Re: [PATCH] vmscan: do not evict inactive pages when skipping an active list scan

[Johannes Weiner <hannes@cmpxchg.org>]

Hello all,

On Wed, Nov 25, 2009 at 01:37:52PM -0500, Rik van Riel wrote:
> In AIM7 runs, recent kernels start swapping out anonymous pages
> well before they should.  This is due to shrink_list falling
> through to shrink_inactive_list if !inactive_anon_is_low(zone, sc),
> when all we really wanted to do is pre-age some anonymous pages to
> give them extra time to be referenced while on the inactive list.

I do not quite understand what changed 'recently'.

That fall-through logic to keep eating inactives when the ratio is off
came in a year ago with the second-chance-for-anon-pages patch..?

> The obvious fix is to make sure that shrink_list does not fall
> through to scanning/reclaiming inactive pages when we called it
> to scan one of the active lists.
>
> This change should be safe because the loop in shrink_zone ensures
> that we will still shrink the anon and file inactive lists whenever
> we should.

It was not so obvious to me ;)

At first, I thought it would make sense to actively rebalance between
the lists if the inactive one grows too large (the fall-through case).

But shrink_zone() does not know about this and although we scan
inactive pages, we do not account for them and decrease the 'nr[lru]'
for active pages instead, effectively shifting the 'active todo' over
to the 'inactive todo'.  I can imagine this going wrong!

So I agree, we should use the inactive_*_is_low() predicate only
passively.
 
> Reported-by: Larry Woodman <lwoodman@redhat.com>
> Signed-off-by: Rik van Riel <riel@redhat.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 777af57..ec4dfda 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1469,13 +1469,15 @@ static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
>  {
>  	int file = is_file_lru(lru);
>  
> -	if (lru == LRU_ACTIVE_FILE && inactive_file_is_low(zone, sc)) {
> -		shrink_active_list(nr_to_scan, zone, sc, priority, file);
> +	if (lru == LRU_ACTIVE_FILE) {
> +		if (inactive_file_is_low(zone, sc))
> +		      shrink_active_list(nr_to_scan, zone, sc, priority, file);
>  		return 0;
>  	}
>  
> -	if (lru == LRU_ACTIVE_ANON && inactive_anon_is_low(zone, sc)) {
> -		shrink_active_list(nr_to_scan, zone, sc, priority, file);
> +	if (lru == LRU_ACTIVE_ANON) {
> +		if (inactive_file_is_low(zone, sc))
> +		      shrink_active_list(nr_to_scan, zone, sc, priority, file);
>  		return 0;
>  	}
>  	return shrink_inactive_list(nr_to_scan, zone, sc, priority, file);
> 
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