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 31E9FC00140 for ; Thu, 18 Aug 2022 12:59:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244846AbiHRM7C (ORCPT ); Thu, 18 Aug 2022 08:59:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48950 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244282AbiHRM7A (ORCPT ); Thu, 18 Aug 2022 08:59:00 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 21C6350724 for ; Thu, 18 Aug 2022 05:58:57 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4M7lGB3xsHzXdSx; Thu, 18 Aug 2022 20:54:42 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Thu, 18 Aug 2022 20:58:55 +0800 Subject: Re: [PATCH 4/6] mm: hugetlb_vmemmap: add missing smp_wmb() before set_pte_at() To: Muchun Song , "Yin, Fengwei" CC: Andrew Morton , Mike Kravetz , Muchun Song , Linux MM , References: <20220816130553.31406-1-linmiaohe@huawei.com> <20220816130553.31406-5-linmiaohe@huawei.com> <0EAF1279-6A1C-41FA-9A32-414C36B3792A@linux.dev> <019c1272-9d01-9d51-91a0-2d656b25c318@intel.com> <18adbf89-473e-7ba6-9a2b-522e1592bdc6@huawei.com> <9c791de0-b702-1bbe-38a4-30e87d9d1b95@intel.com> <931536E2-3948-40AB-88A7-E36F67954AAA@linux.dev> <7be98c64-88a1-3bee-7f92-67bb1f4f495b@huawei.com> <3B1463C2-9DC4-43D0-93EC-2D2334A20502@linux.dev> <7fa5b2b2-dcef-f264-7932-c4fdbb9619d0@intel.com> <7408156a-f708-5e73-d0a2-69b1acca9b96@intel.com> <15DD6DCA-39BC-4EA2-984F-D488E94CC4FF@linux.dev> From: Miaohe Lin Message-ID: <615c8ec8-6977-2ce0-f049-d2ec1619245c@huawei.com> Date: Thu, 18 Aug 2022 20:58:54 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <15DD6DCA-39BC-4EA2-984F-D488E94CC4FF@linux.dev> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/8/18 17:18, Muchun Song wrote: > > >> On Aug 18, 2022, at 16:54, Yin, Fengwei wrote: >> >> >> >> On 8/18/2022 4:40 PM, Muchun Song wrote: >>> >>> >>>> On Aug 18, 2022, at 16:32, Yin, Fengwei wrote: >>>> >>>> >>>> >>>> On 8/18/2022 3:59 PM, Muchun Song wrote: >>>>> >>>>> >>>>>> On Aug 18, 2022, at 15:52, Miaohe Lin wrote: >>>>>> >>>>>> On 2022/8/18 10:47, Muchun Song wrote: >>>>>>> >>>>>>> >>>>>>>> On Aug 18, 2022, at 10:00, Yin, Fengwei wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> On 8/18/2022 9:55 AM, Miaohe Lin wrote: >>>>>>>>>>>> /* >>>>>>>>>>>> * The memory barrier inside __SetPageUptodate makes sure that >>>>>>>>>>>> * preceding stores to the page contents become visible before >>>>>>>>>>>> * the set_pte_at() write. >>>>>>>>>>>> */ >>>>>>>>>>>> __SetPageUptodate(page); >>>>>>>>>>> IIUC, the case here we should make sure others (CPUs) can see new page’s >>>>>>>>>>> contents after they have saw PG_uptodate is set. I think commit 0ed361dec369 >>>>>>>>>>> can tell us more details. >>>>>>>>>>> >>>>>>>>>>> I also looked at commit 52f37629fd3c to see why we need a barrier before >>>>>>>>>>> set_pte_at(), but I didn’t find any info to explain why. I guess we want >>>>>>>>>>> to make sure the order between the page’s contents and subsequent memory >>>>>>>>>>> accesses using the corresponding virtual address, do you agree with this? >>>>>>>>>> This is my understanding also. Thanks. >>>>>>>>> That's also my understanding. Thanks both. >>>>>>>> I have an unclear thing (not related with this patch directly): Who is response >>>>>>>> for the read barrier in the read side in this case? >>>>>>>> >>>>>>>> For SetPageUptodate, there are paring write/read memory barrier. >>>>>>>> >>>>>>> >>>>>>> I have the same question. So I think the example proposed by Miaohe is a little >>>>>>> difference from the case (hugetlb_vmemmap) here. >>>>>> >>>>>> Per my understanding, memory barrier in PageUptodate() is needed because user might access the >>>>>> page contents using page_address() (corresponding pagetable entry already exists) soon. But for >>>>>> the above proposed case, if user wants to access the page contents, the corresponding pagetable >>>>>> should be visible first or the page contents can't be accessed. So there should be a data dependency >>>>>> acting as memory barrier between pagetable entry is loaded and page contents is accessed. >>>>>> Or am I miss something? >>>>> >>>>> Yep, it is a data dependency. The difference between hugetlb_vmemmap and PageUptodate() is that >>>>> the page table (a pointer to the mapped page frame) is loaded by MMU while PageUptodate() is >>>>> loaded by CPU. Seems like the data dependency should be inserted between the MMU access and the CPU >>>>> access. Maybe it is hardware’s guarantee? >>>> I just found the comment in pmd_install() explained why most arch has no read >>> >>> I think pmd_install() is a little different as well. We should make sure the >>> page table walker (like GUP) see the correct PTE entry after they see the pmd >>> entry. >> >> The difference I can see is that pmd/pte thing has both hardware page walker and >> software page walker (like GUP) as read side. While the case here only has hardware >> page walker as read side. But I suppose the memory barrier requirement still apply >> here. > > I am not against this change. Just in order to make me get a better understanding of > hardware behavior. > >> >> Maybe we could do a test: add large delay between reset_struct_page() and set_pte_at? > > Hi Miaohe, > > Would you mind doing this test? One thread do vmemmap_restore_pte(), another thread > detect if it can see a tail page with PG_head after the previous thread has executed > set_pte_at(). Will it be easier to construct the memory reorder manually like below? vmemmap_restore_pte() ... set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot)); /* might a delay. */ copy_page(to, (void *)walk->reuse_addr); reset_struct_pages(to); And another thread detects whether it can see a tail page with some invalid fields? If so, it seems the problem will always trigger? If not, we depend on the observed meory reorder and set_pte_at doesn't contain a memory barrier? Thanks, Miaohe Lin