netdev - Re: TUN problems (regression?)

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Date:	Fri, 21 Dec 2012 12:26:56 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Eric Dumazet <erdnetdev@...il.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	Paul Moore <pmoore@...hat.com>, netdev@...r.kernel.org
Subject: Re: TUN problems (regression?)

On 12/21/2012 11:39 AM, Eric Dumazet wrote:
> On Fri, 2012-12-21 at 11:32 +0800, Jason Wang wrote:
>> On 12/21/2012 07:50 AM, Stephen Hemminger wrote:
>>> On Thu, 20 Dec 2012 15:38:17 -0800
>>> Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>
>>>> On Thu, 2012-12-20 at 18:16 -0500, Paul Moore wrote:
>>>>> [CC'ing netdev in case this is a known problem I just missed ...]
>>>>>
>>>>> Hi Jason,
>>>>>
>>>>> I started doing some more testing with the multiqueue TUN changes and I ran 
>>>>> into a problem when running tunctl: running it once w/o arguments works as 
>>>>> expected, but running it a second time results in failure and a 
>>>>> kmem_cache_sanity_check() failure.  The problem appears to be very repeatable 
>>>>> on my test VM and happens independent of the LSM/SELinux fixup patches.
>>>>>
>>>>> Have you seen this before?
>>>>>
>>>> Obviously code in tun_flow_init() is wrong...
>>>>
>>>> static int tun_flow_init(struct tun_struct *tun)
>>>> {
>>>>         int i;
>>>>
>>>>         tun->flow_cache = kmem_cache_create("tun_flow_cache",
>>>>                                             sizeof(struct tun_flow_entry), 0, 0,
>>>>                                             NULL);
>>>>         if (!tun->flow_cache)
>>>>                 return -ENOMEM;
>>>> ...
>>>> }
>>>>
>>>>
>>>> I have no idea why we would need a kmem_cache per tun_struct,
>>>> and why we even need a kmem_cache.
>>> Normally flow malloc/free should be good enough.
>>> It might make sense to use private kmem_cache if doing hlist_nulls.
>>>
>>>
>>> Acked-by: Stephen Hemminger <shemminger@...tta.com>
>> Should be at least a global cache, I thought I can get some speed-up by
>> using kmem_cache.
>>
>> Acked-by: Jason Wang <jasowang@...hat.com>
> Was it with SLUB or SLAB ?
>
> Using generic kmalloc-64 is better than a dedicated kmem_cache of 48
> bytes per object, as we guarantee each object is on a single cache line.
>
>

Right, thanks for the explanation.

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