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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 13A24C433EF for ; Tue, 7 Jun 2022 02:20:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 641116B0072; Mon, 6 Jun 2022 22:20:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5F1366B0073; Mon, 6 Jun 2022 22:20:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 46AEA6B0074; Mon, 6 Jun 2022 22:20:50 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 30C3B6B0072 for ; Mon, 6 Jun 2022 22:20:50 -0400 (EDT) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 03A3F34EAB for ; Tue, 7 Jun 2022 02:20:49 +0000 (UTC) X-FDA: 79549836660.13.750E113 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by imf02.hostedemail.com (Postfix) with ESMTP id A09A38003E for ; Tue, 7 Jun 2022 02:20:41 +0000 (UTC) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4LHDbb59dDzpW2r; Tue, 7 Jun 2022 10:20:27 +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, 7 Jun 2022 10:20:44 +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> <7ca676a9-1f51-47f7-0245-d041d075a440@huawei.com> <059fe8fe-bd89-477f-2430-277bb738525b@redhat.com> From: Miaohe Lin Message-ID: <4b13e16e-1b66-d49a-da0b-7b29c0be8ace@huawei.com> Date: Tue, 7 Jun 2022 10:20:43 +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: <059fe8fe-bd89-477f-2430-277bb738525b@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: dggems705-chm.china.huawei.com (10.3.19.182) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: A09A38003E Authentication-Results: imf02.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf02.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.189 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com X-Stat-Signature: pppx7cdwq165y11y7xke6f61874af57e X-Rspam-User: X-HE-Tag: 1654568441-907855 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2022/6/2 16:47, David Hildenbrand wrote: > On 02.06.22 09:40, Miaohe Lin wrote: >> On 2022/6/1 18:31, David Hildenbrand wrote: >>> On 31.05.22 14:37, Miaohe Lin wrote: >>>> 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. >>> >>> Right, page is un-isolated. >>> >>>> >>>> 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); >>> >>> Right. >>> >>>> >>>> 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? :) >>> >>> For #1, I'm still not sure what would happen on a speculative reference. >>> >>> It's worth summarizing that >>> >>> a) free_pages_prepare() will clear both flags via page->flags &= >>> ~PAGE_FLAGS_CHECK_AT_PREP; >>> >>> b) free_pages_prepare() will bail out if any flag is set in >>> check_free_page(). >>> >>> As we've never seen b) in the wild, this certainly has low priority, and >>> maybe it really cannot happen right now. >>> >>> However, maybe really allowing these flags to be set when freeing the >>> page and removing the "page_count(page) == 1" case from migration code >>> would be the clean thing to do. >> >> IMHO, check_free_page is used to catch possible problem. There's the comment of PAGE_FLAGS_CHECK_AT_FREE: >> >> /* >> * Flags checked when a page is freed. Pages being freed should not have >> * these flags set. If they are, there is a problem. >> */ >> #define PAGE_FLAGS_CHECK_AT_FREE >> >> There might be an assumption: when page is freed, it shouldn't be an active or unevictable page. It should be >> inactive and evictable. So allowing these flags to be set when freeing the page might not be a good idea? > > Yeah, and we'd be lifting that restriction because there is good reason > to do so. > > Maybe we *could* special case for isolated pages; however, that adds > runtime overhead. Of course, we could perform different checks for e.g., > DEBUG_VM vs !DEBUG_VM. I found there is one assumption about PG_active and PG_unevictable, i.e. in __folio_clear_lru_flags: /* this shouldn't happen, so leave the flags to bad_page() */ if (folio_test_active(folio) && folio_test_unevictable(folio)) return; If PG_active and PG_unevictable are both set, this case will be caught in the bad_page() via check_free_page(). There might be some other assumptions about PG_active and PG_unevictable. So I think it's not safe to lift that restriction. But maybe we could limit this check within DEBUG_VM as you suggested. Am I supposed to do it? Thanks! >