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

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

在 2025/2/14 14:54, Miaohe Lin 写道:
> On 2025/2/13 14:59, Shuai Xue wrote:
>>
>>
>> 在 2025/2/13 11:20, Miaohe Lin 写道:
>>> On 2025/2/12 21:55, Shuai Xue wrote:
>>>>
>>>>
>>>> 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
>>>
>>> Do you mean try_to_unmap?
>>
>> Yes, sorry for the typo.
>>>
>>>> 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.
>>>
>>> If try_to_unmap() fails, the PTE entry will still point to the dirty page. Then in
>>> check_hwpoisoned_entry(), we will have pfn == poisoned_pfn. So walk_page_range()
>>> will return 1 in this case. Or am I miss something?
>>>
>>
>> You’re right; I overlooked the pte_present() branch.
>>
>> Therefore, in the walk_page_range() function:
>> - It returns 0 when the poison page is a clean page.
>> - It returns 1 when CMCI wins, regardless of whether try_to_unmap succeeds
>>    or fails.
>>
>> Then the patch will be like:
>>
>> @@ -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 : 0;
>>
>> Here, returning 0 indicates that memory_failure() successfully handled the
>> error by dropping the clean page.
> 
> I'm not sure whether there's another scene that can make walk_page_range() returns 0. But if the
> only reason for walk_page_range() returning 0 is the poison page is a clean page and it's dropped,
> then this modification should be appropriate. With this change, the callers never send SIGBUS now.
> They might need to be changed too.

Yes, if memory_failure() successfully handled the error, the caller should be nothing.

Thanks.
Shuai