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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id CF844C433F5 for ; Fri, 10 Dec 2021 17:20:30 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 32ECA6B0072; Fri, 10 Dec 2021 12:20:20 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 2DE9B6B0073; Fri, 10 Dec 2021 12:20:20 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1CE086B0074; Fri, 10 Dec 2021 12:20:20 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0160.hostedemail.com [216.40.44.160]) by kanga.kvack.org (Postfix) with ESMTP id 0B0966B0072 for ; Fri, 10 Dec 2021 12:20:20 -0500 (EST) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id BA36186992 for ; Fri, 10 Dec 2021 17:20:09 +0000 (UTC) X-FDA: 78902547738.18.0A2E643 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf10.hostedemail.com (Postfix) with ESMTP id 5AECFC0010 for ; Fri, 10 Dec 2021 17:20:06 +0000 (UTC) Received: from mail.kernel.org (unknown [198.145.29.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 68F8DB8291B; Fri, 10 Dec 2021 17:20:07 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id 9417560C4B; Fri, 10 Dec 2021 17:20:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1639156805; bh=pv3ZYrotcb0uW7lV6hIkQGuAyq/RUhIrnAgbae5vAIY=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=MDFk4gU9evOWaGId7xWHaNPUy1NOOEWwfxBj2LFa+8l7e5EnRxBC8UNL12W55Oz4g 8LeRvgYdlwXrxveGzScE8Mn3xZp6Tg3va1OGbJsSM6bg/xHMYrHGteYEsx62QdPajR 2f0zu0tuHQzkiDxAhJzwskovryvrWq3wFRZs8OSM= Date: Fri, 10 Dec 2021 09:20:03 -0800 From: Andrew Morton To: Hugh Dickins Cc: 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() Message-Id: <20211210092003.cf84354b406a47253afc868c@linux-foundation.org> In-Reply-To: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> References: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 5AECFC0010 Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=MDFk4gU9; spf=pass (imf10.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-Rspamd-Server: rspam04 X-Stat-Signature: ouhsjixei3eqahd8zfbrydd7zhqrbn36 X-HE-Tag: 1639156806-127698 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.