Re: [PATCH -V9 7/9] mm/vmscan: Consider anonymous pages without swap

[Wei Xu]

On Fri, Jun 25, 2021 at 12:33 AM Huang Ying <ying.huang@xxxxxxxxx> wrote:
>
> From: Keith Busch <kbusch@xxxxxxxxxx>
>
> Reclaim anonymous pages if a migration path is available now that
> demotion provides a non-swap recourse for reclaiming anon pages.
>
> Note that this check is subtly different from the
> anon_should_be_aged() checks.  This mechanism checks whether a
> specific page in a specific context *can* actually be reclaimed, given
> current swap space and cgroup limits
>
> anon_should_be_aged() is a much simpler and more preliminary check
> which just says whether there is a possibility of future reclaim.
>
> Cc: Keith Busch <kbusch@xxxxxxxxxx>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
>
> --
>
> Changes since 20210618:
>  * Consider whether demotion is disabled
>
> Changes from Dave 202010:
>  * remove 'total_swap_pages' modification
>
> Changes from Dave 202006:
>  * rename reclaim_anon_pages()->can_reclaim_anon_pages()
>
> Note: Keith's Intel SoB is commented out because he is no
> longer at Intel and his @intel.com mail will bounce.
> ---
>  mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 34 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 55f6192b2a51..fce43c7970d7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -519,6 +519,36 @@ static long add_nr_deferred(long nr, struct shrinker *shrinker,
>         return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
>  }
>
> +static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
> +                                         int node_id,
> +                                         struct scan_control *sc)
> +{
> +       if (memcg == NULL) {
> +               /*
> +                * For non-memcg reclaim, is there
> +                * space in any swap device?
> +                */
> +               if (get_nr_swap_pages() > 0)
> +                       return true;
> +       } else {
> +               /* Is the memcg below its swap limit? */
> +               if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
> +                       return true;
> +       }
> +
> +       /*
> +        * The page can not be swapped.
> +        *
> +        * Can it be reclaimed from this node via demotion?
> +        */
> +       if ((!sc || !sc->no_demotion) &&
> +           next_demotion_node(node_id) != NUMA_NO_NODE)
> +               return true;

It is better to abstract these checks into a function, e.g.
can_demote_anon_pages(), to share with anon_can_be_aged().

> +       /* No way to reclaim anon pages */
> +       return false;
> +}
> +
>  /*
>   * This misses isolated pages which are not accounted for to save counters.
>   * As the data only determines if reclaim or compaction continues, it is
> @@ -530,7 +560,7 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>
>         nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
>                 zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
> -       if (get_nr_swap_pages() > 0)
> +       if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
>                 nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
>                         zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
>
> @@ -2531,6 +2561,7 @@ enum scan_balance {
>  static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>                            unsigned long *nr)
>  {
> +       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>         unsigned long anon_cost, file_cost, total_cost;
>         int swappiness = mem_cgroup_swappiness(memcg);
> @@ -2541,7 +2572,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>         enum lru_list lru;
>
>         /* If we have no swap space, do not bother scanning anon pages. */
> -       if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
> +       if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
>                 scan_balance = SCAN_FILE;
>                 goto out;
>         }
> @@ -2916,7 +2947,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>          */
>         pages_for_compaction = compact_gap(sc->order);
>         inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
> -       if (get_nr_swap_pages() > 0)
> +       if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
>                 inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
>
>         return inactive_lru_pages > pages_for_compaction;
> --
> 2.30.2
>