linux-kernel - Re: [RFC PATCH] mm: net: disable kswapd for high-order network buffer allocation

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Message-ID: <CANn89iKC_y6Fae9E5ETOE46y-RCqD6cLHnp=7GynL_=sh3noKg@mail.gmail.com>
Date: Tue, 14 Oct 2025 03:39:07 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Barry Song <21cnbao@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, netdev@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Barry Song <v-songbaohua@...o.com>, Jonathan Corbet <corbet@....net>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Willem de Bruijn <willemb@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>, 
	Yunsheng Lin <linyunsheng@...wei.com>, Huacai Zhou <zhouhuacai@...o.com>
Subject: Re: [RFC PATCH] mm: net: disable kswapd for high-order network buffer allocation

On Tue, Oct 14, 2025 at 3:19 AM Barry Song <21cnbao@...il.com> wrote:
>
> > >
> > > >
> > > > I think you are missing something to control how much memory  can be
> > > > pushed on each TCP socket ?
> > > >
> > > > What is tcp_wmem on your phones ? What about tcp_mem ?
> > > >
> > > > Have you looked at /proc/sys/net/ipv4/tcp_notsent_lowat
> > >
> > > # cat /proc/sys/net/ipv4/tcp_wmem
> > > 524288  1048576 6710886
> >
> > Ouch. That is insane tcp_wmem[0] .
> >
> > Please stick to 4096, or risk OOM of various sorts.
> >
> > >
> > > # cat /proc/sys/net/ipv4/tcp_notsent_lowat
> > > 4294967295
> > >
> > > Any thoughts on these settings?
> >
> > Please look at
> > https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
> >
> > tcp_notsent_lowat - UNSIGNED INTEGER
> > A TCP socket can control the amount of unsent bytes in its write queue,
> > thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
> > reports POLLOUT events if the amount of unsent bytes is below a per
> > socket value, and if the write queue is not full. sendmsg() will
> > also not add new buffers if the limit is hit.
> >
> > This global variable controls the amount of unsent data for
> > sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
> > to the global variable has immediate effect.
> >
> >
> > Setting this sysctl to 2MB can effectively reduce the amount of memory
> > in TCP write queues by 66 %,
> > or allow you to increase tcp_wmem[2] so that only flows needing big
> > BDP can get it.
>
> We obtained these settings from our hardware vendors.

Tell them they are wrong.

>
> It might be worth exploring these settings further, but I can’t quite see
> their connection to high-order allocations, since high-order allocations are
> kernel macros.
>
> #define SKB_FRAG_PAGE_ORDER     get_order(32768)
> #define PAGE_FRAG_CACHE_MAX_SIZE        __ALIGN_MASK(32768, ~PAGE_MASK)
> #define PAGE_FRAG_CACHE_MAX_ORDER       get_order(PAGE_FRAG_CACHE_MAX_SIZE)
>
> Is there anything I’m missing?

What is your question exactly ? You read these macros just fine. What
is your point ?

We had in the past something dynamic that we removed

commit d9b2938aabf757da2d40153489b251d4fc3fdd18
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Wed Aug 27 20:49:34 2014 -0700

    net: attempt a single high order allocation