In FreeBSD 13.2 and 14.0, the NFS client was optimized to improve the performance of IO_APPEND writes, that is, writes which add data to the end of a file and so extend its size. This uncovered an old bug in some routines which copy userspace data into the kernel. The bug also affects the NFS client's implementation of direct I/O; however, this implementation is disabled by default by the vfs.nfs.nfs_directio_enable sysctl and is only used to handle synchronous writes.
