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 F37CDC433EF for ; Wed, 15 Jun 2022 06:48:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234813AbiFOGsJ (ORCPT ); Wed, 15 Jun 2022 02:48:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34844 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234799AbiFOGsI (ORCPT ); Wed, 15 Jun 2022 02:48:08 -0400 Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 48ABA2124A; Tue, 14 Jun 2022 23:48:05 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=xianting.tian@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0VGRbaRR_1655275679; Received: from B-LB6YLVDL-0141.local(mailfrom:xianting.tian@linux.alibaba.com fp:SMTPD_---0VGRbaRR_1655275679) by smtp.aliyun-inc.com; Wed, 15 Jun 2022 14:48:00 +0800 Subject: Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype To: Zi Yan , Guo Ren Cc: 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> From: Xianting Tian Message-ID: Date: Wed, 15 Jun 2022 14:47:59 +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: <435B45C3-E6A5-43B2-A5A2-318C748691FC@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 2022/6/14 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. 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