Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752948AbcDKKb4 (ORCPT ); Mon, 11 Apr 2016 06:31:56 -0400 Received: from mail-io0-f182.google.com ([209.85.223.182]:34773 "EHLO mail-io0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751216AbcDKKby (ORCPT ); Mon, 11 Apr 2016 06:31:54 -0400 MIME-Version: 1.0 In-Reply-To: <570B758E.7070005@hisilicon.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> Date: Mon, 11 Apr 2016 12:31:53 +0200 Message-ID: Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64 From: Ard Biesheuvel To: Chen Feng , Mark Rutland Cc: Will Deacon , mhocko@suse.com, Laura Abbott , Dan Zhao , Yiping Xu , puck.chen@foxmail.com, albert.lubing@hisilicon.com, Catalin Marinas , suzhuangluan@hisilicon.com, "linux-kernel@vger.kernel.org" , linuxarm@huawei.com, "linux-mm@kvack.org" , kirill.shutemov@linux.intel.com, David Rientjes , oliver.fu@hisilicon.com, Andrew Morton , robin.murphy@arm.com, "linux-arm-kernel@lists.infradead.org" , saberlily.xia@hisilicon.com Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5229 Lines: 138 On 11 April 2016 at 11:59, Chen Feng wrote: > Hi Ard, > > On 2016/4/11 16:00, Ard Biesheuvel wrote: >> On 11 April 2016 at 09:55, Chen Feng wrote: >>> Hi Ard, >>> >>> On 2016/4/11 15:35, Ard Biesheuvel wrote: >>>> On 11 April 2016 at 04:49, Chen Feng wrote: >>>>> Hi will, >>>>> Thanks for review. >>>>> >>>>> On 2016/4/7 22:21, Will Deacon wrote: >>>>>> On Tue, Apr 05, 2016 at 04:22:51PM +0800, Chen Feng wrote: >>>>>>> We can reduce the memory allocated at mem-map >>>>>>> by flatmem. >>>>>>> >>>>>>> currently, the default memory-model in arm64 is >>>>>>> sparse memory. The mem-map array is not freed in >>>>>>> this scene. If the physical address is too long, >>>>>>> it will reserved too much memory for the mem-map >>>>>>> array. >>>>>> >>>>>> Can you elaborate a bit more on this, please? We use the vmemmap, so any >>>>>> spaces between memory banks only burns up virtual space. What exactly is >>>>>> the problem you're seeing that makes you want to use flatmem (which is >>>>>> probably unsuitable for the majority of arm64 machines). >>>>>> >>>>> The root cause we want to use flat-mem is the mam_map alloced in sparse-mem >>>>> is not freed. >>>>> >>>>> take a look at here: >>>>> arm64/mm/init.c >>>>> void __init mem_init(void) >>>>> { >>>>> #ifndef CONFIG_SPARSEMEM_VMEMMAP >>>>> free_unused_memmap(); >>>>> #endif >>>>> } >>>>> >>>>> Memory layout (3GB) >>>>> >>>>> 0 1.5G 2G 3.5G 4G >>>>> | | | | | >>>>> +--------------+------+---------------+--------------+ >>>>> | MEM | hole | MEM | IO (regs) | >>>>> +--------------+------+---------------+--------------+ >>>>> >>>>> >>>>> Memory layout (4GB) >>>>> >>>>> 0 3.5G 4G 4.5G >>>>> | | | | >>>>> +-------------------------------------+--------------+-------+ >>>>> | MEM | IO (regs) | MEM | >>>>> +-------------------------------------+--------------+-------+ >>>>> >>>>> Currently, the sparse memory section is 1GB. >>>>> >>>>> 3GB ddr: the 1.5 ~2G and 3.5 ~ 4G are holes. >>>>> 3GB ddr: the 3.5 ~ 4G and 4.5 ~ 5G are holes. >>>>> >>>>> This will alloc 1G/4K * (struct page) memory for mem_map array. >>>>> >>>> >>>> No, this is incorrect. Sparsemem vmemmap only allocates struct pages >>>> for memory regions that are actually populated. >>>> >>>> For instance, on the Foundation model with 4 GB of memory, you may see >>>> something like this in the boot log >>>> >>>> [ 0.000000] vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000 >>>> ( 8 GB maximum) >>>> [ 0.000000] 0xffffffbdc0000000 - 0xffffffbde2000000 >>>> ( 544 MB actual) >>>> >>>> but in reality, only the following regions have been allocated >>>> >>>> ---[ vmemmap start ]--- >>>> 0xffffffbdc0000000-0xffffffbdc2000000 32M RW NX SHD AF >>>> BLK UXN MEM/NORMAL >>>> 0xffffffbde0000000-0xffffffbde2000000 32M RW NX SHD AF >>>> BLK UXN MEM/NORMAL >>>> ---[ vmemmap end ]--- >>>> >>>> so only 64 MB is used to back 4 GB of RAM with struct pages, which is >>>> minimal. Moving to flatmem will not reduce the memory footprint at >>>> all. >>> >>> Yes,but the populate is section, which is 1GB. Take a look at the above >>> memory layout. >>> >>> The section 1G ~ 2G is a section. But 1.5G ~ 2G is a hole. >>> >>> The section 3G ~ 4G is a section. But 3.5G ~ 4G is a hole. >>>>> 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 ]--- > OK, I see what you mean now. Sorry for taking so long to catch up. > 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. Thanks, Ard.