From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx113.postini.com [74.125.245.113]) by kanga.kvack.org (Postfix) with SMTP id 485026B005D for ; Mon, 10 Dec 2012 06:17:29 -0500 (EST) Message-ID: <50C5C4A2.2070002@huawei.com> Date: Mon, 10 Dec 2012 19:16:50 +0800 From: Xishi Qiu MIME-Version: 1.0 Subject: Re: [PATCH V2] MCE: fix an error of mce_bad_pages statistics References: <50C1AD6D.7010709@huawei.com> <20121207141102.4fda582d.akpm@linux-foundation.org> <20121210083342.GA31670@hacker.(null)> <50C5A62A.6030401@huawei.com> <1355136423.1700.2.camel@kernel.cn.ibm.com> In-Reply-To: <1355136423.1700.2.camel@kernel.cn.ibm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Simon Jeons Cc: Wanpeng Li , Andrew Morton , WuJianguo , Liujiang , Vyacheslav.Dubeyko@huawei.com, Borislav Petkov , andi@firstfloor.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, wency@cn.fujitsu.com 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 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. >> >> 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! >>>> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org