On Mon, Oct 24, 2016 at 02:30:07PM +0800, Xin Long wrote: > [1] > >> This patch doesn't ignore all the ENOMEN cases, only after msg is > >> enqueued in out queue/send queue, in the lower layer, when alloc > >> new skb and copy data from old skb, if it fails to alloc new skb, sctp > >> will ignore this ENOMEM, as this msg will be taken care by retransmit > >> mechanism, it's reasonable and also safe, user can't feel that. > >> > > > > Yes, that part i got. > > > > [2] > >> But for the cases before enqueue, like in sctp_sendmsg, > >> sctp_datamsg_from_user may return ENOMEM, this err will return > >> back to user, and can't be ignored. > >> > > > > The hard part is distinguishing between the above case and real > > failure. > > I am assuming in the case above user is _not_ required to send > > again. But in the general case they are required to send again. > > Correct? > in case [1], user can't see the ENOMEM, ENOMEM is more like > a internal err. > > in case [2], user will got the ENOMEM, they should resend this msg, > It's the the general case mentioned-above > > > > >> So I don't really think we should change something in manpage, what > >> do you think ? maybe a little explanation there is also nice, :) > > > > > > Yes, that would help. In particular it should be clear what user space > > is expected to do. While this is about sctp - I am assuming equivalent > > behavior for all callers of sendxxx() regardless of protocol. > here sctp's behavior is actually same with tcp's, in tcp, tcp_transmit_skb > also may fail to alloc skb, but it doesn't return any err to user, just like > sctp_packet_transmit. That's why I don't think we should change something > in manpage, as here sctp is consistent with tcp now. > > make sense ?
I may be saying what is already understood, but just to be clear, without this patch, there is no consistent way to known if you hit [1] or [2]. Recovering from it then depends on how the protocol above SCTP will handle it, if it can handle duplicate messages or not.
