netdev - Re: [PATCH net-next] tcp: add tcp_add

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Date:   Mon, 29 Aug 2016 12:22:37 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: add tcp_add_backlog()

On Mon, 2016-08-29 at 15:51 -0300, Marcelo Ricardo Leitner wrote:
> 	skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
> 
> Shouldn't __pskb_pull_tail() already fix this? As it seems the expected
> behavior and it would have a more global effect then. For drivers not
> using copybreak, that's needed here anyway, but maybe this help other
> protocols/situations too.

That would be difficult, because some callers do their own truesize
tacking (skb might be attached/charged to a socket, so changing
skb->truesize would need to adjust the amount that was charged)

This is why pskb_expand_head() is not allowed to mess with skb->truesize
(but in the opposite way, since we probably are increasing
skb->truesize)

Not sure it is worth the pain in fast path, where packets are consumed
so fast that their skb->truesize being slightly over estimated is not an
issue.