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

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

On Tue, Dec 11, 2012 at 02:49:06PM +0800, Xishi Qiu wrote:
>>>> Hi Simon,
>
>>>>
>>>> If we use "/sys/devices/system/memory/soft_offline_page" to offline a
>>>> free page, the value of mce_bad_pages will be added. Then the page is marked
>>>> HWPoison, but it is still managed by page buddy alocator.
>>>>
>>>> So if we offline it again, the value of mce_bad_pages will be added again.
>>>> Assume the page is not allocated during this short time.
>>>>
>>>> soft_offline_page()
>>>> 	get_any_page()
>>>> 		"else if (is_free_buddy_page(p))" branch return 0
>>>> 			"goto done";
>>>> 				"atomic_long_add(1, &mce_bad_pages);"
>>>>
>>>> I think it would be better to move "if(PageHWPoison(page))" at the beginning of
>>>> soft_offline_page(). However I don't know what do these words mean,
>>>> "Synchronized using the page lock with memory_failure()"
>> 
>> Hi Xishi,
>> 
>> Unpoison will clear PG_hwpoison flag after hold page lock, memory_failure() and 
>> soft_offline_page() take the lock to avoid unpoison clear the flag behind them.
>> 
>> Regards,
>> Wanpeng Li 
>> 
>
>Hi Wanpeng,
>
>As you mean, it is the necessary to get the page lock first when we check the
>HWPoison flag every time, this is in order to avoid conflict, right?
>

Hi Xishi,

Avoid race.

>So why not use a globe lock here? For example lock_memory_hotplug() is used in
>online_pages() and offline_pages()?

Just for a single page, a global lock maybe more contend.

Regards,
Wanpeng Li

>
>Thanks,
>Xishi Qiu

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