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 70CBAC001DE for ; Sun, 23 Jul 2023 01:16:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229677AbjGWBQh (ORCPT ); Sat, 22 Jul 2023 21:16:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42826 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229476AbjGWBQf (ORCPT ); Sat, 22 Jul 2023 21:16:35 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 607D21B8; Sat, 22 Jul 2023 18:16:31 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.56]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4R7lhN02LKzVj2S; Sun, 23 Jul 2023 09:15:00 +0800 (CST) Received: from [10.174.151.185] (10.174.151.185) 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 15.1.2507.27; Sun, 23 Jul 2023 09:16:28 +0800 Subject: Re: [PATCH] mm/memcg: use get_page() for device private pages in mc_handle_swap_pte() From: Miaohe Lin To: Matthew Wilcox CC: , , , , , , , , References: <20230715032802.2508163-1-linmiaohe@huawei.com> <73b5d7c2-783d-3d75-2c1b-4b91a039df94@huawei.com> Message-ID: <3a86cef4-d1fd-8914-3b0a-2ae95ccff740@huawei.com> Date: Sun, 23 Jul 2023 09:16:28 +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: <73b5d7c2-783d-3d75-2c1b-4b91a039df94@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.151.185] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) 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 2023/7/17 10:28, Miaohe Lin wrote: > On 2023/7/15 11:56, Matthew Wilcox wrote: >> On Sat, Jul 15, 2023 at 11:28:02AM +0800, Miaohe Lin wrote: >>> When page table locked is held, the page can't be freed from under us. >> >> But the page isn't mapped into the page table ... there's a swap entry >> in the page table, so I don't think your logic holds. >> > > IIUC, device_private_entry will hold one page refcnt when it's set to page table. Take remove_migration_pte() as example, it will hold extra one page refcnt when set device private entry: remove_migration_pte() ... folio_get(folio); ... if (unlikely(is_device_private_page(new))) { make_[writable|readable]_device_private_entry(); } ... set_pte_at > And there's similar code in do_swap_page(): > > vm_fault_t do_swap_page(struct vm_fault *vmf) > if (unlikely(non_swap_entry(entry))) { > if (is_device_private_entry(entry)) > /* > * Get a page reference while we know the page can't be > * freed. > */ > get_page(vmf->page); > pte_unmap_unlock(vmf->pte, vmf->ptl); > ret = vmf->page->pgmap->ops->migrate_to_ram(vmf); > put_page(vmf->page); > ... > > If my logic doesn't hold, do_swap_page() will need to fix the code. Or am I miss something? Can I have your opinion? Thanks.