Re: [PATCH v1 4/4] mm/hwpoison: Fix incorrect "not recovered" report for recovered clean pages

[Shuai Xue <xueshuai@linux.alibaba.com>]

在 2025/2/12 16:09, Miaohe Lin 写道:
> On 2025/2/11 14:02, Shuai Xue wrote:
>> When an uncorrected memory error is consumed there is a race between
>> the CMCI from the memory controller reporting an uncorrected error
>> with a UCNA signature, and the core reporting and SRAR signature
>> machine check when the data is about to be consumed.
>>
>> If the CMCI wins that race, the page is marked poisoned when
>> uc_decode_notifier() calls memory_failure(). For dirty pages,
>> memory_failure() invokes try_to_unmap() with the TTU_HWPOISON flag,
>> converting the PTE to a hwpoison entry. However, for clean pages, the
>> TTU_HWPOISON flag is cleared, leaving the PTE unchanged and not converted
>> to a hwpoison entry. Consequently, for an unmapped dirty page, the PTE is
>> marked as a hwpoison entry allowing kill_accessing_process() to:
>>
>> - call walk_page_range() and return 1
>> - call kill_proc() to make sure a SIGBUS is sent
>> - return -EHWPOISON to indicate that SIGBUS is already sent to the process
>>    and kill_me_maybe() doesn't have to send it again.
>>
>> Conversely, for clean pages where PTE entries are not marked as hwpoison,
>> kill_accessing_process() returns -EFAULT, causing kill_me_maybe() to send a
>> SIGBUS.
>>
>> Console log looks like this:
>>
>>      Memory failure: 0x827ca68: corrupted page was clean: dropped without side effects
>>      Memory failure: 0x827ca68: recovery action for clean LRU page: Recovered
>>      Memory failure: 0x827ca68: already hardware poisoned
>>      mce: Memory error not recovered
>>
>> To fix it, return -EHWPOISON if no hwpoison PTE entry is found, preventing
>> an unnecessary SIGBUS.
> 
> Thanks for your patch.
> 
>>
>> Fixes: 046545a661af ("mm/hwpoison: fix error page recovered but reported "not recovered"")
>> Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
>> ---
>>   mm/memory-failure.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 995a15eb67e2..f9a6b136a6f0 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -883,10 +883,9 @@ static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
>>   			      (void *)&priv);
>>   	if (ret == 1 && priv.tk.addr)
>>   		kill_proc(&priv.tk, pfn, flags);
>> -	else
>> -		ret = 0;
>>   	mmap_read_unlock(p->mm);
>> -	return ret > 0 ? -EHWPOISON : -EFAULT;
>> +
>> +	return ret >= 0 ? -EHWPOISON : -EFAULT;
> 
> IIUC, kill_accessing_process() is supposed to return -EHWPOISON to notify that SIGBUS is already
> sent to the process and kill_me_maybe() doesn't have to send it again. But with your change,
> kill_accessing_process() will return -EHWPOISON even if SIGBUS is not sent. Does this break
> the semantics of -EHWPOISON?

Yes, from the comment of kill_me_maybe(),

	 * -EHWPOISON from memory_failure() means that it already sent SIGBUS
	 * to the current process with the proper error info,
	 * -EOPNOTSUPP means hwpoison_filter() filtered the error event,

this patch break the comment.

But the defination of EHWPOISON is quite different from the comment.

  #define EHWPOISON	133	/* Memory page has hardware error */

As for this issue, returning 0 or EHWPOISON can both prevent a SIGBUS signal
from being sent in kill_me_maybe().

Which way do you prefer?

> 
> BTW I scanned the code of walk_page_range(). It seems with implementation of hwpoison_walk_ops
> walk_page_range() will only return 0 or 1, i.e. always >= 0. So kill_accessing_process() will always
> return -EHWPOISON if this patch is applied.
> 
> Correct me if I miss something.

Yes, you are right. Let's count the cases one by one:

1. clean page: try_to_remap(!TTU_HWPOISON), walk_page_range() will return 0 and
we should not send sigbus in kill_me_maybe().

2. dirty page:
2.1 MCE wins race
           CMCI:w/o Action Require         MCE: w/ Action Require
                                       TestSetPageHWPoison
       TestSetPageHWPoison
       return -EHWPOISON
                                       try_to_unmap(TTU_HWPOISON)
                                       kill_proc in hwpoison_user_mappings()

If MCE wins the race, because the flag of memory_fialure() called by CMCI is
not set as MF_ACTION_REQUIRED, everything goes well, kill_proc() will send
SIGBUS in hwpoison_user_mappings().

2.2 CMCI win
           CMCI:w/o Action Require         MCE: w/ Action Require
     TestSetPageHWPoison
     try_to_unmap(TTU_HWPOISON)
                                        walk_page_range() return 1 due to hwpoison PTE entry
                                        kill_proc in kill_accessing_process()

If the CMCI wins the race, we need to kill the process in
kill_accessing_process(). And if try_to_remap() success, everything goes well,
kill_proc() will send SIGBUS in kill_accessing_process().

But if try_to_remap() fails, the PTE entry will not be marked as hwpoison, and
walk_page_range() return 0 as case 1 clean page, NO SIGBUS will be sent.

In summary, hwpoison_walk_ops cannot distinguish between try_to_unmap failing
and causing the PTE entry not to be set to hwpoison, and a clean page that
originally does not have the PTE entry set to hwpoison.

+naoya for orginal patch intend.

Thanks.
Best Regard,
Shuai