linux-kernel - Re: [PATCH 2/2] tuntap: limit the number of flow caches

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Date:	Thu, 24 Jan 2013 11:22:52 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	David Miller <davem@...emloft.net>
CC:	mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stephen@...workplumber.org
Subject: Re: [PATCH 2/2] tuntap: limit the number of flow caches

On 01/24/2013 02:50 AM, David Miller wrote:
> From: Jason Wang <jasowang@...hat.com>
> Date: Wed, 23 Jan 2013 21:59:13 +0800
>
>> We create new flow caches when a new flow is identified by tuntap, This may lead
>> some issues:
>>
>> - userspace may produce a huge amount of short live flows to exhaust host memory
>> - the unlimited number of flow caches may produce a long list which increase the
>>   time in the linear searching
>>
>> Solve this by introducing a limit of total number of flow caches.
>>
>> Cc: Stephen Hemminger <stephen@...workplumber.org>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
> Applied, but really flow caches are an extremely broken concept especially
> when external entities control the population of such caches.
>
> We removed the routing cache of the ipv4 networking code exactly because
> this kind of crap does not work at all.
>
> Next you're going to have to add a delicately managed garbage
> collection scheme for this tuntap flow cache, and that will be tuned
> endlessly, when the real issue is that fundamentally this does not
> work.
>
> Instead, make the full lookup scale properly and use appropriate data
> structures.  It won't be as fast as a simple hash table demux, but
> it'll actually be immune to growth issues and DoS attacks and give
> consistent and repeatable lookup performance regardless of traffic
> patterns.

Ok, I will rework it in 3.9.

Thanks
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