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:

quoted

On 11 April 2016 at 11:59, Chen Feng [off-list ref] wrote:

quoted

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.

quoted

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