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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 D0698C433F5 for ; Mon, 3 Sep 2018 02:52:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7FFD320857 for ; Mon, 3 Sep 2018 02:52:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7FFD320857 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727266AbeICHKD (ORCPT ); Mon, 3 Sep 2018 03:10:03 -0400 Received: from gate.crashing.org ([63.228.1.57]:53971 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725780AbeICHKD (ORCPT ); Mon, 3 Sep 2018 03:10:03 -0400 Received: from localhost (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id w832PAI5005868; Sun, 2 Sep 2018 21:25:11 -0500 Message-ID: Subject: Re: Access to non-RAM pages From: Benjamin Herrenschmidt To: Linus Torvalds Cc: Jiri Kosina , =?ISO-8859-1?Q?J=FCrgen_Gro=DF?= , Linux Kernel Mailing List , Michal Hocko , Naoya Horiguchi , Michael Ellerman , Will Deacon Date: Mon, 03 Sep 2018 12:25:10 +1000 In-Reply-To: References: <3009b28a-971c-920a-9184-900f1f3b2203@suse.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5 (3.28.5-1.fc28) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2018-09-02 at 19:10 -0700, Linus Torvalds wrote: > On Sun, Sep 2, 2018 at 7:01 PM Benjamin Herrenschmidt > wrote: > > > > Still, I can potentially see an issue with DEBUG_PAGEALLOC > > An unmapped page isn't a problem. That's what the whole > load_unaligned_zeropad() is about: it's ok to take a fault on the part > that crosses a page, and we'll just fill the value with zeroes (that's > the "zeropad" part). Ah, my bad reading, I was looking at read_word_at_a_time() instead of load_unaligned_zeropad(). I'm not familiar enough with the dentry qstr stuff, I assume this is safe ? > So as long as it's rare (and it is), it's all fine. > > That said, I think we turn off for DEBUG_PAGEALLOC simply because it's > not rare _enough_. > > And vmalloc() should actually be safe too, simply because I think we > strive for a guard page between vmalloc areas. > > So only a *mapped* page after the page that matters, and only if it's > something you can't read without side effects. > > Which basically doesn't happen on x86 in reality. BIOSes just don't > put MMIO right after the last page of RAM. I think this is why it only > triggered on Xen, due to some crazy "Xen reacts badly" case where we > do the speculation into a balloon address. > > So _practically_ this is just a Xen bug, nothing more. > > But since in _theory_ you could have MMIO abut regular RAM directly, > it's worth maybe making sure it's purely theory.