netdev - Re: [PATCH net-next] net: adjust skb->truesize in pskb_expand

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Message-ID: <1485533695.6360.64.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 27 Jan 2017 08:14:55 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Slava Shwartsman <slavash@...lanox.com>
Subject: Re: [PATCH net-next] net: adjust skb->truesize in pskb_expand_head()

On Fri, 2017-01-27 at 15:46 +0000, David Laight wrote:

> Reads code ...
> My confusion is that the call is specifying the number of EXTRA bytes of head/tail
> room rather than the number of bytes needed.

And the fact that @size is changed in existing code (so not visible in
patch diff) to

size = SKB_WITH_OVERHEAD(ksize(data));


> I was thinking that pskb_expand_head(skb, 16, -16, ...) could be implemented
> (mostly) with memmove().

Yes, might worth doing that if one day a caller tries that ;)

In the meantime, not worth adding code that wont be reached.

Thanks.