This is a note to let you know that I've just added the patch titled
tcp: select sane initial rcvq_space.space for big MSS
to the 5.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tcp-select-sane-initial-rcvq_space.space-for-big-mss.patch
and it can be found in the queue-5.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From foo@baz Sat Dec 19 11:38:44 AM CET 2020
From: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Tue, 8 Dec 2020 08:21:31 -0800
Subject: tcp: select sane initial rcvq_space.space for big MSS
From: Eric Dumazet <edumazet@xxxxxxxxxx>
[ Upstream commit 72d05c00d7ecda85df29abd046da7e41cc071c17 ]
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@xxxxxxxxxx/
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@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
net/ipv4/tcp_input.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -447,7 +447,6 @@ static void tcp_init_buffer_space(struct
if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
tcp_sndbuf_expand(sk);
- tp->rcvq_space.space = min_t(u32, tp->rcv_wnd, TCP_INIT_CWND * tp->advmss);
tcp_mstamp_refresh(tp);
tp->rcvq_space.time = tp->tcp_mstamp;
tp->rcvq_space.seq = tp->copied_seq;
@@ -471,6 +470,8 @@ static void tcp_init_buffer_space(struct
tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
tp->snd_cwnd_stamp = tcp_jiffies32;
+ tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
+ (u32)TCP_INIT_CWND * tp->advmss);
}
/* 4. Recalculate window clamp after socket hit its memory bounds. */
Patches currently in stable-queue which might be from edumazet@xxxxxxxxxx are
queue-5.9/mac80211-mesh-fix-mesh_pathtbl_init-error-path.patch
queue-5.9/tcp-fix-cwnd-limited-bug-for-tso-deferral-where-we-send-nothing.patch
queue-5.9/tcp-select-sane-initial-rcvq_space.space-for-big-mss.patch
