[PATCH v4 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.
A new bitmask no_pagecache is introduced in scan_control for this
purpose, which is 0 by default.

reclaim_state.reclaimed_slab will 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 in node reclaim.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/vmscan.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 281bfa9..da53ed6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -91,6 +91,9 @@ struct scan_control {
 	/* e.g. boosted watermark reclaim leaves slabs alone */
 	unsigned int may_shrinkslab:1;
 
+	/* in node relcaim mode, we may shrink slab only */
+	unsigned int no_pagecache:1;
+
 	/*
 	 * Cgroups are not reclaimed below their configured memory.low,
 	 * unless we threaten to OOM. If any cgroups are skipped due to
@@ -2744,7 +2747,9 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 
 			reclaimed = sc->nr_reclaimed;
 			scanned = sc->nr_scanned;
-			shrink_node_memcg(pgdat, memcg, sc);
+
+			if (!sc->no_pagecache)
+				shrink_node_memcg(pgdat, memcg, sc);
 
 			if (sc->may_shrinkslab) {
 				shrink_slab(sc->gfp_mask, pgdat->node_id,
@@ -4176,6 +4181,10 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
 		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
 		.may_swap = 1,
+		.may_shrinkslab = (node_page_state(pgdat, NR_SLAB_RECLAIMABLE) >
+				  pgdat->min_slab_pages),
+		.no_pagecache = !(node_pagecache_reclaimable(pgdat) >
+				pgdat->min_unmapped_pages),
 		.reclaim_idx = gfp_zone(gfp_mask),
 	};
 
@@ -4194,15 +4203,13 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	reclaim_state.reclaimed_slab = 0;
 	p->reclaim_state = &reclaim_state;
 
-	if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
-		/*
-		 * Free memory by calling shrink node with increasing
-		 * priorities until we have enough memory freed.
-		 */
-		do {
-			shrink_node(pgdat, &sc);
-		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
-	}
+	/*
+	 * Free memory by calling shrink node with increasing
+	 * priorities until we have enough memory freed.
+	 */
+	do {
+		shrink_node(pgdat, &sc);
+	} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
 
 	p->reclaim_state = NULL;
 	current->flags &= ~PF_SWAPWRITE;
-- 
1.8.3.1