Re: [PATCH v1] mm,hwpoison: set PG_hwpoison for busy hugetlb pages

[Mike Kravetz <mike.kravetz@oracle.com>]

On 5/11/22 19:54, Miaohe Lin wrote:
> On 2022/5/12 2:35, Mike Kravetz wrote:
>> On 5/11/22 08:19, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>>
>>> If memory_failure() fails to grab page refcount on a hugetlb page
>>> because it's busy, it returns without setting PG_hwpoison on it.
>>> This not only loses a chance of error containment, but breaks the rule
>>> that action_result() should be called only when memory_failure() do
>>> any of handling work (even if that's just setting PG_hwpoison).
>>> This inconsistency could harm code maintainability.
>>>
>>> So set PG_hwpoison and call hugetlb_set_page_hwpoison() for such a case.
> 
> I'm sorry but where is hugetlb_set_page_hwpoison() defined and used ? I can't find it.
> 
>>>
>>> Fixes: 405ce051236c ("mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()")
>>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>> ---
>>>  include/linux/mm.h  | 1 +
>>>  mm/memory-failure.c | 8 ++++----
>>>  2 files changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index d446e834a3e5..04de0c3e4f9f 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -3187,6 +3187,7 @@ enum mf_flags {
>>>  	MF_MUST_KILL = 1 << 2,
>>>  	MF_SOFT_OFFLINE = 1 << 3,
>>>  	MF_UNPOISON = 1 << 4,
>>> +	MF_NO_RETRY = 1 << 5,
>>>  };
>>>  extern int memory_failure(unsigned long pfn, int flags);
>>>  extern void memory_failure_queue(unsigned long pfn, int flags);
>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>> index 6a28d020a4da..e3269b991016 100644
>>> --- a/mm/memory-failure.c
>>> +++ b/mm/memory-failure.c
>>> @@ -1526,7 +1526,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>>>  			count_increased = true;
>>>  	} else {
>>>  		ret = -EBUSY;
>>> -		goto out;
>>> +		if (!(flags & MF_NO_RETRY))
>>> +			goto out;
>>>  	}
>>
>> Hi Naoya,
>>
>> We are in the else block because !HPageFreed() and !HPageMigratable().
>> IIUC, this likely means the page is isolated.  One common reason for isolation
>> is migration.  So, the page could be isolated and on a list for migration.
>>
>> I took a quick look at the hugetlb migration code and did not see any checks
>> for PageHWPoison after a hugetlb page is isolated.  I could have missed
>> something?  If there are no checks, we will read the PageHWPoison page
>> in kernel mode while copying to the migration target.
>>
>> Is this an issue?  Is is something we need to be concerned with?  Memory
>> errors can happen at any time, and gracefully handling them is best effort.
> 
> It seems HWPoison hugetlb page will still be accessed before this patch. Can we do a
> get_page_unless_zero first here to ensure that hugetlb page migration should fail due
> to this extra page reference and thus not access the page content? If hugetlb page is
> already freezed, corrupted memory will still be consumed though. :(

Right.  This potential issue was not introduced with this patch.
Also, I am not sure but it might be an issue with non-hugetlb pages as well.

As mentioned, memory error handling is a best effort.  Since errors can
happen at any time, we can not handle all cases.  Or, you could spend the
rest of your life trying. :)

The question is, should we worry about errors that happen when a page is
isolated for migration?

-- 
Mike Kravetz