netdev - Re: [PATCH net-next] tcp: reduce skb overhead in selected places

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Message-ID: <1485380412.5145.59.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 25 Jan 2017 13:40:12 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: reduce skb overhead in selected places

On Wed, 2017-01-25 at 14:03 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 25 Jan 2017 10:38:52 -0800
> 
> > Do you think we could change __pskb_pull_tail() right away and fix the
> > few places that would break, or should we add various helpers with extra
> > parameters to take a safe route ?
> 
> It should always be safe as long as we see no socket attached on RX,
> right?
> 
> That's the only real case where truesize adjustments can cause trouble.
Queue can be virtual, as for xmit path, tracking skb->truesize in
sk->sk_wmem_alloc.

If a layer calls pskb_may_pull(), we can not change skb->truesize
without also changing skb->sk->sk_wmem_alloc, or sock_wfree() will
trigger bugs.