On Mon, Dec 10, 2012 at 06:47:44AM -0600, Simon Jeons wrote:
>Cc other guys.
>
>On Mon, 2012-12-10 at 20:40 +0800, Xishi Qiu wrote:
>> On 2012/12/10 19:56, Simon Jeons wrote:
>>
>> > On Mon, 2012-12-10 at 19:16 +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
>> >
>> > What topic? I still can't figure out when this branch can be executed
>> > since hwpoison inject path can't poison free buddy pages.
>> >
>>
>> 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
>>
>> >> 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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