linux-kernel - Re: [PATCH V2] MCE: fix an error of mce_bad

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Date:	Mon, 10 Dec 2012 19:16:50 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Simon Jeons <simon.jeons@...il.com>
CC:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	WuJianguo <wujianguo@...wei.com>,
	Liujiang <jiang.liu@...wei.com>, <Vyacheslav.Dubeyko@...wei.com>,
	Borislav Petkov <bp@...en8.de>, <andi@...stfloor.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<wency@...fujitsu.com>
Subject: Re: [PATCH V2] MCE: fix an error of mce_bad_pages statistics

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@...wei.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.

>>
>> 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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