netdev - Re: [PATCH net] net: allow small head cache usage with large MAX_SKB

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Message-ID: <CANn89iLbe2fpLUvMJk-0Keaz1yvWb7WUe9X-3Gd5wmNQn7DN9w@mail.gmail.com>
Date: Tue, 18 Feb 2025 14:43:25 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>
Subject: Re: [PATCH net] net: allow small head cache usage with large
 MAX_SKB_FRAGS values

On Mon, Feb 17, 2025 at 3:48 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 2/12/25 9:47 PM, Eric Dumazet wrote:
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index 5b2b04835688f65daa25ca208e29775326520e1e..a14ab14c14f1bd6275ab2d1d93bf230b6be14f49
> > 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -56,7 +56,11 @@ DECLARE_PER_CPU(u32, tcp_tw_isn);
> >
> >  void tcp_time_wait(struct sock *sk, int state, int timeo);
> >
> > -#define MAX_TCP_HEADER L1_CACHE_ALIGN(128 + MAX_HEADER)
> > +#define MAX_TCP_HEADER L1_CACHE_ALIGN(64 + MAX_HEADER)
>
> I'm sorry for the latency following-up here, I really want to avoid
> another fiasco.
>
> If I read correctly, you see the warning on top of my patch because you
> have the above chunk in your local tree, am I correct?

Not at all, simply using upstream trees, perhaps a different .config
than yours ?

I think I suggested to change MAX_TCP_HEADER like this because max TCP
header is 60 bytes.

Add to this MAX_HEADER, and round to a cache line, this comes to :

#define MAX_TCP_HEADER L1_CACHE_ALIGN(64 + MAX_HEADER)

This standalone change certainly can be done much later in net-next


>
> If so, would you be ok to split the change in a 'net' patch doing the
> minimal fix (basically the initially posted patch) and following-up on
> net-next to adjust MAX_TCP_HEADER and SKB_SMALL_HEAD_SIZE as you suggest?
>
> I have a vague fear some encap scenario may suffer from the reduced TCP
> headroom, I would refrain from pushing such change on stable, if possible.

Then MAX_HEADER might be too small ?