netdev - Re: [PATCH net] inet: limit length of fragment queue hash table bucket lists

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Message-Id: <20130319.100324.927922515830950770.davem@davemloft.net>
Date:	Tue, 19 Mar 2013 10:03:24 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com, jbrouer@...hat.com
Subject: Re: [PATCH net] inet: limit length of fragment queue hash table
 bucket lists

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Fri, 15 Mar 2013 22:32:30 +0100

> This patch introduces a constant limit of the fragment queue hash
> table bucket list lengths. Currently the limit 128 is choosen somewhat
> arbitrary and just ensures that we can fill up the fragment cache with
> empty packets up to the default ip_frag_high_thresh limits. It should
> just protect from list iteration eating considerable amounts of cpu.
> 
> If we reach the maximum length in one hash bucket a warning is printed.
> This is implemented on the caller side of inet_frag_find to distinguish
> between the different users of inet_fragment.c.
> 
> I dropped the out of memory warning in the ipv4 fragment lookup path,
> because we already get a warning by the slab allocator.
> 
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Jesper Dangaard Brouer <jbrouer@...hat.com>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

This looks mostly fine to me, Eric could you give it a quick review?

Although one comment from me:

> +/* averaged:
> + * max_depth = default ipfrag_high_thresh / INETFRAGS_HASHSZ /
> + *	       rounded up (SKB_TRUELEN(0) + sizeof(struct ipq or
> + *	       struct frag_queue))
> + */
> +#define INETFRAGS_MAXDEPTH		128

If we deem this to be the ideal formula, maybe we can maintain it
accurately and very cheaply at run time.  We'd do this by adding a
handler for the ipfrag_high_thresh sysctl, and use that to recalculate
the maxdepth any time ipfrag_high_thresh is changed by the user.
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