Date: Mon, 11 Apr 2016 11:40:13 +0100
From: Will Deacon <will.deacon@arm.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Chen Feng <puck.chen@hisilicon.com>, Mark Rutland <mark.rutland@arm.com>,
        mhocko@suse.com, Laura Abbott <labbott@redhat.com>,
        Dan Zhao <dan.zhao@hisilicon.com>, Yiping Xu <xuyiping@hisilicon.com>,
        puck.chen@foxmail.com, albert.lubing@hisilicon.com,
        Catalin Marinas <catalin.marinas@arm.com>, suzhuangluan@hisilicon.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linuxarm@huawei.com, "linux-mm@kvack.org" <linux-mm@kvack.org>,
        kirill.shutemov@linux.intel.com, David Rientjes <rientjes@google.com>,
        oliver.fu@hisilicon.com, Andrew Morton <akpm@linux-foundation.org>,
        robin.murphy@arm.com,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        saberlily.xia@hisilicon.com
Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64
Message-ID: <20160411104013.GG15729@arm.com>
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On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote:
> On 11 April 2016 at 11:59, Chen Feng <puck.chen@hisilicon.com> wrote:
> > 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.

You need to be really careful making SECTION_SIZE_BITS smaller because
it has a direct correlation on the use of page->flags and you can end up
running out of bits fairly easily.

Will