netdev - Re: send/sendmsg ENOMEM errors WAS(Re: [PATCH net 6/6] sctp: not return ENOMEM err back in sctp_packet

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Date:   Mon, 24 Oct 2016 14:30:07 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Vlad Yasevich <vyasevic@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Michael Tuexen <Michael.Tuexen@...chi.franken.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Brenda Butler <bjb@...atatu.com>, gabor@...atatu.com
Subject: Re: send/sendmsg ENOMEM errors WAS(Re: [PATCH net 6/6] sctp: not
 return ENOMEM err back in sctp_packet_transmit

[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 ?