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

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Message-Id: <20180216.153242.812854511264084636.davem@davemloft.net>
Date:   Fri, 16 Feb 2018 15:32:42 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     jon.maloy@...csson.com
Cc:     netdev@...r.kernel.org,
        mohan.krishna.ghanta.krishnamurthy@...csson.com,
        tung.q.nguyen@...tech.com.au, hoang.h.le@...tech.com.au,
        canh.d.luu@...tech.com.au, ying.xue@...driver.com,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [net-next v2 1/1] tipc: avoid unnecessary copying of bundled
 messages

From: Jon Maloy <jon.maloy@...csson.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@...tek.com.au>
> Acked-by: Ying Xue <ying.xue@...driver.com>
> Signed-off-by: Jon Maloy <jon.maloy@...csson.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.