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 48EDDC47081 for ; Mon, 9 May 2022 08:57:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237594AbiEII7I (ORCPT ); Mon, 9 May 2022 04:59:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51948 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237172AbiEIIzn (ORCPT ); Mon, 9 May 2022 04:55:43 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 583BA1B7AD for ; Mon, 9 May 2022 01:51:49 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4KxZbC663yzGpfG; Mon, 9 May 2022 16:48:55 +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, 9 May 2022 16:51:42 +0800 Subject: Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and PageMovable check To: David Hildenbrand CC: , , , , , , , , References: <20220425132723.34824-1-linmiaohe@huawei.com> <20220425132723.34824-3-linmiaohe@huawei.com> <525298ad-5e6a-2f8d-366d-4dcb7eebd093@redhat.com> From: Miaohe Lin Message-ID: Date: Mon, 9 May 2022 16:51:41 +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: <525298ad-5e6a-2f8d-366d-4dcb7eebd093@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: dggems701-chm.china.huawei.com (10.3.19.178) 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 2022/4/29 18:07, David Hildenbrand wrote: > On 25.04.22 15:27, Miaohe Lin wrote: >> When non-lru movable page was freed from under us, __ClearPageMovable must >> have been done. Even if it's not done, ClearPageIsolated here won't hurt >> as page will be freed anyway. So we can thus remove unneeded lock page and >> PageMovable check here. >> >> Signed-off-by: Miaohe Lin >> Reviewed-by: Christoph Hellwig >> --- >> mm/migrate.c | 8 ++------ >> 1 file changed, 2 insertions(+), 6 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index b779646665fe..0fc4651b3e39 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1093,12 +1093,8 @@ static int unmap_and_move(new_page_t get_new_page, >> /* page was freed from under us. So we are done. */ >> ClearPageActive(page); >> ClearPageUnevictable(page); >> - if (unlikely(__PageMovable(page))) { >> - lock_page(page); >> - if (!PageMovable(page)) >> - ClearPageIsolated(page); >> - unlock_page(page); >> - } >> + if (unlikely(__PageMovable(page))) >> + ClearPageIsolated(page); >> goto out; >> } > > Hm, that code+change raises a couple of questions. > > We're doing here the same as in putback_movable_pages(). So I guess the > difference here is that the caller did release the reference while the > page was isolated, while we don't assume the same in > putback_movable_pages(). Agree. > > > Shouldn't whoever owned the page have cleared that? IOW, is it even > valid that we see a movable or isolated page here (WARN/BUG?)? > > At least for balloon compaction, I remember that __PageMovable() is > properly cleared before freeing it via balloon_page_delete(). z3fold, zsmalloc will do __ClearPageMovable when the page is going to be released. So I think we shouldn't see a movable page here: void __ClearPageMovable(struct page *page) { VM_BUG_ON_PAGE(!PageMovable(page), page); /* * Clear registered address_space val with keeping PAGE_MAPPING_MOVABLE * flag so that VM can catch up released page by driver after isolation. * With it, VM migration doesn't try to put it back. */ page->mapping = (void *)((unsigned long)page->mapping & PAGE_MAPPING_MOVABLE); } But it seems there is no guarantee for PageIsolated flag. Or am I miss something? > > > 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. Thanks! > >