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

[Catalin Marinas]

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@xxxxxxxxxxxxx> 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