Re: [PATCH 5/3] mm, vmscan: make zone_reclaimable_pages more precise

[Hillf Danton]

> 
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> zone_reclaimable_pages is used in should_reclaim_retry which uses it to
> calculate the target for the watermark check. This means that precise
> numbers are important for the correct decision. zone_reclaimable_pages
> uses zone_page_state which can contain stale data with per-cpu diffs
> not synced yet (the last vmstat_update might have run 1s in the past).
> 
> Use zone_page_state_snapshot in zone_reclaimable_pages instead. None
> of the current callers is in a hot path where getting the precise value
> (which involves per-cpu iteration) would cause an unreasonable overhead.
> 
> Suggested-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---

Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  mm/vmscan.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 489212252cd6..9145e3f89eab 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -196,21 +196,21 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>  {
>  	unsigned long nr;
> 
> -	nr = zone_page_state(zone, NR_ACTIVE_FILE) +
> -	     zone_page_state(zone, NR_INACTIVE_FILE) +
> -	     zone_page_state(zone, NR_ISOLATED_FILE);
> +	nr = zone_page_state_snapshot(zone, NR_ACTIVE_FILE) +
> +	     zone_page_state_snapshot(zone, NR_INACTIVE_FILE) +
> +	     zone_page_state_snapshot(zone, NR_ISOLATED_FILE);
> 
>  	if (get_nr_swap_pages() > 0)
> -		nr += zone_page_state(zone, NR_ACTIVE_ANON) +
> -		      zone_page_state(zone, NR_INACTIVE_ANON) +
> -		      zone_page_state(zone, NR_ISOLATED_ANON);
> +		nr += zone_page_state_snapshot(zone, NR_ACTIVE_ANON) +
> +		      zone_page_state_snapshot(zone, NR_INACTIVE_ANON) +
> +		      zone_page_state_snapshot(zone, NR_ISOLATED_ANON);
> 
>  	return nr;
>  }
> 
>  bool zone_reclaimable(struct zone *zone)
>  {
> -	return zone_page_state(zone, NR_PAGES_SCANNED) <
> +	return zone_page_state_snapshot(zone, NR_PAGES_SCANNED) <
>  		zone_reclaimable_pages(zone) * 6;
>  }
> 
> --
> 2.7.0.rc3