netdev - Re: rhashtable: Add cap on number of elements in hash table

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Message-Id: <20150514.234615.235930228362522399.davem@davemloft.net>
Date:	Thu, 14 May 2015 23:46:15 -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: rhashtable: Add cap on number of elements in hash table

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Fri, 15 May 2015 11:06:23 +0800

> On Thu, May 14, 2015 at 10:22:17PM -0400, David Miller wrote:
>>
>> In my opinion, up to at least 2 X max_size, it's safe to allow the
>> insert.  Assuming a well choosen hash function and a roughly even
>> distribution.
> 
> OK I can make it 2 x max_size/table size.

The rest of my email after what you quoted was intended to get one
to consider this issue generally.  :-)

We wouldn't fail these inserts in any other hash table in the kernel.

Would we stop making new TCP sockets if the TCP ehash chains are 3
entries deep?  4?  5?  The answer to all of those is of course no
for any hash chain length of N whatsoever.

This new rhashtable behavior would be the default, and I seriously
doubt that's a behavior people who use a hash table, generally
speaking, desire or want.

Should there perhaps be hard protections for _extremely_ long hash
chains?  Sure, I'm willing to entertain that kind of idea.  But I
would do so at the very far end of the spectrum.  To the point where
the hash table is degenerating into a linked list.
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