Re: [RFC PATCH v1.1 3/5] mm/damon/vaddr: drop last same folio access check optimization

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Unused parameter and dead code tracking logic left behind after removing optimization.
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--- Patch [3]: [RFC PATCH v1.1 3/5] mm/damon/vaddr: drop last same folio access check optimization ---
commit 40e12defdea3b013644cc026283aa34cdd3ae2fb
Author: SJ Park <[email protected]>
mm/damon/vaddr: drop last same folio access check optimization

This commit removes the last same folio access check optimization from DAMON's
virtual address space primitives. The optimization was found to cause race
conditions when multiple kdamonds are running.

> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index d10b8042adb5b..16fe210d2042a 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c

[ ... ]

> @@ -496,26 +489,15 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
>  static void __damon_va_check_access(struct mm_struct *mm,
>  				struct damon_region *r, bool same_target)
>  {

[Severity: Low]
Does this code still need the same_target parameter?

With the optimization removed below, the parameter is no longer used in
__damon_va_check_access().

The caller, damon_va_check_accesses(), also still computes and tracks the local
same_target variable on every iteration just to pass it here:

	damon_for_each_target(t, ctx) {
		mm = damon_get_mm(t);
		same_target = false;
		damon_for_each_region(r, t) {
			__damon_va_check_access(mm, r, same_target);
			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
			same_target = true;
		}
		if (mm)
			mmput(mm);
	}

Could we remove the parameter and the dead tracking code in the caller as well?

> -	static unsigned long last_addr;
> -	static unsigned long last_folio_sz = PAGE_SIZE;
> -	static bool last_accessed;
> +	bool accessed;
>  
>  	if (!mm) {
>  		damon_update_region_access_rate(r, false);
>  		return;
>  	}
>  
> -	/* If the region is in the last checked page, reuse the result */
> -	if (same_target && (ALIGN_DOWN(last_addr, last_folio_sz) ==
> -				ALIGN_DOWN(r->sampling_addr, last_folio_sz))) {
> -		damon_update_region_access_rate(r, last_accessed);
> -		return;
> -	}
> -
> -	last_accessed = damon_va_young(mm, r->sampling_addr, &last_folio_sz);
> -	damon_update_region_access_rate(r, last_accessed);
> -
> -	last_addr = r->sampling_addr;
> +	accessed = damon_va_young(mm, r->sampling_addr);
> +	damon_update_region_access_rate(r, accessed);
>  }
>  
>  static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=3