Re: [PATCH 3/4] arm64: mm: support large block mapping when rodata=full

[Ryan Roberts <ryan.roberts@arm.com>]

On 05/08/2025 19:53, Yang Shi wrote:

[...]

>>> +    arch_enter_lazy_mmu_mode();
>>> +    ret = split_pgd(pgd_offset_k(start), start, end);
>> My instinct still remains that it would be better not to iterate over the range
>> here, but instead call a "split(start); split(end);" since we just want to split
>> the start and end. So the code would be simpler and probably more performant if
>> we get rid of all the iteration.
> 
> It should be more performant for splitting large range, especially the range
> includes leaf mappings at different levels. But I had some optimization to skip
> leaf mappings in this version, so it should be close to your implementation from
> performance perspective. And it just walks the page table once instead of twice.
> It should be more efficient for small split, for example, 4K.

I guess this is the crux of our disagreement. I think the "walks the table once
for 4K" is a micro optimization, which I doubt we would see on any benchmark
results. In the absence of data, I'd prefer the simpler, smaller, easier to
understand version.

Both implementations are on list now; perhaps the maintainers can steer us.

Thanks,
Ryan