linux-kernel - Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64

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Date:	Mon, 11 Apr 2016 11:40:13 +0100
From:	Will Deacon <will.deacon@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Chen Feng <puck.chen@...ilicon.com>,
	Mark Rutland <mark.rutland@....com>, mhocko@...e.com,
	Laura Abbott <labbott@...hat.com>,
	Dan Zhao <dan.zhao@...ilicon.com>,
	Yiping Xu <xuyiping@...ilicon.com>, puck.chen@...mail.com,
	albert.lubing@...ilicon.com,
	Catalin Marinas <catalin.marinas@....com>,
	suzhuangluan@...ilicon.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxarm@...wei.com, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	kirill.shutemov@...ux.intel.com,
	David Rientjes <rientjes@...gle.com>, oliver.fu@...ilicon.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	robin.murphy@....com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, saberlily.xia@...ilicon.com
Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64

On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote:
> On 11 April 2016 at 11:59, Chen Feng <puck.chen@...ilicon.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