From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by kanga.kvack.org (Postfix) with ESMTP id 5993F8E021D for ; Fri, 14 Dec 2018 19:13:06 -0500 (EST) Received: by mail-wm1-f71.google.com with SMTP id y85so2511749wmc.7 for ; Fri, 14 Dec 2018 16:13:06 -0800 (PST) Received: from merlin.infradead.org (merlin.infradead.org. [2001:8b0:10b:1231::1]) by mx.google.com with ESMTPS id q3si4291933wrx.388.2018.12.14.16.13.04 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 14 Dec 2018 16:13:04 -0800 (PST) Subject: Re: [PATCH v1 9/9] mm: better document PG_reserved References: <20181214111014.15672-1-david@redhat.com> <20181214111014.15672-10-david@redhat.com> From: Randy Dunlap Message-ID: <03f65c6f-1287-4c63-6705-16e58d659d94@infradead.org> Date: Fri, 14 Dec 2018 16:12:22 -0800 MIME-Version: 1.0 In-Reply-To: <20181214111014.15672-10-david@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand , linux-mm@kvack.org Cc: 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 , Matthew Wilcox , Anthony Yznaga , Miles Chen , yi.z.zhang@linux.intel.com, Dan Williams On 12/14/18 3:10 AM, David Hildenbrand wrote: > The usage of PG_reserved and how PG_reserved pages are to be treated is > buried deep down in different parts of the kernel. Let's shine some light > onto these details by documenting current users and expected > behavior. > > Especially, clarify on the "Some of them might not even exist" case. > These are physical memory gaps that will never be dumped as they > are not marked as IORESOURCE_SYSRAM. PG_reserved does in general not > hinder anybody from dumping or swapping. In some cases, these pages > will not be stored in the hibernation image. Hi, Thanks for the doc update. Comments below. > 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 > --- > include/linux/page-flags.h | 33 +++++++++++++++++++++++++++++++-- > 1 file changed, 31 insertions(+), 2 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 808b4183e30d..9de2e941cbd5 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -17,8 +17,37 @@ > /* > * 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. by its owner. > + * Pages marked as PG_reserved include: > + * - Pages part of the kernel image (including vDSO) and similar (e.g. BIOS, > + * initrd, HW tables) > + * - Pages reserved or allocated early during boot (before the page allocator > + * was initialized). This includes (depending on the architecture) the > + * initial vmmap, initial page tables, crashkernel, elfcorehdr, and much VM map, > + * much more. Once (if ever) freed, PG_reserved is cleared and they will > + * be given to the page allocator. > + * - Pages falling into physical memory gaps - not IORESOURCE_SYSRAM. Trying > + * to read/write these pages might end badly. Don't touch! > + * - The zero page(s) > + * - Pages not added to the page allocator when onlining a section because > + * they were excluded via the online_page_callback() or because they are > + * PG_hwpoison. > + * - Pages allocated in the context of kexec/kdump (loaded kernel image, > + * control pages, vmcoreinfo) > + * - MMIO/DMA pages. 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). > + * - Pages part of an offline section (struct pages of offline sections should > + * not be trusted as they will be initialized when first onlined). > + * - MCA pages on ia64 > + * - Pages holding CPU notes for POWER Firmware Assisted Dump > + * - Device memory (e.g. PMEM, DAX, HMM) > + * Some PG_reserved pages will be excluded from the hibernation image. > + * PG_reserved does in general not hinder anybody from dumping or swapping > + * and is no longer required for remap_pfn_range(). ioremap might require it. > + * 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 > cheers. -- ~Randy