Re: [net-next v2 1/1] tipc: avoid unnecessary copying of bundled messages

[David Miller <davem@davemloft.net>]

From: Jon Maloy <jon.maloy@ericsson.com>
Date: Thu, 15 Feb 2018 14:14:37 +0100

> A received sk buffer may contain dozens of smaller 'bundled' messages
> which after extraction go each in their own direction.
> 
> Unfortunately, when we extract those messages using skb_clone() each
> of the extracted buffers inherit the truesize value of the original
> buffer. Apart from causing massive overaccounting of the base buffer's
> memory, this often causes tipc_msg_validate() to come to the false
> conclusion that the ratio truesize/datasize > 4, and perform an
> unnecessary copying of the extracted buffer.
> 
> We now fix this problem by explicitly correcting the truesize value of
> the buffer clones to be the truesize of the clone itself plus a
> calculated fraction of the base buffer's overhead. This change
> eliminates the overaccounting and at least mitigates the occurrence
> of unnecessary buffer copying.
> 
> Reported-by: Hoang Le <hoang.h.le@dektek.com.au>
> Acked-by: Ying Xue <ying.xue@windriver.com>
> Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>

As I explained in my previous two emails, I don't think this method of
accounting is appropriate.

All of your clones must use the same skb->truesize as the original SKB
because each and every one of them keeps the full buffer from being
liberated until they are released.