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

[Xishi Qiu]

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@xxxxxxxxxx> 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()

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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> 
> 
> .
> 



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