,
+        .write          = do_sync_write,
         .aio_read       = generic_file_aio_read,
         .splice_read    = generic_file_splice_read,
         .aio_write      = btrfs_file_aio_write,