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@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()
>>
>> 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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