Re: [PATCH net] tcp: select sane initial rcvq_space.space for big MSS

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue,  8 Dec 2020 08:21:31 -0800

> From: Eric Dumazet <edumazet@google.com>
> 
> Before commit a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
> small tcp_rmem[1] values were overridden by tcp_fixup_rcvbuf() to accommodate various MSS.
> 
> This is no longer the case, and Hazem Mohamed Abuelfotoh reported
> that DRS would not work for MTU 9000 endpoints receiving regular (1500 bytes) frames.
> 
> Root cause is that tcp_init_buffer_space() uses tp->rcv_wnd for upper limit
> of rcvq_space.space computation, while it can select later a smaller
> value for tp->rcv_ssthresh and tp->window_clamp.
> 
> ss -temoi on receiver would show :
> 
> skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) rcv_space:62496 rcv_ssthresh:56596
> 
> This means that TCP can not increase its window in tcp_grow_window(),
> and that DRS can never kick.
> 
> Fix this by making sure that rcvq_space.space is not bigger than number of bytes
> that can be held in TCP receive queue.
> 
> People unable/unwilling to change their kernel can work around this issue by
> selecting a bigger tcp_rmem[1] value as in :
> 
> echo "4096 196608 6291456" >/proc/sys/net/ipv4/tcp_rmem
> 
> Based on an initial report and patch from Hazem Mohamed Abuelfotoh
>  https://lore.kernel.org/netdev/20201204180622.14285-1-abuehaze@amazon.com/
> 
> Fixes: a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
> Fixes: 041a14d26715 ("tcp: start receiver buffer autotuning sooner")
> Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied, thanks Eric.