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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A29B5C4332B for ; Fri, 20 Mar 2020 16:34:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7E32320767 for ; Fri, 20 Mar 2020 16:34:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727587AbgCTQea (ORCPT ); Fri, 20 Mar 2020 12:34:30 -0400 Received: from out30-43.freemail.mail.aliyun.com ([115.124.30.43]:52179 "EHLO out30-43.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727163AbgCTQe3 (ORCPT ); Fri, 20 Mar 2020 12:34:29 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R261e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04391;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0Tt7jLIR_1584722063; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0Tt7jLIR_1584722063) by smtp.aliyun-inc.com(127.0.0.1); Sat, 21 Mar 2020 00:34:25 +0800 Subject: Re: [PATCH] mm: khugepaged: fix potential page state corruption To: "Kirill A. Shutemov" Cc: kirill.shutemov@linux.intel.com, hughd@google.com, aarcange@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1584573582-116702-1-git-send-email-yang.shi@linux.alibaba.com> <20200319001258.creziw6ffw4jvwl3@box> <2cdc734c-c222-4b9d-9114-1762b29dafb4@linux.alibaba.com> <20200319104938.vphyajoyz6ob6jtl@box> <20200320114536.brigxjkgjmxyhdu5@box> From: Yang Shi Message-ID: <552f0725-42ff-c09d-592c-3e433b71edbb@linux.alibaba.com> Date: Fri, 20 Mar 2020 09:34:21 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20200320114536.brigxjkgjmxyhdu5@box> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/20/20 4:45 AM, Kirill A. Shutemov wrote: > On Thu, Mar 19, 2020 at 09:57:47AM -0700, Yang Shi wrote: >> >> On 3/19/20 3:49 AM, Kirill A. Shutemov wrote: >>> On Wed, Mar 18, 2020 at 10:39:21PM -0700, Yang Shi wrote: >>>> On 3/18/20 5:55 PM, Yang Shi wrote: >>>>> On 3/18/20 5:12 PM, Kirill A. Shutemov wrote: >>>>>> On Thu, Mar 19, 2020 at 07:19:42AM +0800, Yang Shi wrote: >>>>>>> When khugepaged collapses anonymous pages, the base pages would >>>>>>> be freed >>>>>>> via pagevec or free_page_and_swap_cache().  But, the anonymous page may >>>>>>> be added back to LRU, then it might result in the below race: >>>>>>> >>>>>>>     CPU A                CPU B >>>>>>> khugepaged: >>>>>>>    unlock page >>>>>>>    putback_lru_page >>>>>>>      add to lru >>>>>>>                 page reclaim: >>>>>>>                   isolate this page >>>>>>>                   try_to_unmap >>>>>>>    page_remove_rmap <-- corrupt _mapcount >>>>>>> >>>>>>> It looks nothing would prevent the pages from isolating by reclaimer. >>>>>> Hm. Why should it? >>>>>> >>>>>> try_to_unmap() doesn't exclude parallel page unmapping. _mapcount is >>>>>> protected by ptl. And this particular _mapcount pin is reachable for >>>>>> reclaim as it's not part of usual page table tree. Basically >>>>>> try_to_unmap() will never succeeds until we give up the _mapcount on >>>>>> khugepaged side. >>>>> I don't quite get. What does "not part of usual page table tree" means? >>>>> >>>>> How's about try_to_unmap() acquires ptl before khugepaged? >>> The page table we are dealing with was detached from the process' page >>> table tree: see pmdp_collapse_flush(). try_to_unmap() will not see the >>> pte. >>> >>> try_to_unmap() can only reach the ptl if split ptl is disabled >>> (mm->page_table_lock is used), but it still will not be able to reach pte. >> Aha, got it. Thanks for explaining. I definitely missed this point. Yes, >> pmdp_collapse_flush() would clear the pmd, then others won't see the page >> table. >> >> However, it looks the vmscan would not stop at try_to_unmap() at all, >> try_to_unmap() would just return true since pmd_present() should return >> false in pvmw. Then it would go all the way down to __remove_mapping(), but >> freezing the page would fail since try_to_unmap() doesn't actually drop the >> refcount from the pte map. > No. try_to_unmap() checks mapcount at the end and only returns true if > it's zero. Aha, yes, you are right. It does check mapcount. It would not go that far. > >> It would not result in any critical problem AFAICT, but suboptimal and it >> may causes some unnecessary I/O due to swap. >> >>>>>> I don't see the issue right away. >>>>>> >>>>>>> The other problem is the page's active or unevictable flag might be >>>>>>> still set when freeing the page via free_page_and_swap_cache(). >>>>>> So what? >>>>> The flags may leak to page free path then kernel may complain if >>>>> DEBUG_VM is set. >>> Could you elaborate on what codepath you are talking about? >> __put_page -> >>     __put_single_page -> >>         free_unref_page -> >>             put_unref_page_prepare -> >>                 free_pcp_prepare -> >>                     free_pages_prepare -> >>                         free_pages_check >> >> This check would just be run when DEBUG_VM is enabled. > I'm not 100% sure, but I belive these flags will ge cleared on adding into > lru: > > release_pte_page() > putback_lru_page() > lru_cache_add() > __lru_cache_add() > __pagevec_lru_add() > __pagevec_lru_add_fn() > __pagevec_lru_add_fn() No, adding into lru would not clear the flags. But, I finally found where they get cleared. They get cleared by: __put_single_page() ->     __page_cache_release() ->         if page is lru         del_page_from_lru_list(page, lruvec, page_off_lru(page)); page_off_lru() would clear active and unevictable flags. The name (__page_cache_release) sounds a little bit confusing I misunderstood it just would done something for page cache. So, it looks the code does depend on putting page back to lru to release it. Nothing is wrong, just a little bit unproductive IMHO. Sorry for the rash patch. And thank you for your time again. >