RFC: overheads of using mb_unmapped_to_ext?

Hi,

I have a simple patch that makes the NFS server generate
read replies as a list of m_extpg mbufs. (The code already
knows how to do this for the KTLS, but this patch enables
it for non-TLS use.)

I am wondering if the overhead of calling
mb_unmapped_to_ext() on the mbuf chain when the
NIC does not have IFCAP_MEXTPG makes doing
this an undesirable alternative to generating a longer
mbuf chain of mbuf clusters?

For my hardware, things just run at wire speed (1Gbps),
so I do not see any difference between the two cases.
I do see a 15% improvement for localhost mounts, but
those hardly matter.

I tester did determine that performance was improved for
a Mellanox NIC that has IFCAP_MEXTPG. (5-15% depending
on setup)

I can enable this two ways:
1 - A sysctl that is enabled/disabled by default (which means most
     cases will call mb_unmapped_to_ext() if enabled).
or
2 - Try and check to see if the outbound NIC has IFCAP_MEXTPG.
    (Note that this does not have to be 100% correct, since the NFS
     reply works either way.  The reviewer was concerned about routing
     changes, etc, affecting the result.)

I implemented #2 using the suggested approach, but the
reviewer (who suggested the approach) decided that doing so
was not recommended. (D55203 on phabricator.)

So, what do you think is the preferred alternative?

rick