Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964997AbcDLO7L (ORCPT ); Tue, 12 Apr 2016 10:59:11 -0400 Received: from foss.arm.com ([217.140.101.70]:56360 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964905AbcDLO7K (ORCPT ); Tue, 12 Apr 2016 10:59:10 -0400 Date: Tue, 12 Apr 2016 15:59:03 +0100 From: Catalin Marinas To: Ard Biesheuvel Cc: Chen Feng , Mark Rutland , Dan Zhao , mhocko@suse.com, Yiping Xu , puck.chen@foxmail.com, "linux-mm@kvack.org" , suzhuangluan@hisilicon.com, Will Deacon , "linux-kernel@vger.kernel.org" , linuxarm@huawei.com, albert.lubing@hisilicon.com, "linux-arm-kernel@lists.infradead.org" , David Rientjes , oliver.fu@hisilicon.com, Andrew Morton , Laura Abbott , robin.murphy@arm.com, kirill.shutemov@linux.intel.com, saberlily.xia@hisilicon.com Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64 Message-ID: <20160412145903.GF8066@e104818-lin.cambridge.arm.com> References: <1459844572-53069-1-git-send-email-puck.chen@hisilicon.com> <20160407142148.GI5657@arm.com> <570B10B2.2000000@hisilicon.com> <570B5875.20804@hisilicon.com> <570B758E.7070005@hisilicon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2976 Lines: 62 On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote: > On 11 April 2016 at 11:59, Chen Feng wrote: > > On 2016/4/11 16:00, Ard Biesheuvel wrote: > >> On 11 April 2016 at 09:55, Chen Feng wrote: > >>> On 2016/4/11 15:35, Ard Biesheuvel wrote: > >>>> On 11 April 2016 at 04:49, Chen Feng wrote: > >>>>> 0 1.5G 2G 3.5G 4G > >>>>> | | | | | > >>>>> +--------------+------+---------------+--------------+ > >>>>> | MEM | hole | MEM | IO (regs) | > >>>>> +--------------+------+---------------+--------------+ > >>> The hole in 1.5G ~ 2G is also allocated mem-map array. And also with the 3.5G ~ 4G. > >>> > >> > >> No, it is not. It may be covered by a section, but that does not mean > >> sparsemem vmemmap will actually allocate backing for it. The > >> granularity used by sparsemem vmemmap on a 4k pages kernel is 128 MB, > >> due to the fact that the backing is performed at PMD granularity. > >> > >> Please, could you share the contents of the vmemmap section in > >> /sys/kernel/debug/kernel_page_tables of your system running with > >> sparsemem vmemmap enabled? You will need to set CONFIG_ARM64_PTDUMP=y > > > > Please see the pg-tables below. > > > > With sparse and vmemmap enable. > > > > ---[ vmemmap start ]--- > > 0xffffffbdc0200000-0xffffffbdc4800000 70M RW NX SHD AF UXN MEM/NORMAL > > ---[ vmemmap end ]--- [...] > > The board is 4GB, and the memap is 70MB > > 1G memory --- 14MB mem_map array. > > No, this is incorrect. 1 GB corresponds with 16 MB worth of struct > pages assuming sizeof(struct page) == 64 > > So you are losing 6 MB to rounding here, which I agree is significant. > I wonder if it makes sense to use a lower value for SECTION_SIZE_BITS > on 4k pages kernels, but perhaps we're better off asking the opinion > of the other cc'ees. IIRC, SECTION_SIZE_BITS was chosen to be the maximum sane value we were thinking of at the time, assuming that 1GB RAM alignment to be fairly normal. For the !SPARSEMEM_VMEMMAP case, we should probably be fine with 29 but, as Will said, we need to be careful with the page flags. At a quick look, we have 25 page flags, 2 bits per zone, NUMA nodes and (48 - section_size_bits) for the section width. We also need to take into account 4 more bits for 52-bit PA support (ARMv8.2). So, without NUMA nodes, we are currently at 49 bits used in page->flags. For the SPARSEMEM_VMEMMAP case, we can decrease the SECTION_SIZE_BITS in the MAX_ORDER limit. An alternative would be to free the vmemmap holes later (but still keep the vmemmap mapping alias). Yet another option would be to change the sparse_mem_map_populate() logic get the actual section end rather than always assuming PAGES_PER_SECTION. But I don't think any of these are worth if we can safely reduce SECTION_SIZE_BITS. -- Catalin