On 04/08/2014 11:26 AM, Daniel Borkmann wrote:
> SCTP charges chunks for wmem accounting via skb->truesize in
> sctp_set_owner_w(), and sctp_wfree() respectively as the
> reverse operation. If a sender runs out of wmem, it needs to
> wait via sctp_wait_for_sndbuf(), and gets woken up by a call
> to __sctp_write_space() mostly via sctp_wfree().
>
> __sctp_write_space() is being called per association. Although
> we assign sk->sk_write_space() to sctp_write_space(), which
> is then being done per socket, it is only used if send space
> is increased per socket option (SO_SNDBUF), as SOCK_USE_WRITE_QUEUE
> is set and therefore not invoked in sock_wfree().
>
> Commit 4c3a5bdae293 ("sctp: Don't charge for data in sndbuf
> again when transmitting packet") fixed an issue where in case
> sctp_packet_transmit() manages to queue up more than sndbuf
> bytes, sctp_wait_for_sndbuf() will never be woken up again
> unless it is interrupted by a signal. However, a still
> remaining issue is that if net.sctp.sndbuf_policy=0, that is
> accounting per socket, and one-to-many sockets are in use,
> the reclaimed write space from sctp_wfree() is 'unfairly'
> handed back on the server to the association that is the lucky
> one to be woken up again via __sctp_write_space(), while
> the remaining associations are never be woken up again
> (unless by a signal).
>
> The effect disappears with net.sctp.sndbuf_policy=1, that
> is wmem accounting per association, as it guarantees a fair
> share of wmem among associations.
>
> Therefore, if we have reclaimed memory in case of per socket
> accounting, wake all related associations to a socket in a
> fair manner, that is, traverse the socket association list
> starting from the current neighbour of the association and
> issue a __sctp_write_space() to everyone until we end up
> waking ourselves. This guarantees that no association is
> preferred over another and even if more associations are
> taken into the one-to-many session, all receivers will get
> messages from the server and are not stalled forever on
> high load. This setting still leaves the advantage of per
> socket accounting in touch as an association can still use
> up global limits if unused by others.
>
> Fixes: 4eb701dfc618 ("[SCTP] Fix SCTP sendbuffer accouting.")
> Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> Cc: Thomas Graf <tgraf@xxxxxxx>
> Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
> Cc: Vlad Yasevich <vyasevic@xxxxxxxxxx>
Acked-by: Vlad Yasevich <vyasevic@xxxxxxxxxx>
-vlad
> ---
> [ When net-next opens up again, we need to think how
> we can ideally make a new list interface and simplify
> both open-coded list traversals. ]
>
> v1->v2:
> - Improved comment, included note about locking
>
> net/sctp/socket.c | 36 +++++++++++++++++++++++++++++++++++-
> 1 file changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 981aaf8..96cfaba 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -6593,6 +6593,40 @@ static void __sctp_write_space(struct sctp_association *asoc)
> }
> }
>
> +static void sctp_wake_up_waiters(struct sock *sk,
> + struct sctp_association *asoc)
> +{
> + struct sctp_association *tmp = asoc;
> +
> + /* We do accounting for the sndbuf space per association,
> + * so we only need to wake our own association.
> + */
> + if (asoc->ep->sndbuf_policy)
> + return __sctp_write_space(asoc);
> +
> + /* Accounting for the sndbuf space is per socket, so we
> + * need to wake up others, try to be fair and in case of
> + * other associations, let them have a go first instead
> + * of just doing a sctp_write_space() call.
> + *
> + * Note that we reach sctp_wake_up_waiters() only when
> + * associations free up queued chunks, thus we are under
> + * lock and the list of associations on a socket is
> + * guaranteed not to change.
> + */
> + for (tmp = list_next_entry(tmp, asocs); 1;
> + tmp = list_next_entry(tmp, asocs)) {
> + /* Manually skip the head element. */
> + if (&tmp->asocs == &((sctp_sk(sk))->ep->asocs))
> + continue;
> + /* Wake up association. */
> + __sctp_write_space(tmp);
> + /* We've reached the end. */
> + if (tmp == asoc)
> + break;
> + }
> +}
> +
> /* Do accounting for the sndbuf space.
> * Decrement the used sndbuf space of the corresponding association by the
> * data size which was just transmitted(freed).
> @@ -6620,7 +6654,7 @@ static void sctp_wfree(struct sk_buff *skb)
> sk_mem_uncharge(sk, skb->truesize);
>
> sock_wfree(skb);
> - __sctp_write_space(asoc);
> + sctp_wake_up_waiters(sk, asoc);
>
> sctp_association_put(asoc);
> }
>
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