netdev - Re: [PATCH net-next] pkt_sched: fq: better control of DDOS traffic

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Message-Id: <20150202.181834.2201965486446161539.davem@davemloft.net>
Date:	Mon, 02 Feb 2015 18:18:34 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] pkt_sched: fq: better control of DDOS traffic

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 30 Jan 2015 06:06:12 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> FQ has a fast path for skb attached to a socket, as it does not
> have to compute a flow hash. But for other packets, FQ being non
> stochastic means that hosts exposed to random Internet traffic
> can allocate million of flows structure (104 bytes each) pretty
> easily. Not only host can OOM, but lookup in RB trees can take
> too much cpu and memory resources.
> 
> This patch adds a new attribute, orphan_mask, that is adding
> possibility of having a stochastic hash for orphaned skb.
> 
> Its default value is 1024 slots.
> 
> This patch also handles the specific case of SYNACK messages:
> 
> They are attached to the listener socket, and therefore all map
> to a single hash bucket. If listener have set SO_MAX_PACING_RATE,
> hoping to have new accepted socket inherit this rate, SYNACK
> might be paced and even dropped.
> 
> This is very similar to an internal patch Google have used more
> than one year.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Can you document the mask value a little bit more?

For example, I don't understand why "(1024 - 1) << 1" means 1024
slots just from looking at this change.

Thanks.
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