llmv tries posix_fallocate() and falls back to ftruncate() if it indicates that the operation is not supported. From llvm-project-llvmorg-11.0.0/llvm/lib/Support/Unix/Path.inc:
Somewhere on the server side OST_FALLOCATE is failing and we are returning ENOTSUPP but we should be returning EOPNOTSUPP.
ENOTSUPP is 524 and defined only in the kernel errno.h and is a NFSv3 specific errno.
EOPNOTSUPP (95) is defined in kernel and userspace errno.h.
ENOTSUP is defined in userspace errno.h as an alias for EOPNOTSUPP.
Ideally we should change every ENOTSUPP to EOPNOTSUPP and add a checkpatch warning. (Or almost every if there's some real NFSv3 interaction.)