From: Eric Dumazet edumazet@google.com
[ Upstream commit f969dc5a885736842c3511ecdea240fbb02d25d9 ]
While commit 24adbc1676af ("tcp: fix SO_RCVLOWAT hangs with fat skbs") fixed an issue vs too small sk_rcvbuf for given sk_rcvlowat constraint, it missed to address issue caused by memory pressure.
1) If we are under memory pressure and socket receive queue is empty. First incoming packet is allowed to be queued, after commit 76dfa6082032 ("tcp: allow one skb to be received per socket under memory pressure")
But we do not send EPOLLIN yet, in case tcp_data_ready() sees sk_rcvlowat is bigger than skb length.
2) Then, when next packet comes, it is dropped, and we directly call sk->sk_data_ready().
3) If application is using poll(), tcp_poll() will then use tcp_stream_is_readable() and decide the socket receive queue is not yet filled, so nothing will happen.
Even when sender retransmits packets, phases 2) & 3) repeat and flow is effectively frozen, until memory pressure is off.
Fix is to consider tcp_under_memory_pressure() to take care of global memory pressure or memcg pressure.
Fixes: 24adbc1676af ("tcp: fix SO_RCVLOWAT hangs with fat skbs") Signed-off-by: Eric Dumazet edumazet@google.com Reported-by: Arjun Roy arjunroy@google.com Suggested-by: Wei Wang weiwan@google.com Reviewed-by: Wei Wang weiwan@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- include/net/tcp.h | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/include/net/tcp.h b/include/net/tcp.h index 444762485615a..f5128bc28bb78 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1380,8 +1380,13 @@ static inline int tcp_full_space(const struct sock *sk) */ static inline bool tcp_rmem_pressure(const struct sock *sk) { - int rcvbuf = READ_ONCE(sk->sk_rcvbuf); - int threshold = rcvbuf - (rcvbuf >> 3); + int rcvbuf, threshold; + + if (tcp_under_memory_pressure(sk)) + return true; + + rcvbuf = READ_ONCE(sk->sk_rcvbuf); + threshold = rcvbuf - (rcvbuf >> 3);
return atomic_read(&sk->sk_rmem_alloc) > threshold; }