On Mon, Feb 1, 2021 at 3:55 PM Jakub Kicinski <k...@kernel.org> wrote: > > On Mon, 1 Feb 2021 13:33:05 -0500 Willem de Bruijn wrote: > > On Mon, Feb 1, 2021 at 11:41 AM Loic Poulain <loic.poul...@linaro.org> > > wrote: > > > On Mon, 1 Feb 2021 at 15:24, Willem de Bruijn wrote: > > > > What is this path to rmnet, if not the usual xmit path / > > > > validate_xmit_skb? > > > > > > I mean, not sure what to do exactly here, instead of using > > > skb_copy_expand to re-aggregate data from the different skbs, Jakub > > > suggests chaining the skbs instead (via 'frag_list' and 'next' pointer > > > I assume), and to push this chained skb to net core via netif_rx. In > > > that case, I assume the de-fragmentation/linearization will happen in > > > the net core, right? If the transported protocol is rmnet, the packet > > > is supposed to reach the rmnet_rx_handler at some point, but rmnet > > > only works with standard/linear skbs. > > > > If it has that limitation, the rx_handler should have a check and linearize. > > Do you mean it's there or we should add it?
I mean that if rmnet cannot handle non-linear skbs, then rmnet_rx_handler should linearize to be safe. It should not just rely on __netif_receive_skb_core passing it only linear packets. That is fragile. I suppose the requirement is due to how rmnet_map_deaggregate splits packets, with bare memcpy + skb_pull. I don't see a linearize call. If only this path is affected, an skb_linearize could also be limited to this branch. Or the code converted to handle skb frags and frag_list. That said, addressing the fragile behavior is somewhat tangential to the aim of this patch: passing such packets to __netif_receive_skb_core. Your comment rightly asked about the logic during allocation failure. If instead the whole packet is just discarded at that point, I suppose the current approach works. > > That is simpler than this skb_copy_expand, and as far as I can see no > > more expensive. > > rx_handler is only used by uppers which are 100% SW. I think the right > path is to fix the upper, rather than add a check to the fastpath, no? Right. I think we're on the same page. I did not mean linearizing before calling any rx_handler. Just specific inside this rx_handler implementation.
