From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id CC2936B74CB for ; Wed, 5 Dec 2018 09:35:13 -0500 (EST) Received: by mail-pf1-f199.google.com with SMTP id b17so16836286pfc.11 for ; Wed, 05 Dec 2018 06:35:13 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id c3si20156079pls.73.2018.12.05.06.35.12 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 05 Dec 2018 06:35:12 -0800 (PST) Date: Wed, 5 Dec 2018 06:35:10 -0800 From: Matthew Wilcox Subject: Re: [PATCH RFC 7/7] mm: better document PG_reserved Message-ID: <20181205143510.GA17232@bombadil.infradead.org> 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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. > +++ 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. So maybe just add one more option to the list.