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

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

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

>- 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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