Re: [PATCH V2] MCE: fix an error of mce_bad_pages statistics

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Mon, Dec 10, 2012 at 07:16:50PM +0800, Xishi Qiu wrote:
>On 2012/12/10 18:47, Simon Jeons wrote:
>
>> On Mon, 2012-12-10 at 17:06 +0800, Xishi Qiu wrote:
>>> On 2012/12/10 16:33, Wanpeng Li wrote:
>>>
>>>> On Fri, Dec 07, 2012 at 02:11:02PM -0800, Andrew Morton wrote:
>>>>> On Fri, 7 Dec 2012 16:48:45 +0800
>>>>> Xishi Qiu <qiuxishi@huawei.com> wrote:
>>>>>
>>>>>> On x86 platform, if we use "/sys/devices/system/memory/soft_offline_page" to offline a
>>>>>> free page twice, the value of mce_bad_pages will be added twice. So this is an error,
>>>>>> since the page was already marked HWPoison, we should skip the page and don't add the
>>>>>> value of mce_bad_pages.
>>>>>>
>>>>>> $ cat /proc/meminfo | grep HardwareCorrupted
>>>>>>
>>>>>> soft_offline_page()
>>>>>> 	get_any_page()
>>>>>> 		atomic_long_add(1, &mce_bad_pages)
>>>>>>
>>>>>> ...
>>>>>>
>>>>>> --- a/mm/memory-failure.c
>>>>>> +++ b/mm/memory-failure.c
>>>>>> @@ -1582,8 +1582,11 @@ int soft_offline_page(struct page *page, int flags)
>>>>>>  		return ret;
>>>>>>
>>>>>>  done:
>>>>>> -	atomic_long_add(1, &mce_bad_pages);
>>>>>> -	SetPageHWPoison(page);
>>>>>>  	/* keep elevated page count for bad page */
>>>>>> +	if (!PageHWPoison(page)) {
>>>>>> +		atomic_long_add(1, &mce_bad_pages);
>>>>>> +		SetPageHWPoison(page);
>>>>>> +	}
>>>>>> +
>>>>>>  	return ret;
>>>>>>  }
>>>>>
>>>>> A few things:
>>>>>
>>>>> - soft_offline_page() already checks for this case:
>>>>>
>>>>> 	if (PageHWPoison(page)) {
>>>>> 		unlock_page(page);
>>>>> 		put_page(page);
>>>>> 		pr_info("soft offline: %#lx page already poisoned\n", pfn);
>>>>> 		return -EBUSY;
>>>>> 	}
>>>>>
>>>>>  so why didn't this check work for you?
>>>>>
>>>>>  Presumably because one of the earlier "goto done" branches was
>>>>>  taken.  Which one, any why?
>>>>>
>>>>>  This function is an utter mess.  It contains six return points
>>>>>  randomly intermingled with three "goto done" return points.
>>>>>
>>>>>  This mess is probably the cause of the bug you have observed.  Can
>>>>>  we please fix it up somehow?  It *seems* that the design (lol) of
>>>>>  this function is "for errors, return immediately.  For success, goto
>>>>>  done".  In which case "done" should have been called "success".  But
>>>>>  if you just look at the function you'll see that this approach didn't
>>>>>  work.  I suggest it be converted to have two return points - one for
>>>>>  the success path, one for the failure path.  Or something.
>>>>>
>>>>> - soft_offline_huge_page() is a miniature copy of soft_offline_page()
>>>>>  and might suffer the same bug.
>>>>>
>>>>> - A cleaner, shorter and possibly faster implementation is
>>>>>
>>>>> 	if (!TestSetPageHWPoison(page))
>>>>> 		atomic_long_add(1, &mce_bad_pages);
>>>>>
>>>>
>>>> Hi Andrew,
>>>>
>>>> Since hwpoison bit for free buddy page has already be set in get_any_page, 
>>>> !TestSetPageHWPoison(page) will not increase mce_bad_pages count even for 
>>>> the first time.
>>>>
>>>> Regards,
>>>> Wanpeng Li
>>>>
>>>
>>> The poisoned page is isolated in bad_page(), I wonder whether it could be isolated
>>> immediately in soft_offline_page() and memory_failure()?
>>>
>>> buffered_rmqueue()
>>> 	prep_new_page()
>>> 		check_new_page()
>>> 			bad_page()
>> 
>> Do you mean else if(is_free_buddy_page(p)) branch is redundancy?
>> 
>
>Hi Simon,
>
>get_any_page() -> "else if(is_free_buddy_page(p))" branch is *not* redundancy.
>
>It is another topic, I mean since the page is poisoned, so why not isolate it
>from page buddy alocator in soft_offline_page() rather than in check_new_page().
>
>I find soft_offline_page() only migrate the page and mark HWPoison, the poisoned
>page is still managed by page buddy alocator.
>

Hi Xishi,

HWPoison delays any action on buddy allocator pages, handling can be safely postponed 
until a later time when the page might be referenced. By delaying, some transient errors 
may not reoccur or may be irrelevant.

Regards,
Wanpeng Li 

>>>
>>> Thanks
>>> Xishi Qiu
>>>
>>>>> - We have atomic_long_inc().  Use it?
>>>>>
>>>>> - Why do we have a variable called "mce_bad_pages"?  MCE is an x86
>>>>>  concept, and this code is in mm/.  Lights are flashing, bells are
>>>>>  ringing and a loudspeaker is blaring "layering violation" at us!
>>>>>
>

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