Date:	Wed, 06 May 2015 09:13:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next] tcp: set SOCK_NOSPACE under memory pressure

On Wed, 2015-05-06 at 15:52 +0000, Jason Baron wrote:
> From: Jason Baron <jbaron@...mai.com>
> 
> Under tcp memory pressure, calling epoll_wait() in edge triggered
> mode after -EAGAIN, can result in an indefinite hang in epoll_wait(),
> even when there is sufficient memory available to continue making
> progress. The problem is that when __sk_mem_schedule() returns 0
> under memory pressure, we do not set the SOCK_NOSPACE flag in the
> tcp write paths (tcp_sendmsg() or do_tcp_sendpages()). Then, since
> SOCK_NOSPACE is used to trigger wakeups when incoming acks create
> sufficient new space in the write queue, all outstanding packets
> are acked, but we never wake up with the the EPOLLOUT that we are
> expecting from epoll_wait().
> 
> This issue is currently limited to epoll() when used in edge trigger
> mode, since 'tcp_poll()', does in fact currently set SOCK_NOSPACE.
> This is sufficient for poll()/select() and epoll() in level trigger
> mode. However, in edge trigger mode, epoll() is relying on the write
> path to set SOCK_NOSPACE. EPOLL(7) says that in edge-trigger mode we
> can only call epoll_wait() after read/write return -EAGAIN. Thus, in
> the case of the socket write, we are relying on the fact that
> tcp_sendmsg()/network write paths are going to issue a wakeup for
> us at some point in the future when we get -EAGAIN.
> 
> Normally, epoll() edge trigger works fine when we've exceeded the
> sk->sndbuf because in that case we do set SOCK_NOSPACE. However, when
> we return -EAGAIN from the write path b/c we are over the tcp memory
> limits and not b/c we are over the sndbuf, we are never going to get
> another wakeup.
> 
> I can reproduce this issue, using SO_SNDBUF, since __sk_mem_schedule()
> will return 0, or failure more readily with SO_SNDBUF:
> 
> 1) create socket and set SO_SNDBUF to N
> 2) add socket as edge trigger
> 3) write to socket and block in epoll on -EAGAIN
> 4) cause tcp mem pressure via: echo "<small val>" > net.ipv4.tcp_mem
> 
> The fix here is simply to set SOCK_NOSPACE in sk_stream_wait_memory()
> when the socket is non-blocking. Note that SOCK_NOSPACE, in addition
> to waking up outstanding waiters is also used to expand the size of
> the sk->sndbuf. However, we will not expand it by setting it in this
> case because tcp_should_expand_sndbuf(), ensures that no expansion
> occurs when we are under tcp memory pressure.
> 
> Note that we could still hang if sk->sk_wmem_queue is 0, when we get
> the -EAGAIN. In this case the SOCK_NOSPACE bit will not help, since we
> are waiting for and event that will never happen. I believe
> that this case is harder to hit (and did not hit in my testing),
> in that over the tcp 'soft' memory limits, we continue to guarantee a
> minimum write buffer size. Perhaps, we could return -ENOSPC in this
> case, or maybe we simply issue a wakeup in this case, such that we
> keep retrying the write. Note that this case is not specific to
> epoll() ET, but rather would affect blocking sockets as well. So I
> view this patch as bringing epoll() edge-trigger into sync with the
> current poll()/select()/epoll() level trigger and blocking sockets
> behavior.
> 
> Signed-off-by: Jason Baron <jbaron@...mai.com>
> ---

Thanks Jason for this extremely clear changelog ;)

Acked-by: Eric Dumazet <edumazet@...gle.com>


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