Linus Torvalds [off-list ref] wrote:

So I think you need to remove the changes you did to
memcpy_from_iter(). The old code was an explicit conditional of direct
calls:

        if (iov_iter_is_copy_mc(i))
                return (void *)copy_mc_to_kernel(to, from, size);
        return memcpy(to, from, size);

and now you do that

                                   iov_iter_is_copy_mc(i) ?
                                   memcpy_from_iter_mc : memcpy_from_iter);

to pass in a function pointer.

Not ok. Not ok at all. It may look clever, but function pointers are
bad. Avoid them like the plague.

Yeah.  I was hoping that the compiler would manage to inline that, but it just
does an indirect call.  I'm trying to avoid passing the iterator as that makes
things bigger.  I think I can probably share the extra argument used for
passing checksums.

David