netdev - Re: rhashtable: Use __vmalloc with GFP

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Date:	Sat, 5 Dec 2015 15:03:54 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, phil@....cc, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, tgraf@...g.ch,
	fengguang.wu@...el.com, wfg@...ux.intel.com, lkp@...org
Subject: Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation

On Fri, Dec 04, 2015 at 04:53:34PM -0500, David Miller wrote:
> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Fri, 4 Dec 2015 22:39:56 +0800
> 
> > When an rhashtable user pounds rhashtable hard with back-to-back
> > insertions we may end up growing the table in GFP_ATOMIC context.
> > Unfortunately when the table reaches a certain size this often
> > fails because we don't have enough physically contiguous pages
> > to hold the new table.
> > 
> > Eric Dumazet suggested (and in fact wrote this patch) using
> > __vmalloc instead which can be used in GFP_ATOMIC context.
> > 
> > Reported-by: Phil Sutter <phil@....cc>
> > Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> > Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> Applied, thanks Herbert.

Sorry Dave but you'll have to revert this because I've been able
to trigger the following crash with the patch:

Testing concurrent rhashtable access from 50 threads
------------[ cut here ]------------
kernel BUG at ../mm/vmalloc.c:1337!
invalid opcode: 0000 [#1] PREEMPT SMP 

The reason is that because I was testing insertions with BH disabled,
and __vmalloc doesn't like that, even with GFP_ATOMIC.  As we
obviously want to continue to support rhashtable users inserting
entries with BH disabled, we'll have to look for an alternate
solution.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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