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

[Miaohe Lin <linmiaohe@huawei.com>]

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.

Thanks.
.

> 
> 
>>>
>>> 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.
>>
>> Is it possible current process is not the one accessing the hwpoisoned page? E.g. memory_failure
>> is deferred and called from kworker context or something like that. If it's possible, this is
>> another scene needs to be considered.
> 
> Yes, it possibale.
> 
> But kill_accessing_process() will only be called with MF_ACTION_REQUIRED.
> MF_ACTION_REQUIRED indates that current process is exactly the one accesing the
> poison data.
> 
> Fox x86 platform, GHES driver may queue a kwoker to defer memory_failure() with
> flag=0.  So kill_accessing_process() will not be called in such case.
> 
> Thanks.
> Shuai
> .