netdev - Re: [PATCH v2 net-next 2/2] tcp: fix possible freeze in tx path under memory pressure

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Date:   Fri, 17 Jun 2022 12:13:24 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 net-next 2/2] tcp: fix possible freeze in tx path
 under memory pressure

On Fri, Jun 17, 2022 at 12:08:14PM +0200, Jason A. Donenfeld wrote:
> Hi,
> 
> On Tue, Jun 14, 2022 at 10:17:34AM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > Blamed commit only dealt with applications issuing small writes.
> > 
> > Issue here is that we allow to force memory schedule for the sk_buff
> > allocation, but we have no guarantee that sendmsg() is able to
> > copy some payload in it.
> > 
> > In this patch, I make sure the socket can use up to tcp_wmem[0] bytes.
> > 
> > For example, if we consider tcp_wmem[0] = 4096 (default on x86),
> > and initial skb->truesize being 1280, tcp_sendmsg() is able to
> > copy up to 2816 bytes under memory pressure.
> > 
> > Before this patch a sendmsg() sending more than 2816 bytes
> > would either block forever (if persistent memory pressure),
> > or return -EAGAIN.
> > 
> > For bigger MTU networks, it is advised to increase tcp_wmem[0]
> > to avoid sending too small packets.
> > 
> > v2: deal with zero copy paths.
> 
> I think this might have gotten double applied:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=849b425cd091e1804af964b771761cfbefbafb43
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=f54755f6a11accb2db5ef17f8f75aad0875aefdc
> 
> and now net-next builds are broken:
> 
> ../../../../../../../../net/ipv4/tcp.c:971:12: error: redefinition of ‘tcp_wmem_schedule’
>   971 | static int tcp_wmem_schedule(struct sock *sk, int copy)                                |            ^~~~~~~~~~~~~~~~~
> ../../../../../../../../net/ipv4/tcp.c:954:12: note: previous definition of ‘tcp_wmem_schedule’ with type ‘int(struct sock *, int)’
>   954 | static int tcp_wmem_schedule(struct sock *sk, int copy)                                |            ^~~~~~~~~~~~~~~~~
> ../../../../../../../../net/ipv4/tcp.c:954:12: warning: ‘tcp_wmem_schedule’ defined but not used [-Wunused-function]                                                              make[5]: *** [/home/wgci/tmp/2813390.12234/tmp.0PMBO65tGf/scripts/Makefile.build:249: net
> /ipv4/tcp.o] Error 1                                                                     make[4]: *** [/home/wgci/tmp/2813390.12234/tmp.0PMBO65tGf/scripts/Makefile.build:466: net/ipv4] Error 2
> make[4]: *** Waiting for unfinished jobs....

Ah, fixed already:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=fd8b330ce1bb79ba00047435cc213b14f886bf1f

> 
> Jason