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 4ADB0C433F5 for ; Thu, 12 May 2022 13:27:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354418AbiELN1Q (ORCPT ); Thu, 12 May 2022 09:27:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60454 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1354384AbiELN1D (ORCPT ); Thu, 12 May 2022 09:27:03 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DE6C149F97 for ; Thu, 12 May 2022 06:26:47 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4KzXbr19WqzgYsx; Thu, 12 May 2022 21:26:16 +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, 12 May 2022 21:26:45 +0800 Subject: Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and PageMovable check To: David Hildenbrand CC: , , , , , , , , , Minchan Kim References: <20220425132723.34824-1-linmiaohe@huawei.com> <20220425132723.34824-3-linmiaohe@huawei.com> <525298ad-5e6a-2f8d-366d-4dcb7eebd093@redhat.com> <4cf144a9-fff5-d993-4fcb-7f2dfa6e71bb@redhat.com> <924de987-202b-a97e-e6d2-6bdab530f190@huawei.com> From: Miaohe Lin Message-ID: <025d0dc8-a446-b720-14a8-97c041055f48@huawei.com> Date: Thu, 12 May 2022 21:26:44 +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: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) 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 2022/5/12 15:10, David Hildenbrand wrote: >>> If PG_isolated is still set, it will get cleared in the buddy when >>> freeing the page via >>> >>> page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP; >> >> Yes, check_free_page only complains about flags belonging to PAGE_FLAGS_CHECK_AT_FREE and PG_isolated >> will be cleared in the buddy when freeing the page. But it might not be a good idea to reply on this ? >> IMHO, it should be better to clear the PG_isolated explicitly ourselves. > > I think we can pretty much rely on this handling in the buddy :) So is the below code change what you're suggesting? if (page_count(page) == 1) { /* page was freed from under us. So we are done. */ ClearPageActive(page); ClearPageUnevictable(page); - if (unlikely(__PageMovable(page))) - ClearPageIsolated(page); goto out; } > >> >>> >>>> >>>>> >>>>> >>>>> Also, I am not sure how reliable that page count check is here: if we'd >>>>> have another speculative reference to the page, we might see >>>>> "page_count(page) > 1" and not take that path, although the previous >>>>> owner released the last reference. >>>> >>>> IIUC, there should not be such speculative reference. The driver should have taken care >>>> of it. >>> >>> How can you prevent any kind of speculative references? >>> >>> See isolate_movable_page() as an example, which grabs a speculative >>> reference to then find out that the page is already isolated by someone >>> else, to then back off. >> >> You're right. isolate_movable_page will be an speculative references case. But the page count check here >> is just an optimization. If we encounter speculative references, it still works with useless effort of >> migrating to be released page. > > > Not really. The issue is that PAGE_FLAGS_CHECK_AT_FREE contains > PG_active and PG_unevictable. > > We only clear those 2 flags if "page_count(page) == 1". Consequently, > with a speculative reference, we'll run into the check_free_page_bad() > when dropping the last reference. It seems if a speculative reference happens after the "page_count(page) == 1" check, it's ok because we cleared the PG_active and PG_unevictable. And if it happens before the check, this code block is skipped and the page will be freed after migration. The PG_active and PG_unevictable will be correctly cleared when page is actually freed via __folio_clear_active. (Please see below comment) > > This is just shaky. Special casing on "page_count(page) == 1" for > detecting "was this freed by the owner" is not 100% water proof. > > In an ideal world, we'd just get rid of that whole block of code and let > the actual freeing code clear PG_active and PG_unevictable. But that > would require changes to free_pages_prepare(). > > > Now I do wonder, if we ever even have PG_active or PG_unevictable still > set when the page was freed by the owner in this code. IOW, maybe that > is dead code as well and we can just remove the whole shaky > "page_count(page) == 1" code block. Think about below common scene: Anonymous page is actively used by the sole owner process, so it will have PG_active set. Then process exited while vm tries to migrate that page. So the page should have refcnt == 1 while PG_active is set? Note normally PG_active should be cleared when the page is released: __put_single_page PageLRU __clear_page_lru_flags __folio_clear_active __folio_clear_unevictable But for isolated page, PageLRU is cleared. So when the isolated page is released, __clear_page_lru_flags won't be called. So we have to clear the PG_active and PG_unevictable here manully. So I think this code block works. Or am I miss something again? Thanks! > > Ccing Minchan, who added clearing of the pageflags at that point. >