netdev - Re: [net-next PATCH V2 1/9] net: frag evictor, avoid killing warm frag queues

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Date:	Thu, 29 Nov 2012 15:32:16 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	David Miller <davem@...emloft.net>, fw@...len.de,
	netdev@...r.kernel.org, pablo@...filter.org, tgraf@...g.ch,
	amwang@...hat.com, kaber@...sh.net, paulmck@...ux.vnet.ibm.com,
	herbert@...dor.hengli.com.au
Subject: Re: [net-next PATCH V2 1/9] net: frag evictor, avoid killing warm
 frag queues

On Thu, 2012-11-29 at 23:17 +0100, Jesper Dangaard Brouer wrote:
> On Thu, 2012-11-29 at 12:44 -0500, David Miller wrote:

> It is VERY important that you understand/realize that throwing more
> memory at the problem is NOT the solution.  It a classical queue theory
> situation where the arrival rate is bigger than the processing
> capabilities. Thus, we must drop packets, or else the NIC will do it for
> us... for fragments we need do this "dropping" more intelligent.

Thats the typical head/tail drop choice. There is no bad/good choice.

Implementing head drop or tail drop is not a matter of only dealing with
regular traffic. We also can face DOS attacks, with packets of different
sizes and have to choose a compromise.

For example, it seems we have no protection against an attack using
small frags (but big truesize). So a particular 'packet' could consume
all the truesize we allowed for the whole frags queue.

skb_try_coalesce() cant always deal with this kind of thing.

Basically, thats why using frags in the first place is a bad choice.

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