btrfs: fix readdir deadlock with pagefault

Readdir does dir_emit while under the btree lock.  dir_emit can trigger
the page fault which means we can deadlock.  Fix this by allocating a
buffer on opening a directory and copying the readdir into this buffer
and doing dir_emit from outside of the tree lock.

Thread A
readdir  <holding tree lock>
  dir_emit
    <page fault>
      down_read(mmap_sem)

Thread B
mmap write
  down_write(mmap_sem)
    page_mkwrite
      wait_ordered_extents

Process C
finish_ordered_extent
  insert_reserved_file_extent
   try to lock leaf <hang>

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ copy the deadlock scenario to changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 15 insertions, 7 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 7a3c56c17e00..b21558bb1294 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3966,6 +3966,7 @@ static long btrfs_ioctl_trans_start(struct file *file)
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_trans_handle *trans;
+	struct btrfs_file_private *private;
 	int ret;
 	static bool warned = false;
 
@@ -3984,8 +3985,16 @@ static long btrfs_ioctl_trans_start(struct file *file)
 	}
 
 	ret = -EINPROGRESS;
-	if (file->private_data)
+	private = file->private_data;
+	if (private && private->trans)
 		goto out;
+	if (!private) {
+		private = kzalloc(sizeof(struct btrfs_file_private),
+				  GFP_KERNEL);
+		if (!private)
+			return -ENOMEM;
+		file->private_data = private;
+	}
 
 	ret = -EROFS;
 	if (btrfs_root_readonly(root))
@@ -4002,7 +4011,7 @@ static long btrfs_ioctl_trans_start(struct file *file)
 	if (IS_ERR(trans))
 		goto out_drop;
 
-	file->private_data = trans;
+	private->trans = trans;
 	return 0;
 
 out_drop:
@@ -4257,14 +4266,13 @@ long btrfs_ioctl_trans_end(struct file *file)
 {
 	struct inode *inode = file_inode(file);
 	struct btrfs_root *root = BTRFS_I(inode)->root;
-	struct btrfs_trans_handle *trans;
+	struct btrfs_file_private *private = file->private_data;
 
-	trans = file->private_data;
-	if (!trans)
+	if (!private || !private->trans)
 		return -EINVAL;
-	file->private_data = NULL;
 
-	btrfs_end_transaction(trans);
+	btrfs_end_transaction(private->trans);
+	private->trans = NULL;
 
 	atomic_dec(&root->fs_info->open_ioctl_trans);