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 prelimiary check
which just says whether there is a possibility of future reclaim.

#Signed-off-by: Keith Busch [off-list ref]
Cc: Keith Busch <kbusch@kernel.org>
[vishal: fixup the migration->demotion rename]
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Yang Shi <redacted>
Cc: David Rientjes <rientjes@google.com>
Cc: Huang Ying <redacted>
Cc: Dan Williams <redacted>
Cc: David Hildenbrand <redacted>
Cc: osalvador <osalvador@suse.de>

--

Changes from Dave 10/2020:
 * remove 'total_swap_pages' modification

Changes from Dave 06/2020:
 * 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 bouncee
---

 b/mm/vmscan.c |   35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff -puN mm/vmscan.c~0009-mm-vmscan-Consider-anonymous-pages-without-swap mm/vmscan.c

--- a/mm/vmscan.c~0009-mm-vmscan-Consider-anonymous-pages-without-swap  2021-01-25 16:23:18.106866688 -0800
+++ b/mm/vmscan.c       2021-01-25 16:23:18.111866688 -0800

@@ -289,6 +289,34 @@ static bool writeback_throttling_sane(st
 }
 #endif

+static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
+                                         int node_id)
+{
+       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 (next_demotion_node(node_id) >= 0)
+               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

@@ -300,7 +328,7 @@ unsigned long zone_reclaimable_pages(str

        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)))
                nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
                        zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);

@@ -2323,6 +2351,7 @@ 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);

@@ -2333,7 +2362,7 @@ static void get_scan_count(struct lruvec
        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)) {

                scan_balance = SCAN_FILE;
                goto out;
        }

@@ -2708,7 +2737,7 @@ static inline bool should_continue_recla
         */
        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))
                inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);

        return inactive_lru_pages > pages_for_compaction;

_