Re: [PATCH] mm/memory-failure: fix hardware poison check in unpoison_memory()

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

On 7/17/23 5:39 PM, Naoya Horiguchi wrote:
> On Tue, Jul 18, 2023 at 09:14:09AM +0900, Naoya Horiguchi wrote:
>> On Mon, Jul 17, 2023 at 11:18:12AM -0700, Sidhartha Kumar wrote:
>>> It was pointed out[1] that using folio_test_hwpoison() is wrong
>>> as we need to check the indiviual page that has poison.
>>> folio_test_hwpoison() only checks the head page so go back to using
>>> PageHWPoison().
>>>
>>> Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>>> Fixes: a6fddef49eef ("mm/memory-failure: convert unpoison_memory() to folios")
>>> Cc: stable@vger.kernel.org #v6.4
>>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
>>>
>>> [1]: https://lore.kernel.org/lkml/ZLIbZygG7LqSI9xe@casper.infradead.org/
>>> ---
>>>   mm/memory-failure.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>> index 02b1d8f104d51..a114c8c3039cd 100644
>>> --- a/mm/memory-failure.c
>>> +++ b/mm/memory-failure.c
>>> @@ -2523,7 +2523,7 @@ int unpoison_memory(unsigned long pfn)
>>>   		goto unlock_mutex;
>>>   	}
>>>   
>>> -	if (!folio_test_hwpoison(folio)) {
>>> +	if (!PageHWPoison(p)) {
>>
>>
>> I don't think this works for hwpoisoned hugetlb pages that have PageHWPoison
>> set on the head page, rather than on the raw subpage. In the case of
>> hwpoisoned thps, PageHWPoison is set on the raw subpage, not on the head
>> pages.  (I believe this is not detected because no one considers the
>> scenario of unpoisoning hwpoisoned thps, which is a rare case).  Perhaps the
>> function is_page_hwpoison() would be useful for this purpose?
> 
> Sorry, I was wrong.  Checking PageHWPoison() is fine because the users of
> unpoison should know where the PageHWPoison is set via /proc/kpageflags.
> So this patch is OK to me after comments from other reviewers are resolved.
> 

Hi Naoya,

While taking a closer at the patch, later in unpoison_memory() there is 
also:

-               ret = TestClearPageHWPoison(page) ? 0 : -EBUSY;
+               ret = folio_test_clear_hwpoison(folio) ? 0 : -EBUSY;

I thought this folio conversion would be safe because page is the result 
of a compound_head() call but I'm wondering if the same issue exists 
here and we should be calling TestClearPageHWPoison() on the specific 
subpage by doing TestClearPageHWPoison(p).

Thanks,
Sidhartha Kumar

> Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> Thanks,
> Naoya Horiguchi
>