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 BC482C433EF for ; Tue, 31 May 2022 12:38:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344333AbiEaMiE (ORCPT ); Tue, 31 May 2022 08:38:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37166 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344322AbiEaMh4 (ORCPT ); Tue, 31 May 2022 08:37:56 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8843A3BA58 for ; Tue, 31 May 2022 05:37:54 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4LCBd23WFmzDqZk; Tue, 31 May 2022 20:37:42 +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; Tue, 31 May 2022 20:37:52 +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> <6ba7e2bd-28c1-53ff-a6b7-072c79714dee@huawei.com> <0724b4c4-15f6-e429-f945-f57c619c7270@redhat.com> From: Miaohe Lin Message-ID: <7ca676a9-1f51-47f7-0245-d041d075a440@huawei.com> Date: Tue, 31 May 2022 20:37:51 +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: <0724b4c4-15f6-e429-f945-f57c619c7270@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 2022/5/31 19:59, David Hildenbrand wrote: > Sorry for the late reply, was on vacation. That's all right. Hope you have a great time. ;) > >>>> >>>> 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? > > Maybe > > a) we were lucky so far and didn't trigger it > b) the whole code block is dead code because we are missing something > c) we are missing something else :) I think I found the things we missed in another email [1]. [1]: https://lore.kernel.org/all/948ea45e-3b2b-e16c-5b8c-4c34de0ea593@huawei.com/ Paste the main content of [1] here: " There are 3 cases in unmap_and_move: 1.page is freed through "if (page_count(page) == 1)" code block. This works as PG_active and PG_unevictable are cleared here. 2. Failed to migrate the page. The page won't be release so we don't care about it. 3. The page is migrated successfully. The PG_active and PG_unevictable are cleared via folio_migrate_flags(): if (folio_test_clear_active(folio)) { VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio); folio_set_active(newfolio); } else if (folio_test_clear_unevictable(folio)) folio_set_unevictable(newfolio); For the above race case, the page won't be freed through "if (page_count(page) == 1)" code block. It will just be migrated and freed via put_page() after folio_migrate_flags() having cleared PG_active and PG_unevictable. " Or Am I miss something again? :) > >> >>> >>> 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? > > I wonder if we should simply teach actual freeing code to simply clear > both flags when freeing an isolated page? IOW, to detect "isolated LRU" > is getting freed and fixup? IMHO, clearing both flags are done by the caller indeed. Another example I found when I read the khugepaged code recently is pasted below: collapse_file ... page_ref_unfreeze(page, 1); ClearPageActive(page); ClearPageUnevictable(page); unlock_page(page); put_page(page); index++; ... Thanks! >