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=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 02301C433EF for ; Fri, 24 Sep 2021 02:43:47 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9B756610F7 for ; Fri, 24 Sep 2021 02:43:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 9B756610F7 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id E6A1A6B006C; Thu, 23 Sep 2021 22:43:45 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E19F5900002; Thu, 23 Sep 2021 22:43:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CE1FF6B0073; Thu, 23 Sep 2021 22:43:45 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0254.hostedemail.com [216.40.44.254]) by kanga.kvack.org (Postfix) with ESMTP id BB82D6B006C for ; Thu, 23 Sep 2021 22:43:45 -0400 (EDT) Received: from smtpin28.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 75E84180DC6F0 for ; Fri, 24 Sep 2021 02:43:45 +0000 (UTC) X-FDA: 78620921610.28.5D5C4D5 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf05.hostedemail.com (Postfix) with ESMTP id 2B38250645B7 for ; Fri, 24 Sep 2021 02:43:45 +0000 (UTC) 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 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2B38250645B7 X-Stat-Signature: njctwfxgk5tksbrtqptbneb75wn71p56 Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=NF4px9H0; spf=pass (imf05.hostedemail.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-HE-Tag: 1632451425-528881 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang wrote: >=20 >=20 > > On Sep 22, 2021, at 7:37 PM, Matthew Wilcox wro= te: > >=20 > > On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote: > >> Transparent huge page has supported read-only non-shmem files. The f= ile- > >> backed THP is collapsed by khugepaged and truncated when written (fo= r > >> shared libraries). > >>=20 > >> However, there is race in two possible places. > >>=20 > >> 1) multiple writers truncate the same page cache concurrently; > >> 2) collapse_file rolls back when writer truncates the page cache; > >=20 > > 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 =E2=80=9Cmm, thp: relax the VM_DE= NYWRITE constraint on file-backed THPs" > Introduced file-backed THPs for DSO. It is possible {very rarely} for D= SO to be opened in writeable way. > > ... > > > https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@casper.infradead.or= g/ > All in all, what you mean is that we should solve this race at the sour= ce? 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.=20 Perhaps we should permit the open-writeably attempt to appear to succeed, but to really return a read-only fd?