linux-kernel - Re: [PATCH v3 15/28] slub: always get the cache from its page in kfree

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Message-ID: <4FC4F273.6060807@parallels.com>
Date:	Tue, 29 May 2012 19:59:47 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
	<linux-mm@...ck.org>, <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Greg Thelen <gthelen@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, <devel@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v3 15/28] slub: always get the cache from its page in
 kfree

On 05/29/2012 06:42 PM, Christoph Lameter wrote:
> On Fri, 25 May 2012, Glauber Costa wrote:
>
>> struct page already have this information. If we start chaining
>> caches, this information will always be more trustworthy than
>> whatever is passed into the function
>
> Yes but the lookup of the page struct also costs some cycles. SLAB in
> !NUMA mode and SLOB avoid these lookups and can improve their freeing
> speed because of that.

But for our case, I don't really see a way around. What I can do, is 
wrap it further, so when we're not using it, code goes exactly the same 
way as before, instead of always calculating the page. Would it be better?

>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0eb9e72..640872f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2598,10 +2598,14 @@ redo:
>>   void kmem_cache_free(struct kmem_cache *s, void *x)
>>   {
>>   	struct page *page;
>> +	bool slab_match;
>>
>>   	page = virt_to_head_page(x);
>>
>> -	slab_free(s, page, x, _RET_IP_);
>> +	slab_match = (page->slab == s) | slab_is_parent(page->slab, s);
>> +	VM_BUG_ON(!slab_match);
>
> Why add a slab_match bool if you do not really need it?

style. I find aux variables a very human readable way to deal with the 
80-col limitation.
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