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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS 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 CF914C433F5 for ; Fri, 24 Sep 2021 02:43:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B1EB761100 for ; Fri, 24 Sep 2021 02:43:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243937AbhIXCpR (ORCPT ); Thu, 23 Sep 2021 22:45:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:54454 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233314AbhIXCpQ (ORCPT ); Thu, 23 Sep 2021 22:45:16 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 181AA610CB; Fri, 24 Sep 2021 02:43:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1632451424; bh=F3QkKswuGwQOCXfk/SaqNE24XSVoy4mqskLjR596bIw=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=NF4px9H06Hlw8rFhwiBjM+5UT5NYYROaIqe5rA0lXMeYHV596JNvS19bxrNuJ6BRc iUbLfYv3mjkROkmuJYuau4wv41xtutbQu3+yrXSgYoovteZJlKV/4N904TYIvHkzcY QgYhPsbQnRMtaW2FSXSwzshtXK4bIbaxk3WykHYE= Date: Thu, 23 Sep 2021 19:43:43 -0700 From: Andrew Morton To: Rongwei Wang Cc: Matthew Wilcox , Linux MM , Linux Kernel Mailing List , song@kernel.org, william.kucharski@oracle.com, Hugh Dickins Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache Message-Id: <20210923194343.ca0f29e1c4d361170343a6f2@linux-foundation.org> In-Reply-To: References: <20210906121200.57905-1-rongwei.wang@linux.alibaba.com> <20210922070645.47345-2-rongwei.wang@linux.alibaba.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=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang wrote: > > > > On Sep 22, 2021, at 7:37 PM, Matthew Wilcox wrote: > > > > On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote: > >> Transparent huge page has supported read-only non-shmem files. The file- > >> backed THP is collapsed by khugepaged and truncated when written (for > >> shared libraries). > >> > >> However, there is race in two possible places. > >> > >> 1) multiple writers truncate the same page cache concurrently; > >> 2) collapse_file rolls back when writer truncates the page cache; > > > > As I've said before, the bug here is that somehow there is a writable fd > > to a file with THPs. That's what we need to track down and fix. > Hi, Matthew > I am not sure get your means. We know “mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs" > Introduced file-backed THPs for DSO. It is possible {very rarely} for DSO to be opened in writeable way. > > ... > > > https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@casper.infradead.org/ > All in all, what you mean is that we should solve this race at the source? Matthew is being pretty clear here: we shouldn't be permitting userspace to get a writeable fd for a thp-backed file. Why are we permitting the DSO to be opened writeably? If there's a legitimate case for doing this then presumably "mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs: should be fixed or reverted. If there is no legitimate use case for returning a writeable fd for a thp-backed file then we should fail such an attempt at open(). This approach has back-compatibility issues which need to be thought about. Perhaps we should permit the open-writeably attempt to appear to succeed, but to really return a read-only fd?