Re: mm, something wring in page_lock_anon_vma_read()?

[Xishi Qiu]

On 2017/5/20 6:00, Hugh Dickins wrote:

> On Fri, 19 May 2017, Xishi Qiu wrote:
>> On 2017/5/19 16:52, Xishi Qiu wrote:
>>> On 2017/5/18 17:46, Xishi Qiu wrote:
>>>
>>>> Hi, my system triggers this bug, and the vmcore shows the anon_vma seems be freed.
>>>> The kernel is RHEL 7.2, and the bug is hard to reproduce, so I don't know if it
>>>> exists in mainline, any reply is welcome!
>>>>
>>>
>>> When we alloc anon_vma, we will init the value of anon_vma->root,
>>> so can we set anon_vma->root to NULL when calling
>>> anon_vma_free -> kmem_cache_free(anon_vma_cachep, anon_vma);
>>>
>>> anon_vma_free()
>>> 	...
>>> 	anon_vma->root = NULL;
>>> 	kmem_cache_free(anon_vma_cachep, anon_vma);
>>>
>>> I find if we do this above, system boot failed, why?
>>>
>>
>> If anon_vma was freed, we should not to access the root_anon_vma, because it maybe also
>> freed(e.g. anon_vma == root_anon_vma), right?
>>
>> page_lock_anon_vma_read()
>> 	...
>> 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
>> 	root_anon_vma = ACCESS_ONCE(anon_vma->root);
>> 	if (down_read_trylock(&root_anon_vma->rwsem)) {  // it's not safe
>> 	...
>> 	if (!atomic_inc_not_zero(&anon_vma->refcount)) {  // check anon_vma was not freed
>> 	...
>> 	anon_vma_lock_read(anon_vma);  // it's safe
>> 	...
> 
> You're ignoring the rcu_read_lock() on entry to page_lock_anon_vma_read(),
> and the SLAB_DESTROY_BY_RCU (recently renamed SLAB_TYPESAFE_BY_RCU) nature
> of the anon_vma_cachep kmem cache.  It is not safe to muck with anon_vma->
> root in anon_vma_free(), others could still be looking at it.
> 
> Hugh
> 

Hi Hugh,

Thanks for your reply.

SLAB_DESTROY_BY_RCU will let it call call_rcu() in free_slab(), but if the
anon_vma *reuse* by someone again, access root_anon_vma is not safe, right?

e.g. if I clean the root pointer before free it, then access root_anon_vma
in page_lock_anon_vma_read() is NULL pointer access, right?

anon_vma_free()
	...
	anon_vma->root = NULL;
	kmem_cache_free(anon_vma_cachep, anon_vma);
	...

Thanks,
Xishi Qiu

> .
>