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

[Eric Dumazet]

On Tue, Oct 14, 2025 at 3:19 AM Barry Song <21cnbao@xxxxxxxxx> 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@xxxxxxxxxx>
Date:   Wed Aug 27 20:49:34 2014 -0700

    net: attempt a single high order allocation