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

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Message-ID: <Yvis1knnMomoeuAx@hyeyoo>
Date:   Sun, 14 Aug 2022 17:05:42 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Xin Hao <xhao@...ux.alibaba.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

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.

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
> 

-- 
Thanks,
Hyeonggon