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 DE821C433EF for ; Wed, 8 Jun 2022 14:10:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241019AbiFHOJ7 (ORCPT ); Wed, 8 Jun 2022 10:09:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60634 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241014AbiFHOJ6 (ORCPT ); Wed, 8 Jun 2022 10:09:58 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2854B1952E6 for ; Wed, 8 Jun 2022 07:09:56 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4LJ8HL0qmrz8wsk; Wed, 8 Jun 2022 22:09:34 +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; Wed, 8 Jun 2022 22:09:52 +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: Date: Wed, 8 Jun 2022 22:09:52 +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: 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 2022/5/27 23:02, Matthew Wilcox wrote: > On Fri, May 27, 2022 at 04:04:51PM +0800, Miaohe Lin wrote: >> If folios were freed from under us, there's no need to reclaim them. Skip >> these folios to save lots of cpu cycles and avoid possible unnecessary >> disk IO. >> >> Signed-off-by: Miaohe Lin >> --- >> mm/vmscan.c | 8 +++++++- >> 1 file changed, 7 insertions(+), 1 deletion(-) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index f7d9a683e3a7..646dd1efad32 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -1556,12 +1556,18 @@ static unsigned int shrink_page_list(struct list_head *page_list, >> folio = lru_to_folio(page_list); >> list_del(&folio->lru); >> >> + nr_pages = folio_nr_pages(folio); >> + if (folio_ref_count(folio) == 1) { >> + /* folio was freed from under us. So we are done. */ >> + WARN_ON(!folio_put_testzero(folio)); > > What? No. This can absolutely happen. We have a refcount on the folio, > which means that any other thread can temporarily raise the refcount, > so this WARN_ON can trigger. Also, we don't hold the folio locked, > or an extra reference, so nr_pages is unstable because it can be split. When I reread the code, I found caller holds an extra reference to the folio when calling isolate_lru_pages(), so folio can't be split and thus nr_pages should be stable indeed? Or am I miss something again? Thanks! > >> + goto free_it; >> + } >> + >> if (!folio_trylock(folio)) >> goto keep; >> >> VM_BUG_ON_FOLIO(folio_test_active(folio), folio); >> >> - nr_pages = folio_nr_pages(folio); >> >> /* Account the number of base pages */ >> sc->nr_scanned += nr_pages; >> -- >> 2.23.0 >> >> > > . >