Re: [PATCH V2 1/1] mm/slub: release kobject if kobject_init_and_add failed in sysfs_slab_add

[haoxin]

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@xxxxxxxxxxxxxxxxx>
> > ---
> >   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