From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0EDEDC43334 for ; Wed, 15 Jun 2022 16:19:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344363AbiFOQTA (ORCPT ); Wed, 15 Jun 2022 12:19:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355290AbiFOQSR (ORCPT ); Wed, 15 Jun 2022 12:18:17 -0400 Received: from out199-6.us.a.mail.aliyun.com (out199-6.us.a.mail.aliyun.com [47.90.199.6]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7BCA452E70; Wed, 15 Jun 2022 09:16:39 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046049;MF=xianting.tian@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0VGTX79V_1655309728; Received: from 192.168.1.6(mailfrom:xianting.tian@linux.alibaba.com fp:SMTPD_---0VGTX79V_1655309728) by smtp.aliyun-inc.com; Thu, 16 Jun 2022 00:15:30 +0800 Subject: Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype To: Zi Yan Cc: Guo Ren , Greg Kroah-Hartman , Andrew Morton , Vlastimil Babka , Linux-MM , Linux Kernel Mailing List , stable@vger.kernel.org, huanyi.xj@alibaba-inc.com, zjb194813@alibaba-inc.com, tianhu.hh@alibaba-inc.com, Hanjun Guo , Joonsoo Kim , Laura Abbott References: <20220613131046.3009889-1-xianting.tian@linux.alibaba.com> <0262A4FB-5A9B-47D3-8F1A-995509F56279@nvidia.com> <435B45C3-E6A5-43B2-A5A2-318C748691FC@nvidia.com> <18330D9A-F433-4136-A226-F24173293BF3@nvidia.com> From: Xianting Tian Message-ID: <5526fab6-c7e1-bddc-912b-e4d9b2769d4e@linux.alibaba.com> Date: Thu, 16 Jun 2022 00:15:28 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <18330D9A-F433-4136-A226-F24173293BF3@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org 在 2022/6/15 下午9:55, Zi Yan 写道: > On 15 Jun 2022, at 2:47, Xianting Tian wrote: > >> 在 2022/6/14 上午8:14, Zi Yan 写道: >>> On 13 Jun 2022, at 19:47, Guo Ren wrote: >>> >>>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan wrote: >>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote: >>>>> >>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan wrote: >>>>>>> Hi Xianting, >>>>>>> >>>>>>> Thanks for your patch. >>>>>>> >>>>>>> On 13 Jun 2022, at 9:10, Xianting Tian wrote: >>>>>>> >>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.") >>>>>>>> added buddy check code. But unfortunately, this fix isn't backported to >>>>>>>> linux-5.17.y and the former stable branches. The reason is it added wrong >>>>>>>> fixes message: >>>>>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable >>>>>>>> pageblocks with others") >>>>>>> No, the Fixes tag is right. The commit above does need to validate buddy. >>>>>> I think Xianting is right. The “Fixes:" tag is not accurate and the >>>>>> page_is_buddy() is necessary here. >>>>>> >>>>>> This patch could be applied to the early version of the stable tree >>>>>> (eg: Linux-5.10.y, not the master tree) >>>>> This is quite misleading. Commit 787af64d05cd applies does not mean it is >>>>> intended to fix the preexisting bug. Also it does not apply cleanly >>>>> to commit d9dddbf55667, there is a clear indentation mismatch. At best, >>>>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667. >>>>> There is no way you can apply 787af64d05cd to earlier trees and call it a day. >>>>> >>>>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is >>>>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying >>>>> the fixes message is wrong just misleads people, making them think there is >>>>> no bug in 1dd214b8f21c. We need to be clear about this. >>>> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The >>>> origin fixes could cover the Linux-5.0.y tree if they give the >>>> accurate commit number and that is the cause we want to point out. >>> Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes >>> the issue introduced by d9dddbf55667. But my point is that 787af64d05cd >>> is not intended to fix d9dddbf55667 and saying it has a wrong fixes >>> message is misleading. This is the point I want to make. >>> >>>> Second, if the patch is for d9dddbf55667 then it could cover any tree >>>> in the stable repo. Actually, we only know Linux-5.10.y has the >>>> problem. >>> But it is not and does not apply to d9dddbf55667 cleanly. >>> >>>> Maybe, Gregkh could help to direct us on how to deal with the issue: >>>> (Fixup a bug which only belongs to the former stable branch.) >>>> >>> I think you just need to send this patch without saying “commit >>> 787af64d05cd fixes message is wrong” would be a good start. You also >>> need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17 >>> (inclusive). So there will need to be two patches: >>> >>> 1) your patch to stable tree prior to 5.15 and >>> >>> 2) your patch with an additional mm/page_isolation.c fix to stable tree >>> between 5.15 and 5.17. >>> >>>>> Also, you will need to fix the mm/page_isolation.c code too to make this patch >>>>> complete, unless you can show that PFN=0x1000 is never going to be encountered >>>>> in the mm/page_isolation.c code I mentioned below. >>>> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had >>>> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent >>>> buddy_pfn=0. >>>> The root cause comes from __find_buddy_pfn(): >>>> return page_pfn ^ (1 << order); >>> Right. But pfn_valid_within() was removed since 5.15. So your fix is >>> required for kernels between 5.15 and 5.17 (inclusive). >>> >>>> When page_pfn is the same as the order size, it will return the >>>> previous buddy not the next. That is the only exception for this >>>> algorithm, right? >>>> >>>> >>>> >>>> >>>> In fact, the bug is a very long time to reproduce and is not easy to >>>> debug, so we want to contribute it to the community to prevent other >>>> guys from wasting time. Although there is no new patch at all. >>> Thanks for your reporting and sending out the patch. I really >>> appreciate it. We definitely need your inputs. Throughout the email >>> thread, I am trying to help you clarify the bug and how to fix it >>> properly: >>> >>> 1. The commit 787af64d05cd does not apply cleanly to commits >>> d9dddbf55667, meaning you cannot just cherry-pick that commit to >>> fix the issue. That is why we need your patch to fix the issue. >>> And saying it has a wrong fixes message in this patch’s git log is >>> misleading. >>> >>> 2. For kernels between 5.15 and 5.17 (inclusive), an additional fix >>> to mm/page_isolation.c is also needed, since pfn_valid_within() was >>> removed since 5.15 and the issue can appear during page isolation. >>> >>> 3. For kernels before 5.15, this patch will apply. >> Zi Yan, Guo Ren, >> >> I think we still need some imporvemnt for MASTER branch, as we discussed above, we will get an illegal buddy page if buddy_pfn is 0, >> >> within page_is_buddy(), it still use the illegal buddy page to do the check. I think in most of cases, page_is_buddy() can return false,  but it still may return true with very low probablity. > Can you elaborate more on this? What kind of page can lead to page_is_buddy() > returning true? You said it is buddy_pfn is 0, but if the page is reserved, > if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false. > Maybe show us the dump_page() that offending page. > > Thanks. Let‘s take the issue we met on RISC-V arch for example, pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 0xffffffe07e205000, which is the page address of PFN 512. __find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd. We know PFN 0 is not a valid pfn for buddy system, because 512 is the first PFN for buddy system. Then it use below code to get buddy page with buddy_pfn 0: buddy = page + (buddy_pfn - pfn); So buddy page address is: 0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000) we can know this buddy page's address is less than mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000), actually 0xffffffe07e1fe000 is not a valid page's address. If we use 0xffffffe07e1fe000 as the page's address to extract the value of a member in 'struct page', we may get an uncertain value. That's why I say page_is_buddy() may return true with very low probablity. So I think we need to add the code the verify buddy_pfn in the first place: pfn_valid(buddy_pfn) >> I think we need to add some code to verify buddy_pfn in the first place. >> >> Could you give some suggestions for this idea? >> >>>>>>>> Actually, this issue is involved by commit: >>>>>>>> commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>>> >>>>>>>> For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 512, >>>>>>>> but it got buddy PFN 0 for PFN 0x2000: >>>>>>>> 0 = 0x2000 ^ (1 << 12) >>>>>>>> With the illegal buddy PFN 0, it got an illegal buddy page, which caused >>>>>>>> crash in __get_pfnblock_flags_mask(). >>>>>>> It seems that the RISC-V arch reveals a similar bug from d9dddbf55667. >>>>>>> Basically, this bug will only happen when PFN=0x2000 is merging up and >>>>>>> there are some isolated pageblocks. >>>>>> Not PFN=0x2000, it's PFN=0x1000, I guess. >>>>>> >>>>>> RISC-V's first 2MB RAM could reserve for opensbi, so it would have >>>>>> riscv_pfn_base=512 and mem_map began with 512th PFN when >>>>>> CONFIG_FLATMEM=y. >>>>>> (Also, csky has the same issue: a non-zero pfn_base in some scenarios.) >>>>>> >>>>>> But __find_buddy_pfn algorithm thinks the start address is 0, it could >>>>>> get 0 pfn or less than the pfn_base value. We need another check to >>>>>> prevent that. >>>>>> >>>>>>> BTW, what does first reserved 2MB imply? All 4KB pages from first 2MB are >>>>>>> set to PageReserved? >>>>>>> >>>>>>>> With the patch, it can avoid the calling of get_pageblock_migratetype() if >>>>>>>> it isn't buddy page. >>>>>>> You might miss the __find_buddy_pfn() caller in unset_migratetype_isolate() >>>>>>> from mm/page_isolation.c, if you are talking about linux-5.17.y and former >>>>>>> version. There, page_is_buddy() is also not called and is_migrate_isolate_page() >>>>>>> is called, which calls get_pageblock_migratetype() too. >>>>>>> >>>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>>> Cc: stable@vger.kernel.org >>>>>>>> Reported-by: zjb194813@alibaba-inc.com >>>>>>>> Reported-by: tianhu.hh@alibaba-inc.com >>>>>>>> Signed-off-by: Xianting Tian >>>>>>>> --- >>>>>>>> mm/page_alloc.c | 3 +++ >>>>>>>> 1 file changed, 3 insertions(+) >>>>>>>> >>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>>>>> index b1caa1c6c887..5b423caa68fd 100644 >>>>>>>> --- a/mm/page_alloc.c >>>>>>>> +++ b/mm/page_alloc.c >>>>>>>> @@ -1129,6 +1129,9 @@ static inline void __free_one_page(struct page *page, >>>>>>>> >>>>>>>> buddy_pfn = __find_buddy_pfn(pfn, order); >>>>>>>> buddy = page + (buddy_pfn - pfn); >>>>>>>> + >>>>>>>> + if (!page_is_buddy(page, buddy, order)) >>>>>>>> + goto done_merging; >>>>>>>> buddy_mt = get_pageblock_migratetype(buddy); >>>>>>>> >>>>>>>> if (migratetype != buddy_mt >>>>>>>> -- >>>>>>>> 2.17.1 >>>>>>> -- >>>>>>> Best Regards, >>>>>>> Yan, Zi >>>>>> >>>>>> -- >>>>>> Best Regards >>>>>> Guo Ren >>>>>> >>>>>> ML: https://lore.kernel.org/linux-csky/ >>>>> -- >>>>> Best Regards, >>>>> Yan, Zi >>>> >>>> -- >>>> Best Regards >>>> Guo Ren >>>> >>>> ML: https://lore.kernel.org/linux-csky/ >>> -- >>> Best Regards, >>> Yan, Zi > -- > Best Regards, > Yan, Zi