From: Sagi Grimberg <sagi@grimberg.me>
Date: Mon, 19 Nov 2018 15:14:44 -0800

Also, looking a bit closer there is a slight difference between the
copy vs. the copy_and_csum variants. copy allows for a short_copy if
we copy less than we expect while the csum faults it. I'm thinking
that the copy_and_hash variant should also fault? Although I'm not
sure I understand the fault entirely as csum is supposed to be
cumulative, any insight?

When we are writing and signal an error, sockets have this recurring
pattern where we return immediately the amount of bytes successfully
transferred.  Then on the next sendmsg() call we give the error.

I don't know if that is what is influencing the behavior here or not
but it could be.