On 27/09/18 - 16:12:12, Mat Martineau wrote:
On Thu, 27 Sep 2018, Christoph Paasch wrote:
> I am trying to rework our handling of the skb-building in tcp_sendmsg
> (avoiding custom function-pointers for select_size,...) to reduce our impact
> on regular TCP.
> So, I started looking into what it takes to use do_tcp_sendpages when
> pushing from the MPTCP-level to the subflow-level.
> A problem is that do_tcp_sendpages() only works on subflows that have
> SG-support (see the check in tcp_sendpage_locked()).
> So, that would be kind of problematic, no?
> I'm also not 100% sure how that is handled for the in-kernel TLS :-/
Hi Christoph -
I'm not quite sure why tcp_sendpage_locked() has that check. It may even
force extra copying by GSO. If the net device does not have NETIF_F_SG when
an skb is to be sent to the device, the skb will get linearized just before
getting pushed to the device (see __dev_queue_xmit() and
What's more, NETIF_F_SG is not necessarily fixed for a given device over
time (see commit message for 113f99c3358564a0647d444c2ae34e8b1abfd5b9), so
the stack must be able to handle scatter/gather skbs from do_tcp_sendpages
even if the device turns off that feature while those skbs are queued above
the device layer.
GSO being always enabled now may also be a factor in not requiring
This is what I've pieced together this afternoon, anyway - if anyone has
better information, please share!
yes you are probably right in the sense that it does not matter now with GSO
always-on. I just got confused because tcp_sendmsg is still checking for SG
to decide whether or not to do zero-copy.