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.
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