Re: [PATCH 5/7] mm, vmscan: extract shrink_page_list reclaim counters into a struct

[Vlastimil Babka <vbabka@suse.cz>]

On 01/04/2017 11:19 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> shrink_page_list returns quite some counters back to its caller. Extract
> the existing 5 into struct reclaim_stat because this makes the code
> easier to follow and also allows further counters to be returned.
> 
> While we are at it, make all of them unsigned rather than unsigned long
> as we do not really need full 64b for them (we never scan more than
> SWAP_CLUSTER_MAX pages at once). This should reduce some stack space.
> 
> This patch shouldn't introduce any functional change.

[...]

> @@ -1266,11 +1270,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  	list_splice(&ret_pages, page_list);
>  	count_vm_events(PGACTIVATE, pgactivate);
>  
> -	*ret_nr_dirty += nr_dirty;
> -	*ret_nr_congested += nr_congested;
> -	*ret_nr_unqueued_dirty += nr_unqueued_dirty;
> -	*ret_nr_writeback += nr_writeback;
> -	*ret_nr_immediate += nr_immediate;
> +	if (stat) {
> +		stat->nr_dirty = nr_dirty;
> +		stat->nr_congested = nr_congested;
> +		stat->nr_unqueued_dirty = nr_unqueued_dirty;
> +		stat->nr_writeback = nr_writeback;
> +		stat->nr_immediate = nr_immediate;
> +	}

This change of '+=' to '=' raised my eybrows, but it seems both callers
don't care so this is indeed no functional change and potentially faster.