[PATCH RESEND] 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.
Once __node_reclaim() is called, either the reclaimable pagecache is
greater than min_unmapped_pages or reclaimable slab is greater than
min_slab_pages, that is ensured in function node_reclaim(). So wen can
remove the if statement in __node_reclaim().

sc.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 (they can only be shrunk in kswapd until
the watermark is reached).

Regarding vm.zone_reclaim_mode, we always set it to zero to disable node
reclaim. Someone may prefer to enable it if their different workloads work
on different nodes.

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

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 47aa215..1e410ef 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
@@ -2831,7 +2834,9 @@ static bool 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,
@@ -4268,6 +4273,10 @@ 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),
 	};
 
@@ -4285,15 +4294,13 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	p->flags |= PF_SWAPWRITE;
 	set_task_reclaim_state(p, &sc.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);
 
 	set_task_reclaim_state(p, NULL);
 	current->flags &= ~PF_SWAPWRITE;
-- 
1.8.3.1