On 11 April 2016 at 04:49, Chen Feng [off-list ref] wrote:

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.

No, this is incorrect. Sparsemem vmemmap only allocates struct pages
for memory regions that are actually populated.

For instance, on the Foundation model with 4 GB of memory, you may see
something like this in the boot log

[    0.000000]     vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000
(     8 GB maximum)
[    0.000000]               0xffffffbdc0000000 - 0xffffffbde2000000
(   544 MB actual)

but in reality, only the following regions have been allocated

---[ vmemmap start ]---
0xffffffbdc0000000-0xffffffbdc2000000          32M       RW NX SHD AF
      BLK UXN MEM/NORMAL
0xffffffbde0000000-0xffffffbde2000000          32M       RW NX SHD AF
      BLK UXN MEM/NORMAL
---[ vmemmap end ]---

so only 64 MB is used to back 4 GB of RAM with struct pages, which is
minimal. Moving to flatmem will not reduce the memory footprint at
all.