From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1756964AbbIVLRc (ORCPT <rfc822;w@1wt.eu>);
	Tue, 22 Sep 2015 07:17:32 -0400
Received: from eu-smtp-delivery-143.mimecast.com ([146.101.78.143]:45997 "EHLO
	eu-smtp-delivery-143.mimecast.com" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1751481AbbIVLRa convert rfc822-to-8bit
	(ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Tue, 22 Sep 2015 07:17:30 -0400
Date: Tue, 22 Sep 2015 12:17:27 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: chenfeng <puck.chen@hisilicon.com>
Cc: catalin.marinas@arm.com, will.deacon@arm.com, mark.rutland@arm.com,
        ard.biesheuvel@linaro.org, lauraa@codeaurora.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Dan zhao <dan.zhao@hisilicon.com>, Yiping Xu <xuyiping@hisilicon.com>,
        suzhuangluan@hisilicon.com, qijiwen <qijiwen@hisilicon.com>,
        oliver.fu@hisilicon.com,
        Peter Panshilin <peter.panshilin@hisilicon.com>
Subject: Re: Question about the sparse memory section size
Message-ID: <20150922111723.GI6281@e103592.cambridge.arm.com>
References: <56010DDD.3080803@hisilicon.com>
MIME-Version: 1.0
In-Reply-To: <56010DDD.3080803@hisilicon.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-OriginalArrivalTime: 22 Sep 2015 11:17:27.0279 (UTC) FILETIME=[43720FF0:01D0F528]
X-MC-Unique: NZzFniCuSy6crudTds03Xg-1
Content-Type: text/plain; charset=WINDOWS-1252
Content-Transfer-Encoding: 8BIT
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Sep 22, 2015 at 04:14:21PM +0800, chenfeng wrote:
> Hi all,
> The sparse memory section size, SECTION_SIZE_BITS, currently is 1GB
> for arm64 by default. However, it might generate wasted memmap memory
> space for those memory sections less than 1GB. e.g.
> 
> for 512MB memory section, still 14MB(sizeof(struct page) *
> PAGES_PER_SECTION) memmap needs to be reserved. The wasted memmap
> space could be eliminated by changing memory section size from 1GB to
> 512M, but still some questions to be answered,
> 
> 1) why arm64 uses 1GB as default setting?
> 2) any risk to change section size from 1GB to 512MB? like, any
> impact to performance since memory section number is increased.

For arm64 we have SPARSEMEM_VMEMMAP enabled by default, which enables
much of the wasted memmap backing memory to be reclaimed.

Take a look at arch/arm64/mm/init.c:free_unused_memmap().

This should reduce the amount of actual memory wasted on unused parts
of memmap.  The virtual space stays wasted as you describe, but that's
plentiful on 64-bit arches.

You could try sticking some printks in there is you want to see how much
of the memmap the code successfully frees.

Cheers
---Dave