On 8/6/25 12:20 AM, Ryan Roberts wrote:

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

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+    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.

I did a simple benchmark with module stressor from stress-ng. I used the 
below command line:
stress-ng --module 1 --module-name loop --module-ops 1000

It basically loads loop module 1000 times. I saw a slight slowdown (2% - 
3% slowdown, average time spent in 5 iterations) with your 
implementation on my AmpereOne machine. It shouldn't result in any 
noticeable slowdown for real life workloads per the data.

Thanks,
Yang

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

Thanks,
Ryan