David Hildenbrand <david@xxxxxxxxxx> writes:
> On 01.06.24 08:01, Yu Zhao wrote:
>> On Wed, May 15, 2024 at 4:06 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
...
>>
>> Your system has 2GB memory and it uses zswap with zsmalloc (which is
>> good since it can allocate from the highmem zone) and zstd/lzo (which
>> doesn't matter much). Somehow -- I couldn't figure out why -- it
>> splits the 2GB into a 0.25GB DMA zone and a 1.75GB highmem zone:
>>
>> [ 0.000000] Zone ranges:
>> [ 0.000000] DMA [mem 0x0000000000000000-0x000000002fffffff]
>> [ 0.000000] Normal empty
>> [ 0.000000] HighMem [mem 0x0000000030000000-0x000000007fffffff]
>
> That's really odd. But we are messing with "PowerMac3,6", so I don't
> really know what's right or wrong ...
The DMA zone exists because 9739ab7eda45 ("powerpc: enable a 30-bit
ZONE_DMA for 32-bit pmac") selects it.
It's 768MB (not 0.25GB) because it's clamped at max_low_pfn:
#ifdef CONFIG_ZONE_DMA
max_zone_pfns[ZONE_DMA] = min(max_low_pfn,
1UL << (zone_dma_bits - PAGE_SHIFT));
#endif
Which comes eventually from CONFIG_LOWMEM_SIZE, which defaults to 768MB.
I think it's 768MB because the user:kernel split is 3G:1G, and then the
kernel needs some of that 1G virtual space for vmalloc/ioremap/highmem,
so it splits it 768M:256M.
Then ZONE_NORMAL is empty because it is also limited to max_low_pfn:
max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
The rest of RAM is highmem.
So I think that's all behaving as expected, but I don't know 32-bit /
highmem stuff that well so I could be wrong.
cheers
