Re: [PATCH v8 1/3] mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/4/15 1:56, Mike Kravetz wrote:
> On 4/8/22 06:53, Naoya Horiguchi wrote:
>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>
>> There is a race condition between memory_failure_hugetlb() and hugetlb
>> free/demotion, which causes setting PageHWPoison flag on the wrong page.
>> The one simple result is that wrong processes can be killed, but another
>> (more serious) one is that the actual error is left unhandled, so no one
>> prevents later access to it, and that might lead to more serious results
>> like consuming corrupted data.
>>
>> Think about the below race window:
>>
>>   CPU 1                                   CPU 2
>>   memory_failure_hugetlb
>>   struct page *head = compound_head(p);
>>                                           hugetlb page might be freed to
>>                                           buddy, or even changed to another
>>                                           compound page.
>>
>>   get_hwpoison_page -- page is not what we want now...
>>
>> The current code first does prechecks roughly and then reconfirms
>> after taking refcount, but it's found that it makes code overly
>> complicated, so move the prechecks in a single hugetlb_lock range.
>>
>> A newly introduced function, try_memory_failure_hugetlb(), always
>> takes hugetlb_lock (even for non-hugetlb pages).  That can be
>> improved, but memory_failure() is rare in principle, so should
>> not be a big problem.
...
> 
> The above code works as designed, but may be a bit confusing.  If HPageFreed()
> we KNOW ref count is zero, so no need to even call get_page_unless_zero() as
> it will always return false in this case.  It might be more clear if written
> as separate else if statements such as:
> 
> 	} else if (HPageFreed(head)) {
> 		ret = 0;
> 	} else if (HPageMigratable(head)) {
> 		ret = get_page_unless_zero(head);
> 		if (ret)
> 			count_increased = true;
> 

This code here is consistent with the logic in get_hwpoison_huge_page. If change is required,
they might need to be changed together.

BTW: They look a bit confusing for me at first but I get used to it later. ;)

Thanks!

> Not insisting this be changed.  Just easier to understand IMO.
> 
> Again, thanks for your work on this!
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
>