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>

fs/btrfs/ctree.h

@@ -1264,6 +1264,11 @@ struct btrfs_root {
 	atomic64_t qgroup_meta_rsv;
 };
 
+struct btrfs_file_private {
+	struct btrfs_trans_handle *trans;
+	void *filldir_buf;
+};
+
 static inline u32 btrfs_inode_sectorsize(const struct inode *inode)
 {
 	return btrfs_sb(inode->i_sb)->sectorsize;

fs/btrfs/file.c

@@ -1990,8 +1990,15 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 
 int btrfs_release_file(struct inode *inode, struct file *filp)
 {
-	if (filp->private_data)
+	struct btrfs_file_private *private = filp->private_data;
+
+	if (private && private->trans)
 		btrfs_ioctl_trans_end(filp);
+	if (private && private->filldir_buf)
+		kfree(private->filldir_buf);
+	kfree(private);
+	filp->private_data = NULL;
+
 	/*
 	 * ordered_data_close is set by settattr when we are about to truncate
 	 * a file from a non-zero size to a zero size.  This tries to

fs/btrfs/inode.c

@@ -5876,25 +5876,74 @@ unsigned char btrfs_filetype_table[] = {
 	DT_UNKNOWN, DT_REG, DT_DIR, DT_CHR, DT_BLK, DT_FIFO, DT_SOCK, DT_LNK
 };
 
+/*
+ * All this infrastructure exists because dir_emit can fault, and we are holding
+ * the tree lock when doing readdir.  For now just allocate a buffer and copy
+ * our information into that, and then dir_emit from the buffer.  This is
+ * similar to what NFS does, only we don't keep the buffer around in pagecache
+ * because I'm afraid I'll mess that up.  Long term we need to make filldir do
+ * copy_to_user_inatomic so we don't have to worry about page faulting under the
+ * tree lock.
+ */
+static int btrfs_opendir(struct inode *inode, struct file *file)
+{
+	struct btrfs_file_private *private;
+
+	private = kzalloc(sizeof(struct btrfs_file_private), GFP_KERNEL);
+	if (!private)
+		return -ENOMEM;
+	private->filldir_buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!private->filldir_buf) {
+		kfree(private);
+		return -ENOMEM;
+	}
+	file->private_data = private;
+	return 0;
+}
+
+struct dir_entry {
+	u64 ino;
+	u64 offset;
+	unsigned type;
+	int name_len;
+};
+
+static int btrfs_filldir(void *addr, int entries, struct dir_context *ctx)
+{
+	while (entries--) {
+		struct dir_entry *entry = addr;
+		char *name = (char *)(entry + 1);
+
+		ctx->pos = entry->offset;
+		if (!dir_emit(ctx, name, entry->name_len, entry->ino,
+			      entry->type))
+			return 1;
+		addr += sizeof(struct dir_entry) + entry->name_len;
+		ctx->pos++;
+	}
+	return 0;
+}
+
 static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct inode *inode = file_inode(file);
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	struct btrfs_root *root = BTRFS_I(inode)->root;
+	struct btrfs_file_private *private = file->private_data;
 	struct btrfs_dir_item *di;
 	struct btrfs_key key;
 	struct btrfs_key found_key;
 	struct btrfs_path *path;
+	void *addr;
 	struct list_head ins_list;
 	struct list_head del_list;
 	int ret;
 	struct extent_buffer *leaf;
 	int slot;
-	unsigned char d_type;
-	int over = 0;
-	char tmp_name[32];
 	char *name_ptr;
 	int name_len;
+	int entries = 0;
+	int total_len = 0;
 	bool put = false;
 	struct btrfs_key location;
 
@@ -5905,12 +5954,14 @@ static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
 	if (!path)
 		return -ENOMEM;
 
+	addr = private->filldir_buf;
 	path->reada = READA_FORWARD;
 
 	INIT_LIST_HEAD(&ins_list);
 	INIT_LIST_HEAD(&del_list);
 	put = btrfs_readdir_get_delayed_items(inode, &ins_list, &del_list);
 
+again:
 	key.type = BTRFS_DIR_INDEX_KEY;
 	key.offset = ctx->pos;
 	key.objectid = btrfs_ino(BTRFS_I(inode));
@@ -5920,6 +5971,8 @@ static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
 		goto err;
 
 	while (1) {
+		struct dir_entry *entry;
+
 		leaf = path->nodes[0];
 		slot = path->slots[0];
 		if (slot >= btrfs_header_nritems(leaf)) {
@@ -5941,41 +5994,43 @@ static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
 			goto next;
 		if (btrfs_should_delete_dir_index(&del_list, found_key.offset))
 			goto next;
-
-		ctx->pos = found_key.offset;
-
 		di = btrfs_item_ptr(leaf, slot, struct btrfs_dir_item);
 		if (verify_dir_item(fs_info, leaf, slot, di))
 			goto next;
 
 		name_len = btrfs_dir_name_len(leaf, di);
-		if (name_len <= sizeof(tmp_name)) {
+		if ((total_len + sizeof(struct dir_entry) + name_len) >=
-			name_ptr = tmp_name;
+		    PAGE_SIZE) {
-		} else {
+			btrfs_release_path(path);
-			name_ptr = kmalloc(name_len, GFP_KERNEL);
+			ret = btrfs_filldir(private->filldir_buf, entries, ctx);
-			if (!name_ptr) {
+			if (ret)
-				ret = -ENOMEM;
+				goto nopos;
-				goto err;
+			addr = private->filldir_buf;
-			}
+			entries = 0;
+			total_len = 0;
+			goto again;
 		}
+
+		entry = addr;
+		entry->name_len = name_len;
+		name_ptr = (char *)(entry + 1);
 		read_extent_buffer(leaf, name_ptr, (unsigned long)(di + 1),
 				   name_len);
-
+		entry->type = btrfs_filetype_table[btrfs_dir_type(leaf, di)];
-		d_type = btrfs_filetype_table[btrfs_dir_type(leaf, di)];
 		btrfs_dir_item_key_to_cpu(leaf, di, &location);
-
+		entry->ino = location.objectid;
-		over = !dir_emit(ctx, name_ptr, name_len, location.objectid,
+		entry->offset = found_key.offset;
-				 d_type);
+		entries++;
-
+		addr += sizeof(struct dir_entry) + name_len;
-		if (name_ptr != tmp_name)
+		total_len += sizeof(struct dir_entry) + name_len;
-			kfree(name_ptr);
-
-		if (over)
-			goto nopos;
-		ctx->pos++;
 next:
 		path->slots[0]++;
 	}
+	btrfs_release_path(path);
+
+	ret = btrfs_filldir(private->filldir_buf, entries, ctx);
+	if (ret)
+		goto nopos;
 
 	ret = btrfs_readdir_delayed_dir_index(ctx, &ins_list);
 	if (ret)
@@ -10779,6 +10834,7 @@ static const struct file_operations btrfs_dir_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= generic_read_dir,
 	.iterate_shared	= btrfs_real_readdir,
+	.open		= btrfs_opendir,
 	.unlocked_ioctl	= btrfs_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= btrfs_compat_ioctl,

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 (!private || !private->trans)
-	if (!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);