diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 848cb2c3d9dd..78c4a3765002 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1200,7 +1200,7 @@ ssize_t btrfs_buffered_write(struct kiocb *iocb, struct iov_iter *i)
 	ssize_t ret;
 	bool only_release_metadata = false;
 	bool force_page_uptodate = false;
-	loff_t old_isize = i_size_read(inode);
+	loff_t old_isize;
 	unsigned int ilock_flags = 0;
 	const bool nowait = (iocb->ki_flags & IOCB_NOWAIT);
 	unsigned int bdp_flags = (nowait ? BDP_ASYNC : 0);
@@ -1212,6 +1212,13 @@ ssize_t btrfs_buffered_write(struct kiocb *iocb, struct iov_iter *i)
 	if (ret < 0)
 		return ret;
 
+	/*
+	 * We can only trust the isize with inode lock held, or it can race with
+	 * other buffered writes and cause incorrect call of
+	 * pagecache_isize_extended() to overwrite existing data.
+	 */
+	old_isize = i_size_read(inode);
+
 	ret = generic_write_checks(iocb, i);
 	if (ret <= 0)
 		goto out;