From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) by kanga.kvack.org (Postfix) with ESMTP id ED41A8E0001 for ; Mon, 17 Dec 2018 04:35:07 -0500 (EST) Received: by mail-qk1-f198.google.com with SMTP id a199so14427830qkb.23 for ; Mon, 17 Dec 2018 01:35:07 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id s30si3294572qtj.60.2018.12.17.01.35.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 17 Dec 2018 01:35:06 -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> <03f65c6f-1287-4c63-6705-16e58d659d94@infradead.org> From: David Hildenbrand Message-ID: Date: Mon, 17 Dec 2018 10:34:59 +0100 MIME-Version: 1.0 In-Reply-To: <03f65c6f-1287-4c63-6705-16e58d659d94@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Randy Dunlap , 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 15.12.18 01:12, Randy Dunlap wrote: > 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. Indeed. > >> + * 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, This should actually be vmemmap (aka struct pages). > >> + * 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. > Thanks! -- Thanks, David / dhildenb