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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 08817C04EB9 for ; Wed, 5 Dec 2018 17:32:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C490B20879 for ; Wed, 5 Dec 2018 17:32:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="SDVRmLJo" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C490B20879 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.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 S1728183AbeLERcE (ORCPT ); Wed, 5 Dec 2018 12:32:04 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:48150 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727475AbeLERcE (ORCPT ); Wed, 5 Dec 2018 12:32:04 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=RdrY3P1cNmaV9ItIQ4yjB9qAqpkVJ/dzzD9kY8jmt5c=; b=SDVRmLJotdymrXNnDKMsNc3TZ aILEa+7Dn/LsKJz2L8u+TeGH3qaM9XHE85ijGedcmvmC4UvYmkNs+aGM91FbEpfWmVonSkLeWZxHD 5DhlHwsLLJ2FnzM/jbr/LIS/4uuxvrlKsWQaBf/sojsW9vphIyoCS/DUBuNIMDg1z0N0gZnp9fG/J zcAR/q3jM+D3P6PzFTGgB6wNf6VgkFs1mhRKPbiCbytkMQSB0DoN85a2BPUUrbuVTc8pFtKPVFH72 +nQ409q6abyprryoQ4m8DqmcDS1WhrYIsQhGk10/0j/El7cHOKapqQvmJxu8eCNSx8vdJ1BPBrCS3 ybCbzuULw==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gUb1l-00088G-HT; Wed, 05 Dec 2018 17:32:01 +0000 Date: Wed, 5 Dec 2018 09:32:01 -0800 From: Matthew Wilcox To: David Hildenbrand Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-mediatek@lists.infradead.org, Andrew Morton , Stephen Rothwell , Pavel Tatashin , Michal Hocko , Alexander Duyck , Anthony Yznaga , Miles Chen , yi.z.zhang@linux.intel.com, Dan Williams Subject: Re: [PATCH RFC 7/7] mm: better document PG_reserved Message-ID: <20181205173201.GA11646@bombadil.infradead.org> References: <20181205122851.5891-1-david@redhat.com> <20181205122851.5891-8-david@redhat.com> <20181205143510.GA17232@bombadil.infradead.org> <46d0e90f-f0bb-815e-7a5b-4429de1c502a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46d0e90f-f0bb-815e-7a5b-4429de1c502a@redhat.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 05, 2018 at 04:05:12PM +0100, David Hildenbrand wrote: > On 05.12.18 15:35, Matthew Wilcox wrote: > > On Wed, Dec 05, 2018 at 01:28:51PM +0100, David Hildenbrand wrote: > >> I don't see a reason why we have to document "Some of them might not even > >> exist". If there is a user, we should document it. E.g. for balloon > >> drivers we now use PG_offline to indicate that a page might currently > >> not be backed by memory in the hypervisor. And that is independent from > >> PG_reserved. > > > > I think you're confused by the meaning of "some of them might not even > > exist". What this means is that there might not be memory there; maybe > > writes to that memory will be discarded, or maybe they'll cause a machine > > check. Maybe reads will return ~0, or 0, or cause a machine check. > > We just don't know what's there, and we shouldn't try touching the memory. > > If there are users, let's document it. And I need more details for that :) > > 1. machine check: if there is a HW error, we set PG_hwpoison (except > ia64 MCA, see the list) > > 2. Writes to that memory will be discarded > > Who is the user of that? When will we have such pages right now? > > 3. Reads will return ~0, / 0? > > I think this is a special case of e.g. x86? But where do we have that, > are there any user? When there are gaps in the physical memory. As in, if you put that physical address on the bus (or in a packet), no device will respond to it. Look: 00000000-00000fff : Reserved 00001000-00057fff : System RAM 00058000-00058fff : Reserved 00059000-0009dfff : System RAM 0009e000-000fffff : Reserved Those examples I gave are examples of how various different architectures respond to "no device responded to this memory access".