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

On Apr 11, 2016, at 11:49 AM, Chen Feng wrote:

Dear Chen,

Hi will,
Thanks for review.

On 2016/4/7 22:21, Will Deacon wrote:

quoted

On Tue, Apr 05, 2016 at 04:22:51PM +0800, Chen Feng wrote:

quoted

We can reduce the memory allocated at mem-map
by flatmem.

currently, the default memory-model in arm64 is
sparse memory. The mem-map array is not freed in
this scene. If the physical address is too long,
it will reserved too much memory for the mem-map
array.

Can you elaborate a bit more on this, please? We use the vmemmap, so any
spaces between memory banks only burns up virtual space. What exactly is
the problem you're seeing that makes you want to use flatmem (which is
probably unsuitable for the majority of arm64 machines).

The root cause we want to use flat-mem is the mam_map alloced in sparse-mem
is not freed.

take a look at here:
arm64/mm/init.c
void __init mem_init(void)
{
#ifndef CONFIG_SPARSEMEM_VMEMMAP
	free_unused_memmap();
#endif
}

Memory layout (3GB)

0             1.5G    2G             3.5G            4G
|              |      |               |              |
+--------------+------+---------------+--------------+
|    MEM       | hole |     MEM       |   IO (regs)  |
+--------------+------+---------------+--------------+


Memory layout (4GB)

0                                    3.5G            4G    4.5G
|                                     |              |       |
+-------------------------------------+--------------+-------+
|                   MEM               |   IO (regs)  |  MEM  |
+-------------------------------------+--------------+-------+

Currently, the sparse memory section is 1GB.

3GB ddr: the 1.5 ~2G and 3.5 ~ 4G are holes.
3GB ddr: the 3.5 ~ 4G and 4.5 ~ 5G are holes.

This will alloc 1G/4K * (struct page) memory for mem_map array.

We want to use flat-mem to reduce the alloced mem_map.

I don't know why you tell us the flatmem is unsuitable for the
majority of arm64 machines. Can tell us the reason of it?

And we are not going to limit the memdel in arm64, we just want to
make the flat-mem is an optional item in arm64.

I've experienced the same problem and considered the ideas mentioned
in this thread: flatmem and small SECTION_SIZE_BITS. However, I was
reluctant to post any patch since the issue is highly related to memory
map design document, [1], saying 1GB aligned RAM. The majority of arm64
platforms might follow the information although it is not spec. IOW,
a machine I've played was at least unusual *at that time*, so I didn't
consider upstream work.

[1] http://infocenter.arm.com/help/topic/com.arm.doc.den0001c/DEN0001C_principles_of_arm_memory_maps.pdf  

Best Regards
Jungseok Lee