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 BD642C433F5 for ; Sat, 28 May 2022 06:24:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345565AbiE1GYm (ORCPT ); Sat, 28 May 2022 02:24:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40608 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230407AbiE1GYl (ORCPT ); Sat, 28 May 2022 02:24:41 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4514E3616F for ; Fri, 27 May 2022 23:24:39 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4L9BS52BdBz1JBYf; Sat, 28 May 2022 14:23:01 +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; Sat, 28 May 2022 14:24:35 +0800 Subject: Re: [PATCH] mm/vmscan: don't try to reclaim freed folios To: Matthew Wilcox CC: , , References: <20220527080451.48549-1-linmiaohe@huawei.com> From: Miaohe Lin Message-ID: <354f9b86-44fe-493b-eac4-07c5eeb573cf@huawei.com> Date: Sat, 28 May 2022 14:24:34 +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: 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/5/28 11:13, Matthew Wilcox wrote: > On Sat, May 28, 2022 at 10:52:11AM +0800, Miaohe Lin wrote: >> On 2022/5/27 23:02, Matthew Wilcox wrote: >>> What? No. This can absolutely happen. We have a refcount on the folio, >>> which means that any other thread can temporarily raise the refcount, >> >> IIUC, the folio is only in the isolated page_list now and it's not in the page cache, swap cache, pagetable or >> under any use. So there should be no way that any other thread can temporarily raise the refcount when >> folio_ref_count == 1. Or am I miss something? > > Take a look at something like GUP (fast). If this page _was_ mapped to > userspace, something like this can happen: > > Thread A Thread B > load PTE > unmap page > refcount goes to 1 > vmscan sees the page > try_get_ref > refcount is now 2. WARN_ON. > > Thread A will see that the PTE has changed and will now drop its > reference, but Thread B already spat out the WARN. > > A similar thing can happen with the page cache. Oh, I see. Many thanks for your patient explanation! :) > > If this is a worthwhile optimisation (does it happen often that we find > a refcount == 1 page?), we could do something like ... No, It should be rare. > > if (folio_ref_freeze(folio, 1)) { > nr_pages = folio_nr_pages(folio); > goto free_it; > } > > ... or ... > > if (folio_ref_count(folio) == 1 && > folio_ref_freeze(folio, 1)) { > > ... if we want to test-and-test-and-clear These proposed code changes look good to me. > > But this function is far too complicated already. I really want to > see numbers that proves the extra complexity is worth it. This optimization can save lots of cpu cycles and avoid possible disk I/O in that specified case. But that is a somewhat rare case. So there's no numbers that proves the extra complexity is worth it. Should I drop this patch or proceed with the proposed code changes above in next version? :) Many thanks! > > > . >