netdev - Re: [PATCH net] gve: Fix an edge case for TSO skb validity check

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Message-ID: <361c9ebd-d161-41a9-8690-b9e115836666@redhat.com>
Date: Tue, 23 Jul 2024 12:20:42 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Bailey Forrest <bcf@...gle.com>
Cc: Praveen Kaligineedi <pkaligineedi@...gle.com>, netdev@...r.kernel.org,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 willemb@...gle.com, shailend@...gle.com, hramamurthy@...gle.com,
 csully@...gle.com, jfraker@...gle.com, stable@...r.kernel.org,
 Jeroen de Borst <jeroendb@...gle.com>
Subject: Re: [PATCH net] gve: Fix an edge case for TSO skb validity check

On 7/21/24 21:10, Willem de Bruijn wrote:
> On Fri, Jul 19, 2024 at 9:56 AM Bailey Forrest <bcf@...gle.com> wrote:
>> If last_frag_size is evenly divisible by GVE_TX_MAX_BUF_SIZE_DQO, it
>> doesn't hit the edge case we're looking for.
>>
>> - If it's evenly divisible, then we know it will use exactly
>> (last_frag_size / GVE_TX_MAX_BUF_SIZE_DQO) descriptors
> 
> This assumes that gso_segment start is aligned with skb_frag
> start. That is not necessarily true, right?
> 
> If headlen minus protocol headers is 1B, then the first segment
> will have two descriptors { 1B, 9KB - 1 }. And the next segment
> can have skb_frag_size - ( 9KB - 1).
> 
> I think the statement is correct, but because every multiple
> of 16KB is so much larger than the max gso_size of ~9KB,
> that a single segment will never include more than two
> skb_frags.

I'm a bit lost, but what abut big TCP? that should allow (considerably) 
more than 2 frags 16K each per segment.

In any case it looks like this needs at least some comments clarification.

Thanks,

Paolo