Re: [PATCH 3/4] mm: move lazy free pages to inactive list

Andrew Morton
2015-03-20T22:44:01Z

On Wed, 11 Mar 2015 10:20:37 +0900 Minchan Kim <minchan@kernel.org> wrote:

> MADV_FREE is hint that it's okay to discard pages if there is
> memory pressure and we uses reclaimers(ie, kswapd and direct reclaim)
> to free them so there is no worth to remain them in active anonymous LRU
> so this patch moves them to inactive LRU list's head.
>
> This means that MADV_FREE-ed pages which were living on the inactive list
> are reclaimed first because they are more likely to be cold rather than
> recently active pages.
>
> A arguable issue for the approach would be whether we should put it to
> head or tail in inactive list. I selected *head* because kernel cannot
> make sure it's really cold or warm for every MADV_FREE usecase but
> at least we know it's not *hot* so landing of inactive head would be
> comprimise for various usecases.
>
> This is fixing a suboptimal behavior of MADV_FREE when pages living on
> the active list will sit there for a long time even under memory
> pressure while the inactive list is reclaimed heavily. This basically
> breaks the whole purpose of using MADV_FREE to help the system to free
> memory which is might not be used.
>
> @@ -789,6 +790,23 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
> update_page_reclaim_stat(lruvec, file, 0);
> }
>
> +
> +static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
> + void *arg)
>
> ...
>
> @@ -844,6 +866,18 @@ void deactivate_file_page(struct page *page)
> }
> }
>
> +void deactivate_page(struct page *page)
> +{

lru_deactivate_file_fn() and deactivate_file_page() are carefully
documented and lru_deactivate_fn() and deactivate_page() should
be as well. In fact it becomes more important now that we have two
similar-looking things.