Re: [PATCH] mm: hugetlb: fix a race between memory-failure/soft_offline and gather_surplus_pages

[Mike Kravetz]

On 4/21/21 1:33 AM, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Wed, Apr 21, 2021 at 10:03:34AM +0200, Michal Hocko wrote:
>> [Cc Naoya]
>>
>> On Wed 21-04-21 14:02:59, Muchun Song wrote:
>>> The possible bad scenario:
>>>
>>> CPU0:                           CPU1:
>>>
>>>                                 gather_surplus_pages()
>>>                                   page = alloc_surplus_huge_page()
>>> memory_failure_hugetlb()
>>>   get_hwpoison_page(page)
>>>     __get_hwpoison_page(page)
>>>       get_page_unless_zero(page)
>>>                                   zero = put_page_testzero(page)
>>>                                   VM_BUG_ON_PAGE(!zero, page)
>>>                                   enqueue_huge_page(h, page)
>>>   put_page(page)
>>>
>>> The refcount can possibly be increased by memory-failure or soft_offline
>>> handlers, we can trigger VM_BUG_ON_PAGE and wrongly add the page to the
>>> hugetlb pool list.
>>
>> The hwpoison side of this looks really suspicious to me. It shouldn't
>> really touch the reference count of hugetlb pages without being very
>> careful (and having hugetlb_lock held).
> 
> I have the same feeling, there is a window where a hugepage is refcounted
> during converting from buddy free pages into free hugepage, so refcount
> alone is not enough to prevent the race.  hugetlb_lock is retaken after
> alloc_surplus_huge_page returns, so simply holding hugetlb_lock in
> get_hwpoison_page() seems not work.  Is there any status bit to show that a
> hugepage is just being initialized (not in free hugepage pool or in use)?
> 

It seems we can also race with the code that makes a compound page a
hugetlb page.  The memory failure code could be called after allocating
pages from buddy and before setting compound page DTOR.  So, the memory
handling code will process it as a compound page.

Just thinking that this may not be limited to the hugetlb specific memory
failure handling?
-- 
Mike Kravetz