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

From: Keith Busch <kbusch@kernel.org>

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@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: "Huang, Ying" <redacted>
Reviewed-by: Yang Shi <redacted>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Wei Xu <weixugc@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dan Williams <redacted>
Cc: David Hildenbrand <redacted>

--

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;

+       /* 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