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=-8.5 required=3.0 tests=INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,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 3FAF6C04EB9 for ; Wed, 5 Dec 2018 13:00:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0FFE92081B for ; Wed, 5 Dec 2018 13:00:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0FFE92081B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.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 S1727919AbeLENAB (ORCPT ); Wed, 5 Dec 2018 08:00:01 -0500 Received: from mx2.suse.de ([195.135.220.15]:56054 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726944AbeLENAB (ORCPT ); Wed, 5 Dec 2018 08:00:01 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 7CA82B0E5; Wed, 5 Dec 2018 12:59:58 +0000 (UTC) Date: Wed, 5 Dec 2018 13:59:58 +0100 From: Michal Hocko 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 , Alexander Duyck , Matthew Wilcox , 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: <20181205125957.GN1286@dhcp22.suse.cz> References: <20181205122851.5891-1-david@redhat.com> <20181205122851.5891-8-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181205122851.5891-8-david@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 05-12-18 13:28:51, David Hildenbrand wrote: > The usage of PG_reserved and how PG_reserved pages are to be treated is > burried deep down in different parts of the kernel. Let's shine some light > onto these details by documenting (most?) current users and expected > behavior. > > 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. > > Cc: Andrew Morton > Cc: Stephen Rothwell > Cc: Pavel Tatashin > Cc: Michal Hocko > Cc: Alexander Duyck > Cc: Matthew Wilcox > Cc: Anthony Yznaga > Cc: Miles Chen > Cc: yi.z.zhang@linux.intel.com > Cc: Dan Williams > Signed-off-by: David Hildenbrand This looks like an improvement. The essential part is that PG_reserved page belongs to its user and no generic code should touch it. The rest is a description of current users which I haven't checked due to to lack of time but yeah, I like the updated wording because I have seen multiple people confused from the swapped out part which is not true for many many years. I have tried to dig out when it was actually the case but failed. So I cannot give my Ack because I didn't really do a real review but I like this FWIW. > --- > include/linux/page-flags.h | 18 ++++++++++++++++-- > 1 file changed, 16 insertions(+), 2 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 68b8495e2fbc..112526f5ba61 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -17,8 +17,22 @@ > /* > * Various page->flags bits: > * > - * PG_reserved is set for special pages, which can never be swapped out. Some > - * of them might not even exist... > + * PG_reserved is set for special pages. The "struct page" of such a page > + * should in general not be touched (e.g. set dirty) except by their owner. > + * Pages marked as PG_reserved include: > + * - Kernel image (including vDSO) and similar (e.g. BIOS, initrd) > + * - Pages allocated early during boot (bootmem, memblock) > + * - Zero pages > + * - Pages that have been associated with a zone but are not available for > + * the page allocator (e.g. excluded via online_page_callback()) > + * - Pages to exclude from the hibernation image (e.g. loaded kexec images) > + * - MMIO pages (communicate with a device, special caching strategy needed) > + * - MCA pages on ia64 (pages with memory errors) > + * - Device memory (e.g. PMEM, DAX, HMM) > + * Some architectures don't allow to ioremap pages that are not marked > + * PG_reserved (as they might be in use by somebody else who does not respect > + * the caching strategy). Consequently, PG_reserved for a page mapped into > + * user space can indicate the zero page, the vDSO, MMIO pages or device memory. > * > * The PG_private bitflag is set on pagecache pages if they contain filesystem > * specific data (which is normally at page->private). It can be used by > -- > 2.17.2 > -- Michal Hocko SUSE Labs