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 DE0B3C433FE for ; Fri, 10 Dec 2021 19:19:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343601AbhLJTWu (ORCPT ); Fri, 10 Dec 2021 14:22:50 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41260 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1343555AbhLJTWs (ORCPT ); Fri, 10 Dec 2021 14:22:48 -0500 Received: from mail-qt1-x835.google.com (mail-qt1-x835.google.com [IPv6:2607:f8b0:4864:20::835]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 656BFC0617A1 for ; Fri, 10 Dec 2021 11:19:13 -0800 (PST) Received: by mail-qt1-x835.google.com with SMTP id n15so9422862qta.0 for ; Fri, 10 Dec 2021 11:19:13 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:in-reply-to:message-id:references :mime-version; bh=uPGW5ROteU5Fpf16TNGTuiysra125ZlvW3Q6FYu12Ms=; b=iPHEafCooGeQCkTPhS32asxz8gKfEZ0aooITZcjULaTirjc5u5YarrPCgxX2TWK4EX HwNQKwY1aJDPxGP4hobw0YUsiA49xEoFilYIRWMzGAcCS85ZW0sqUTmMPfNufaqRXFSQ imHOMwznVuvRk7MWOiawkN+cv+Sz/Z+VOR+HNcpTKecM9sCgC5rduMrlMsVkJgHtj6pa 9cYkn9Gj/WUHxRreDsaKKTWqWxQW5orCBGWGOpFt0bXseu5wnkKi/J6Ue24jn3e0I2ku G2U4Y5sh0YK+xM8wuFcDIFeyGlq1OmV38D6bkJQDNFXVSFqNt/en743D8/zzcZiEmsvu UuEg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:in-reply-to:message-id :references:mime-version; bh=uPGW5ROteU5Fpf16TNGTuiysra125ZlvW3Q6FYu12Ms=; b=P/QY4K4Ko4TMMSQtUowEjzeMlYBE7erPEiq2sxEADnn2PEZeKVzALVbCuR1VzgZ48p dTmis6P2N2hQe3FDC45xMSgQBM6oxSobxWHKxQGxyV5E1RHKVKomjmOHcmEiKzEAWnLg c/ds86u9guiQ0Pv9dDPJPFrEiACzUwV9eGdp//IGFMKTG0LKK2M9S0RPMHAIHHiop1Uf 3NJFy87c28Bz4cY4YwS/LrBXkTVgESg8SXA6qTCz5y+A17pgcMnBsHKQwB6AxtAGcx2R OgH0MEeQGLLozaeIhIMy4GxAwkxSaSqlIhDD0DeSq6+o6iEbOg7QWNxDi+JKkPp3Es+E Vnhw== X-Gm-Message-State: AOAM53361Aoq0TjxiclRZpvQ+pmRp9tvcOMZVlQtO9vscn8aYZi2xAQ2 pB0pXc7QXQFL3qB0//Q/9BnKvg== X-Google-Smtp-Source: ABdhPJzoGmsQ9RX8SkvmJQMMv/lm4Mg8AzKdjOAay9UJKgN2FM00BrE2R0eQ/+j/lW2UZ5ub0gliOQ== X-Received: by 2002:a05:622a:1d4:: with SMTP id t20mr29279044qtw.84.1639163952383; Fri, 10 Dec 2021 11:19:12 -0800 (PST) Received: from [192.168.1.227] (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id f18sm1787318qko.34.2021.12.10.11.19.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Dec 2021 11:19:12 -0800 (PST) Date: Fri, 10 Dec 2021 11:18:59 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Andrew Morton cc: Hugh Dickins , Matthew Wilcox , Vlastimil Babka , William Kucharski , Christoph Hellwig , "Kirill A. Shutemov" , Mike Rapoport , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative() In-Reply-To: <20211210092003.cf84354b406a47253afc868c@linux-foundation.org> Message-ID: <7e783f99-148f-c7aa-08a-4c4c45c5506a@google.com> References: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> <20211210092003.cf84354b406a47253afc868c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 10 Dec 2021, Andrew Morton wrote: > On Wed, 8 Dec 2021 23:19:18 -0800 (PST) Hugh Dickins wrote: > > > It is not easily reproducible, but on 5.16-rc I have several times hit > > the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative(): > > usually from filemap_get_read_batch() for an ext4 read, yesterday from > > next_uptodate_page() from filemap_map_pages() for a shmem fault. > > > > That BUG used to be placed where page_ref_add_unless() had succeeded, > > but now it is placed before folio_ref_add_unless() is attempted: that > > is not safe, since it is only the acquired reference which makes the > > page safe from racing THP collapse or split. > > > > We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu() > > has succeeded; but I don't think it adds much value - just delete it. > > > > Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()") > > Signed-off-by: Hugh Dickins > > I added cc:stable to this. Thanks, but no, cc:stable not needed: the fixed commit went into 5.16-rc1, and did not go to stable itself. There was an identical VM_BUG_ON_PAGE in the old __page_cache_add_speculative(), but that one was correctly placed, so there's no need for the old one to be removed. Hugh