netdev - Re: [PATCH net-next] net: Adjust sk_gso_max

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Message-ID: <621185c7-be06-12a8-fe75-f544b392fb06@gmail.com>
Date:   Tue, 25 Jan 2022 16:49:03 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: Adjust sk_gso_max_size once when set

On 1/25/22 10:20 AM, Eric Dumazet wrote:
>> The git history does not explain why MAX_TCP_HEADER is used to lower
>> sk_gso_max_size. Do you recall the history on it?
> 
> Simply that max IP datagram size is 64K
> 
> And TCP is sizing its payload size there (eg in  tcp_tso_autosize()),
> when skb only contains payload.
> 
> Headers are added later in various xmit layers.
> 
> MAX_TCP_HEADER is chosen to avoid re-allocs of skb->head in typical workload.

>From what I can tell skb->head is allocated based on MAX_TCP_HEADER, and
payload is added as frags for TSO.

I was just curious because I noticed a few MTUs (I only looked multiples
of 100 from 1500 to 9000) can get an extra segment in a TSO packet and
stay under the 64kB limit if that offset had better information of the
actual header size needed (if any beyond network + tcp).