RFC: What to do about Allocate in the NFS server for FreeBSD13?

Hi,

There are two problems with Allocate in the NFSv4.2 server in FreeBSD13:
1 - It uses the thread credentials instead of the ones for the RPC.
2 - It does not ensure that file changes are committed to stable storage.
These problems are fixed by commit f0c9847a6c47 in main, which added
ioflag and cred arguments to VOP_ALLOCATE().

I can think of 3 ways to fix Allocate in FreeBSD13:
1 - Apply a *hackish* patch like this:
+	savcred = p->td_ucred;
+	td->td_ucred = cred;
 	do {
 		olen = len;
 		error = VOP_ALLOCATE(vp, &off, &len);
 		if (error == 0 && len > 0 && olen > len)
 			maybe_yield();
 	} while (error == 0 && len > 0 && olen > len);
+	p->td_ucred = savcred;
 	if (error == 0 && len > 0)
 		error = NFSERR_IO;
+	if (error == 0)
+		error = VOP_FSYNC(vp, MNT_WAIT, p);
The worst part of it is temporarily setting td_ucred to cred.

2 - MFC'ng commit f0c9847a6c47. Normally changes to the
     VOP/VFS are not MFC'd. However, in this case, it might be
     ok to do so, since it is unlikely there is an out of source tree
     file system with a custom VOP_ALLOCATE() method?

3 - Just disable it. Currently it is disabled by default and it
     could just be wired down disabled.
     Allocate is not that useful, since ZFS does not support it.

As an aside to this, the IETF NFSv4 working group seems to
have agreed that NFS4ERR_NOTSUPP can be returned by a
NFSv4.2 server on a 'per file system basis" instead of globally,
since the Linux knfsd already does this.
--> As such, Allocate can be enabled by default in main and
       could be enabled by default in FreeBSD13, if #1 or #2 was
       done.
       --> It still would not work for ZFS exports.

So, what do you think is the preferred alternative?

rick