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 63DB0C433F5 for ; Mon, 16 May 2022 02:44:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239460AbiEPCor (ORCPT ); Sun, 15 May 2022 22:44:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60856 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233714AbiEPCon (ORCPT ); Sun, 15 May 2022 22:44:43 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3690C2DD41 for ; Sun, 15 May 2022 19:44:41 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4L1k88382Cz1JC8s; Mon, 16 May 2022 10:43:20 +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; Mon, 16 May 2022 10:44:38 +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> <025d0dc8-a446-b720-14a8-97c041055f48@huawei.com> <143ab5dd-85a9-3338-53b7-e46c9060b20e@redhat.com> From: Miaohe Lin Message-ID: <6ba7e2bd-28c1-53ff-a6b7-072c79714dee@huawei.com> Date: Mon, 16 May 2022 10:44:37 +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: <143ab5dd-85a9-3338-53b7-e46c9060b20e@redhat.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) 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 2022/5/13 0:50, David Hildenbrand wrote: > On 12.05.22 15:26, Miaohe Lin wrote: >> 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; >> } > > Yeah, unless I am missing something important :) > >>> >>>> >>>>> >>>>>> >>>>>>> >>>>>>> >>>>>>> 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? > > Let's assume the following: page as freed by the owner and we enter > unmap_and_move(). > > > #1: enter unmap_and_move() // page_count is 1 > #2: enter isolate_movable_page() // page_count is 1 > #2: get_page_unless_zero() // page_count is now 2 > #1: if (page_count(page) == 1) { // does not trigger > #2: put_page(page); // page_count is now 1 > #1: put_page(page); // page_count is now 0 -> freed > > > #1 will trigger __put_page() -> __put_single_page() -> > __page_cache_release() will not clear the flags because it's not an LRU > page at that point in time, right (-> isolated)? Sorry, you're right. I thought the old page will be freed via putback_lru_page which will set PageLRU back instead of put_page directly. So if the above race occurs, PG_active and PG_unevictable will remain set while page goes to the buddy and check_free_page will complain about it. But it seems this is never witnessed? > > We did not run that code block that would clear PG_active and > PG_unevictable. > > Which still leaves the questions: > > a) If PG_active and PG_unevictable was cleared, where? For LRU pages, PG_active and PG_unevictable are cleared via __page_cache_release. And for isolated (LRU) pages, PG_active and PG_unevictable should be cleared ourselves? > b) Why is that code block that conditionally clears the flags of any > value and why can't we simply drop it? > To fix the issue, should we clear PG_active and PG_unevictable unconditionally here? Thanks a lot!