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

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Message-ID: <23251c74-cc50-012c-409f-c45117b52b16@huawei.com>
Date: Thu, 13 Feb 2025 11:20:03 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
CC: <tglx@...utronix.de>, <mingo@...hat.com>, <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, <hpa@...or.com>, <akpm@...ux-foundation.org>,
	<linux-edac@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <baolin.wang@...ux.alibaba.com>,
	<tianruidong@...ux.alibaba.com>, <tony.luck@...el.com>, <bp@...en8.de>,
	"nao.horiguchi@...il.com" <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1 4/4] mm/hwpoison: Fix incorrect "not recovered" report
 for recovered clean pages

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@...ux.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?

> 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?

> 
> 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.

Thanks.
.