Re: [PATCH v2 1/3] mm/memory-failure.c: fix race with changing page compound again

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/3/15 2:20, Mike Kravetz wrote:
> On 3/11/22 23:46, Miaohe Lin wrote:
>> There is a race window where we got the compound_head, the hugetlb page
>> could be freed to buddy, or even changed to another compound page just
>> before we try to get hwpoison page. 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...
>>
>> If this race happens, just bail out. Also MF_MSG_DIFFERENT_PAGE_SIZE is
>> introduced to record this event.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  include/linux/mm.h      |  1 +
>>  include/ras/ras_event.h |  1 +
>>  mm/memory-failure.c     | 12 ++++++++++++
>>  3 files changed, 14 insertions(+)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index c9bada4096ac..ef98cff2b253 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3253,6 +3253,7 @@ enum mf_action_page_type {
>>  	MF_MSG_BUDDY,
>>  	MF_MSG_DAX,
>>  	MF_MSG_UNSPLIT_THP,
>> +	MF_MSG_DIFFERENT_PAGE_SIZE,
>>  	MF_MSG_UNKNOWN,
>>  };
>>  
>> diff --git a/include/ras/ras_event.h b/include/ras/ras_event.h
>> index d0337a41141c..1e694fd239b9 100644
>> --- a/include/ras/ras_event.h
>> +++ b/include/ras/ras_event.h
>> @@ -374,6 +374,7 @@ TRACE_EVENT(aer_event,
>>  	EM ( MF_MSG_BUDDY, "free buddy page" )				\
>>  	EM ( MF_MSG_DAX, "dax page" )					\
>>  	EM ( MF_MSG_UNSPLIT_THP, "unsplit thp" )			\
>> +	EM ( MF_MSG_DIFFERENT_PAGE_SIZE, "different page size" )	\
>>  	EMe ( MF_MSG_UNKNOWN, "unknown page" )
>>  
>>  /*
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 5444a8ef4867..dabecd87ad3f 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -733,6 +733,7 @@ static const char * const action_page_types[] = {
>>  	[MF_MSG_BUDDY]			= "free buddy page",
>>  	[MF_MSG_DAX]			= "dax page",
>>  	[MF_MSG_UNSPLIT_THP]		= "unsplit thp",
>> +	[MF_MSG_DIFFERENT_PAGE_SIZE]	= "different page size",
>>  	[MF_MSG_UNKNOWN]		= "unknown page",
>>  };
>>  
>> @@ -1534,6 +1535,17 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
>>  	}
>>  
>>  	lock_page(head);
>> +
>> +	/**
>> +	 * The page could have changed compound pages due to race window.
>> +	 * If this happens just bail out.
>> +	 */
>> +	if (!PageHuge(p) || compound_head(p) != head) {
>> +		action_result(pfn, MF_MSG_DIFFERENT_PAGE_SIZE, MF_IGNORED);
>> +		res = -EBUSY;
> 
> We have discussed this race in other versions of the patch.  When we encounter
> the race, we have likely marked poison on the wrong page.  Correct?
> 

Many thanks for comment.
I assume that Naoya's patch "mm/hwpoison: set PageHWPoison after taking page lock
in memory_failure_hugetlb()" would set the PageHWPoison after the above check.
So I think the below operation is not needed as PageHWPoison is not set yet.
Does this makes sense for you?

Thanks.

> Instead of printing a "different page size", would it be better to perhaps:
> - Print a message that wrong page may be marked for poison?
> - Clear the poison flag in the "head page" previously set?
>