Re: [PATCH 1/2] sched/numa: do not let the per-VMA PID filter gate promotion

[Gregory Price]

>  		/*
>  		 * Do not scan the VMA if task has not accessed it, unless no other
>  		 * VMA candidate exists.
> +		 *
> +		 * Force-scan only slow-tier folios when in tiering mode, as a large
> +		 * VMA can cause others to become "permanently unaccessed" if a scan
> +		 * cycle is consumed entirely by the large VMA.
>  		 */
> +		vma->numab_state->slow_only = false;
>  		if (!vma_pids_forced && !vma_is_accessed(mm, vma)) {
> -			vma_pids_skipped = true;
> -			trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_PID_INACTIVE);
> -			continue;
> +			if (!(sysctl_numa_balancing_mode &
> +			      NUMA_BALANCING_MEMORY_TIERING)) {
> +				vma_pids_skipped = true;
> +				trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_PID_INACTIVE);
> +				continue;
> +			}
> +			vma->numab_state->slow_only = true;
>  		}

sashiko (correctly) reported that slow_only=false set unconditionally
here causes a VMA that's scanned over two passes to be prematurely
re-admitted regardless of vma_is_accessed() activity.

Patch looks simple:

+               if (mm->numa_scan_offset <= vma->vm_start)
+                       vma->numab_state->slow_only = false;

and on patch two:

-                       vma->numab_state->slow_only = false;
+                       vma->numab_state->slow_only = ro_file;

I'll follow up with a v2 next week after I test it.

~Gregory