Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression

[Linus Torvalds]

On Sun, 4 Aug 2024 at 01:59, kernel test robot <oliver.sang@xxxxxxxxx> wrote:
>
> kernel test robot noticed a -4.4% regression of stress-ng.pagemove.page_remaps_per_sec on
> commit 8be7258aad44 ("mseal: add mseal syscall")

Ok, it's basically just the vma walk in can_modify_mm():

>       1.06            +0.1        1.18        perf-profile.self.cycles-pp.mas_next_slot
>       1.50            +0.5        1.97        perf-profile.self.cycles-pp.mas_find
>       0.00            +1.4        1.35        perf-profile.self.cycles-pp.can_modify_mm
>       3.13            +2.0        5.13        perf-profile.self.cycles-pp.mas_walk

and looks like it's two different pathways. We have __do_sys_mremap ->
mremap_to -> do_munmap -> do_vmi_munmap -> can_modify_mm for the
destination mapping, but we also have mremap_to() calling
can_modify_mm() directly for the source mapping.

And then do_vmi_munmap() will do it's *own* vma_find() after having
done arch_unmap().

And do_munmap() will obviously do its own vma lookup as part of
calling vma_to_resize().

So it looks like a large portion of this regression is because the
mseal addition just ends up walking the vma list way too much.

              Linus