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

[Chen Feng]

Hi Ard,

On 2016/4/11 15:35, Ard Biesheuvel wrote:
> On 11 April 2016 at 04:49, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote:
>> Hi will,
>> Thanks for review.
>>
>> On 2016/4/7 22:21, Will Deacon wrote:
>>> On Tue, Apr 05, 2016 at 04:22:51PM +0800, Chen Feng wrote:
>>>> 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.

Yes,but the populate is section, which is 1GB. Take a look at the above
memory layout.

The section 1G ~ 2G is a section. But 1.5G ~ 2G is a hole.

The section 3G ~ 4G is a section. But 3.5G ~ 4G is a hole.
>>  0             1.5G    2G             3.5G            4G
>>  |              |      |               |              |
>>  +--------------+------+---------------+--------------+
>>  |    MEM       | hole |     MEM       |   IO (regs)  |
>>  +--------------+------+---------------+--------------+
The hole in 1.5G ~ 2G is also allocated mem-map array. And also with the 3.5G ~ 4G.

We want free the the mem-map array. With flat-mem we can work with this scene very well.

Thanks,


> 
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