Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752088AbcDKHfr (ORCPT ); Mon, 11 Apr 2016 03:35:47 -0400 Received: from mail-ig0-f176.google.com ([209.85.213.176]:38563 "EHLO mail-ig0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750971AbcDKHfq (ORCPT ); Mon, 11 Apr 2016 03:35:46 -0400 MIME-Version: 1.0 In-Reply-To: <570B10B2.2000000@hisilicon.com> References: <1459844572-53069-1-git-send-email-puck.chen@hisilicon.com> <20160407142148.GI5657@arm.com> <570B10B2.2000000@hisilicon.com> Date: Mon, 11 Apr 2016 09:35:45 +0200 Message-ID: Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64 From: Ard Biesheuvel To: Chen Feng Cc: Will Deacon , mhocko@suse.com, Laura Abbott , Mark Rutland , 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, yudongbin@hislicon.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: 2780 Lines: 80 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.