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

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Message-ID: <20160412144434.GE8066@e104818-lin.cambridge.arm.com>
Date:	Tue, 12 Apr 2016 15:44:35 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Dan Zhao <dan.zhao@...ilicon.com>, mhocko@...e.com,
	Yiping Xu <xuyiping@...ilicon.com>, puck.chen@...mail.com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Chen Feng <puck.chen@...ilicon.com>,
	suzhuangluan@...ilicon.com, David Rientjes <rientjes@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxarm@...wei.com, albert.lubing@...ilicon.com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, oliver.fu@...ilicon.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Laura Abbott <labbott@...hat.com>, robin.murphy@....com,
	kirill.shutemov@...ux.intel.com, saberlily.xia@...ilicon.com
Subject: Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64

On Mon, Apr 11, 2016 at 11:40:13AM +0100, Will Deacon wrote:
> 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.

With SPARSEMEM_VMEMMAP, SECTION_SIZE_BITS no longer affect the page
flags since we no longer need to encode the section number in
page->flags.

-- 
Catalin