netdev - Re: [v2 PATCH] rhashtable: Add cap on number of elements in hash table

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Date:	Sat, 16 May 2015 18:08:43 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	johannes@...solutions.net, netdev@...r.kernel.org, kaber@...sh.net,
	tgraf@...g.ch, johannes.berg@...el.com
Subject: Re: [v2 PATCH] rhashtable: Add cap on number of elements in hash
 table

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Fri, 15 May 2015 11:30:47 +0800

> We currently have no limit on the number of elements in a hash table.
> This is a problem because some users (tipc) set a ceiling on the
> maximum table size and when that is reached the hash table may
> degenerate.  Others may encounter OOM when growing and if we allow
> insertions when that happens the hash table perofrmance may also
> suffer.
> 
> This patch adds a new paramater insecure_max_entries which becomes
> the cap on the table.  If unset it defaults to max_size * 2.  If
> it is also zero it means that there is no cap on the number of
> elements in the table.  However, the table will grow whenever the
> utilisation hits 100% and if that growth fails, you will get ENOMEM
> on insertion.
> 
> As allowing oversubscription is potentially dangerous, the name
> contains the word insecure.
> 
> Note that the cap is not a hard limit.  This is done for performance
> reasons as enforcing a hard limit will result in use of atomic ops
> that are heavier than the ones we currently use.
> 
> The reasoning is that we're only guarding against a gross over-
> subscription of the table, rather than a small breach of the limit.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Applied, thanks Herbert.
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