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=-0.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS 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 8649DC433F5 for ; Sat, 1 Sep 2018 17:30:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 222982077C for ; Sat, 1 Sep 2018 17:30:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="U8rXc9Z6" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 222982077C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.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 S1727357AbeIAVkd (ORCPT ); Sat, 1 Sep 2018 17:40:33 -0400 Received: from mail-io0-f174.google.com ([209.85.223.174]:37968 "EHLO mail-io0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727079AbeIAVkd (ORCPT ); Sat, 1 Sep 2018 17:40:33 -0400 Received: by mail-io0-f174.google.com with SMTP id y3-v6so12890505ioc.5 for ; Sat, 01 Sep 2018 10:27:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=QyhfrD3N4GyWMZGeUE19ZxMHb/p/v3MdWJ2DiB0Nn68=; b=U8rXc9Z604dkSmQ72uePHxutI5qBMgxtVWNy0uOIYbK7LwvRz6qExDLqnRjndZUpHB dvzQz8sajBAOa/N8n0IlmrOqR2sV6RSgWCtvSKSmU67wK+El7BtHULxdai6/6+avL/4C bG7aTlSEk+PMHLNUazFPcOWzEfl//wjA0D1c4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=QyhfrD3N4GyWMZGeUE19ZxMHb/p/v3MdWJ2DiB0Nn68=; b=fR6cbdAicZT2WRa40CFgwxk5pAq+FRtU/wnlBdQtNb5MQtETwiG/Rf4q8E2p0bXTKY kYD+NWAgRykWBuRxt1IuPiTMuEyqSxh5KhWdaDrCa6GMyRnfmnP/x5kLfLfx46ym14dl kR57RfaImmVIifhzgOQxpb8R0dID3ObS/IPVIvEPHZdD4Wvy6ELrZcZK6i/7wn5l5eAY HN0eBG6sw2Ejzqe1CaVV1amBFM6ah6r3jYibS+7RWX95IGekKc+TLoHQlMX3WJS3GI/C ZfrZQcNA6RaL5QFII9O6FXuXqgLBGjqNELVIMQVv4m17hfuXte0Z2CT4fBPlHXigycQ9 0OXA== X-Gm-Message-State: APzg51CmEZIugmYzuwNdPcWOed0HTca2J+OgftzZrQtoE8Qbm2iPsrc6 msI33XmCcCIUK+ZDPrIoRQ/8fyuZdSQImQO31pw= X-Google-Smtp-Source: ANB0VdaqX7bkesv2vRNB1mJiBDJg5RPY7r0ZZaU6G0OQWPijRDrF6w15DabD5/aLvvyjfyXL48lffKfLNgHJSouZU8c= X-Received: by 2002:a6b:f815:: with SMTP id o21-v6mr16293783ioh.203.1535822871023; Sat, 01 Sep 2018 10:27:51 -0700 (PDT) MIME-Version: 1.0 References: <3009b28a-971c-920a-9184-900f1f3b2203@suse.com> In-Reply-To: From: Linus Torvalds Date: Sat, 1 Sep 2018 10:27:39 -0700 Message-ID: Subject: Re: Access to non-RAM pages To: Jiri Kosina Cc: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Linux Kernel Mailing List , Michal Hocko Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 31, 2018 at 2:18 PM Jiri Kosina wrote: > > If noone has any clever idea how to work this around (I don't), I am > afraid we'd have to ditch the whole DCACHE_WORD_ACCESS optimization, as > it's silently dangerous. No way in hell will I apply such a stupid patch. It is NOT dangerous. If you have a machine with RAM that touches IO, you need to disable the last page, exactly the same way we disable and marked reserved the first page at zero. I thought we already did that. I suspect this is a Xen bug, where the fake BIOS sets up a garbage description of the hardware that is simply not realistic. I don't think I've ever seen a machine that didn't have some reserved memory at the top, but hey, if we don't expressly mark the last page reserved already, doing so should be trivial. No way do we disable the word accesses just because of some crazy corner case that doesn't matter and doesn't happen in reality. Linus