[PATCH 10/18] nvme-tcp: fixup send workflow for kTLS

[Sagi Grimberg]

>>>> Some of the flags are call specific, others may be internal to the
>>>> networking stack (e.g. the DECRYPTED flag). Old protocols didn't do
>>>> any validation because people coded more haphazardly in the 90s.
>>>> This lack of validation is a major source of technical debt :(
>>>
>>> A-ha. So what is the plan?
>>> Should the stack validate flags?
>>> And should the rules for validating be the same for all protocols?
>>
>> MSG_SENDPAGE_NOTLAST is not an internal flag, I thought it was
>> essentially similar semantics to MSG_MORE but for sendpage. It'd
>> be great if this can be allowed in tls (again, at the very least
>> don't fail but continue as if it wasn't passed).
> 
> .. but.. MSG_SENDPAGE_NOTLAST is supported in TLS, isn't it?
> Why are we talking about it?

Ah, right.

What I'm assuming that Hannes is tripping on is that tls does
not accept when this flag is sent to sock_no_sendpage, which
is simply calling sendmsg. TLS will not accept this flag when
passed to sendmsg IIUC.

Today the rough logic in nvme send path is:

	if (more_coming(queue)) {
		flags = MSG_MORE | MSG_SENDPAGE_NOTLAST;
	} else {
		flags = MSG_EOR;
	}

	if (!sendpage_ok(page)) {
		kernel_sendpage();
	} else {
		sock_no_sendpage();
	}

This pattern (note that sock_no_sednpage was added later following bug
reports where nvme attempted to sendpage a slab allocated page), is
perfectly acceptable with normal sockets, but not with TLS.

So there are two options:
1. have tls accept MSG_SENDPAGE_NOTLAST in sendmsg (called from
    sock_no_sendpage)
2. Make nvme set MSG_SENDPAGE_NOTLAST only when calling
    kernel_sendpage and clear it when calling sock_no_sendpage

If you say that MSG_SENDPAGE_NOTLAST must be cleared when calling
sock_no_sendpage and it is a bug that it isn't enforced for normal tcp
sockets, then we need to change nvme, but I did not find
any documentation that indicates it, and right now, normal sockets
behave differently than tls sockets (wrt this flag in particular).

Hope this clarifies.