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Message-ID: <1355143664.1821.8.camel@kernel.cn.ibm.com>
Date: Mon, 10 Dec 2012 06:47:44 -0600
From: Simon Jeons <simon.jeons@...il.com>
To: Xishi Qiu <qiuxishi@...wei.com>
Cc: Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
Simon Jeons <simon.jeons@...il.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
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@...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
> >
> > 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()"
>
> >> 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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