[PATCH v3 3/3] mm/vmscan: shrink slab in node reclaim

[Yafang Shao <laoar.shao@gmail.com>]

In the node reclaim, may_shrinkslab is 0 by default,
hence shrink_slab will never be performed in it.
While shrik_slab should be performed if the relcaimable slab is over
min slab limit.

If reclaimable pagecache is less than min_unmapped_pages while
reclaimable slab is greater than min_slab_pages, we only shrink slab.
Otherwise the min_unmapped_pages will be useless under this condition.

reclaim_state.reclaimed_slab is to tell us how many pages are
reclaimed in shrink slab.

This issue is very easy to produce, first you continuously cat a random
non-exist file to produce more and more dentry, then you read big file
to produce page cache. And finally you will find that the denty will
never be shrunk.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/vmscan.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e0c5669..d52014f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4157,6 +4157,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	p->reclaim_state = &reclaim_state;
 
 	if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
+		sc.may_shrinkslab = (pgdat->min_slab_pages <
+				node_page_state(pgdat, NR_SLAB_RECLAIMABLE));
 		/*
 		 * Free memory by calling shrink node with increasing
 		 * priorities until we have enough memory freed.
@@ -4164,6 +4166,28 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 		do {
 			shrink_node(pgdat, &sc);
 		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
+	} else {
+		/*
+		 * If the reclaimable pagecache is not greater than
+		 * min_unmapped_pages, only reclaim the slab.
+		 */
+		struct mem_cgroup *memcg;
+		struct mem_cgroup_reclaim_cookie reclaim = {
+			.pgdat = pgdat,
+		};
+
+		do {
+			reclaim.priority = sc.priority;
+			memcg = mem_cgroup_iter(NULL, NULL, &reclaim);
+			do {
+				shrink_slab(sc.gfp_mask, pgdat->node_id,
+					    memcg, sc.priority);
+			} while ((memcg = mem_cgroup_iter(NULL, memcg,
+							  &reclaim)));
+
+			sc.nr_reclaimed += reclaim_state.reclaimed_slab;
+			reclaim_state.reclaimed_slab = 0;
+		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
 	}
 
 	p->reclaim_state = NULL;
-- 
1.8.3.1