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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C796CC388F9 for ; Thu, 22 Oct 2020 16:46:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 61C732463F for ; Thu, 22 Oct 2020 16:46:35 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="PSyTwhKE" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2902018AbgJVQqe (ORCPT ); Thu, 22 Oct 2020 12:46:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54636 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2897884AbgJVQqd (ORCPT ); Thu, 22 Oct 2020 12:46:33 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A44B0C0613CE; Thu, 22 Oct 2020 09:46:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=xro6DWExEiJhODwXOAby9O4a+I7/GqhPz/+TxrJCSCI=; b=PSyTwhKEJI7x6Yu1qPkYUlYXbj 3NqqmLBtG03MbjTHdDGJBrpt5Ev5AwfD6v1ObDzWTzNpwECQsirZtMgIFPFMcvVoDS+LxyiaDvJVa yQQgYFDKrPbnKsQgzHlZ+UBttzVn4O8gnIaS4m1e4A7cpCdedooUcoKaFbVlb20zznFVPu8+U/hD6 TxA0vBUNaYfrcx55mgpAT7H5P3GLgxVdcQOKi55Ta65jj2LWvJek6vxpiYtgYOMau2fIXK3Up7P6B 6QhfrNiW51zGsPK4jwNiBLey0oCMNYPtzHSN4XMbmo5zMKUcDlGTYuO68J31gzhjz+4qL/cXqZiWx tvHZG5Hw==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kVdjM-0003Cv-50; Thu, 22 Oct 2020 16:46:24 +0000 Date: Thu, 22 Oct 2020 17:46:24 +0100 From: Matthew Wilcox To: William Kucharski Cc: Qian Cai , Christoph Hellwig , "Darrick J. Wong" , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Jens Axboe , linux-mm@kvack.org Subject: Re: kernel BUG at mm/page-writeback.c:2241 [ BUG_ON(PageWriteback(page); ] Message-ID: <20201022164624.GW20115@casper.infradead.org> References: <645a3f332f37e09057c10bc32f4f298ce56049bb.camel@lca.pw> <20201022004906.GQ20115@casper.infradead.org> <361D9B8E-CE8F-4BA0-8076-8384C2B7E860@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <361D9B8E-CE8F-4BA0-8076-8384C2B7E860@oracle.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 22, 2020 at 07:23:33AM -0600, William Kucharski wrote: > > > > On Oct 21, 2020, at 6:49 PM, Matthew Wilcox wrote: > > > > On Wed, Oct 21, 2020 at 08:30:18PM -0400, Qian Cai wrote: > >> Today's linux-next starts to trigger this wondering if anyone has any clue. > > > > I've seen that occasionally too. I changed that BUG_ON to VM_BUG_ON_PAGE > > to try to get a clue about it. Good to know it's not the THP patches > > since they aren't in linux-next. > > > > I don't understand how it can happen. We have the page locked, and then we do: > > > > if (PageWriteback(page)) { > > if (wbc->sync_mode != WB_SYNC_NONE) > > wait_on_page_writeback(page); > > else > > goto continue_unlock; > > } > > > > VM_BUG_ON_PAGE(PageWriteback(page), page); > > > > Nobody should be able to put this page under writeback while we have it > > locked ... right? The page can be redirtied by the code that's supposed > > to be writing it back, but I don't see how anyone can make PageWriteback > > true while we're holding the page lock. > > Looking at __test_set_page_writeback(), I see that it (and most other > callers to lock_page_memcg()) do the following: lock_page_memcg() is, unfortunately, completely unrelated to lock_page(). I believe all callers of __test_set_page_writeback() have the page lock held already, but I'm going to put in an assert to that effect.