linux-kernel - Re: [PATCH V2 1/1] mm/slub: release kobject if kobject_init_and_add failed in sysfs_slab

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Date:   Sun, 14 Aug 2022 21:48:54 +0800
From:   haoxin <xhao@...ux.alibaba.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
        roman.gushchin@...ux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/1] mm/slub: release kobject if kobject_init_and_add
 failed in sysfs_slab_add


在 2022/8/14 下午4:05, Hyeonggon Yoo 写道:
> On Thu, Aug 11, 2022 at 03:18:44PM +0800, Xin Hao wrote:
>> In kobject_init_and_add() function, the refcount is setted by calling
>> kobject_init() function, regardless of whether the return value is zero
>> or not, therefore, we must call kobject_del(&s->kobj) to prevent memory
>> of s->kobj is leaked.
> TL;DR: IIUC current code works just fine
>
> After thinking more, I don't think the memory leak you said exist.
> The space for s->kobj is freed in create_cache() when __kmem_cache_create() failed.

Yes, Agree what you explain,  but  in slab_sysfs_init() function, it 
also call slab_sysfs_add() and  there no other function to release it,  
so i think the memory leak still exist.

> The situation here is:
>
> create_cache() {
> 	s = kmem_cache_alloc(kmem_cache, GFP_KERNEL)
> 	err = __kmem_cache_create()
> 	if (err)
> 		goto out_free_cache;
> 	
> out_free_cache:
> 	kmem_cache_free(s) // s is freed here (including its kobject)
> 	[...]
> }
>
> __kmem_cache_create() {
> 	[...]
>
> 	err = sysfs_slab_add();
> 	if (err) {
> 		__kmem_cache_release(s);
> 		return err;
> 	}
> }
>
> The primary goal of kobject_put() is to call release() function
> of kobj_type (when reference becomes zero), which is kmem_cache_release().
>
> kmem_cache_release() {
> 	__kmem_cache_release(s)
> 	kfree_const(s->name)
> 	kmem_cache_free(s)
> }
>
> But when slab_sysfs_add() failed, __kmem_cache_release() and
> create_cache() releases resources related to the cache.
> (Also its name is freed in kmem_cache_create_usercopy().)
>
> So IIUC current code works just fine!
>
>> Signed-off-by: Xin Hao <xhao@...ux.alibaba.com>
>> ---
>>   mm/slub.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index b1281b8654bd..940a3f52e07c 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5981,19 +5981,18 @@ static int sysfs_slab_add(struct kmem_cache *s)
>>   
>>   	err = sysfs_create_group(&s->kobj, &slab_attr_group);
>>   	if (err)
>> -		goto out_del_kobj;
>> +		goto out;
>>   
>>   	if (!unmergeable) {
>>   		/* Setup first alias */
>>   		sysfs_slab_alias(s, s->name);
>>   	}
>> +	return err;
>>   out:
>>   	if (!unmergeable)
>>   		kfree(name);
>> +	kobject_put(&s->kobj);
>>   	return err;
>> -out_del_kobj:
>> -	kobject_del(&s->kobj);
> So related resources are released in create_cache(), instead of by
> calling kobject_put().
>
> But kobject_del() is still needed because it should unlink kobject
> hierarchy when kobject_add() succeeded but sysfs_create_group() failed!
>
>> -	goto out;
>>   }
>>   
>>   void sysfs_slab_unlink(struct kmem_cache *s)
>> -- 
>> 2.31.0
>>