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Date:   Tue, 11 Aug 2020 09:50:10 +0800
From:   Abel Wu <wuyun.wu@...wei.com>
To:     David Rientjes <rientjes@...gle.com>
CC:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <liu.xiang6@....com.cn>,
        "open list:SLAB ALLOCATOR" <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/slub: remove useless kmem_cache_debug



On 2020/8/11 9:29, Abel Wu wrote:
> 
> 
> On 2020/8/11 3:44, David Rientjes wrote:
>> On Mon, 10 Aug 2020, wuyun.wu@...wei.com wrote:
>>
>>> From: Abel Wu <wuyun.wu@...wei.com>
>>>
>>> The commit below is incomplete, as it didn't handle the add_full() part.
>>> commit a4d3f8916c65 ("slub: remove useless kmem_cache_debug() before remove_full()")
>>>
>>> Signed-off-by: Abel Wu <wuyun.wu@...wei.com>
>>> ---
>>>  mm/slub.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index fe81773..0b021b7 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -2182,7 +2182,8 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>>>  		}
>>>  	} else {
>>>  		m = M_FULL;
>>> -		if (kmem_cache_debug(s) && !lock) {
>>> +#ifdef CONFIG_SLUB_DEBUG
>>> +		if (!lock) {
>>>  			lock = 1;
>>>  			/*
>>>  			 * This also ensures that the scanning of full
>>> @@ -2191,6 +2192,7 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>>>  			 */
>>>  			spin_lock(&n->list_lock);
>>>  		}
>>> +#endif
>>>  	}
>>>
>>>  	if (l != m) {
>>
>> This should be functionally safe, I'm wonder if it would make sense to 
>> only check for SLAB_STORE_USER here instead of kmem_cache_debug(), 
>> however, since that should be the only context in which we need the 
>> list_lock for add_full()?  It seems more explicit.
>> .
>>
> Yes, checking for SLAB_STORE_USER here can also get rid of noising macros.
> I will resend the patch later.
> 
> Thanks,
> 	Abel
> .
> 
Wait... It still needs CONFIG_SLUB_DEBUG to wrap around, but can avoid
locking overhead when SLAB_STORE_USER is not set (as what you said).
I will keep the CONFIG_SLUB_DEBUG in my new patch.

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