mm: vmalloc: add a shrinker to drain vmap pools

The added shrinker is used to return back current cached VAs into a global
vmap space, when a system enters into a low memory mode.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
Cc: Kazuhito Hagio <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Joel Fernandes (Google) <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Oleksiy Avramchenko <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/vmalloc.c

@@ -4917,8 +4917,37 @@ static void vmap_init_nodes(void)
 	}
 }
 
+static unsigned long
+vmap_node_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
+{
+	unsigned long count;
+	struct vmap_node *vn;
+	int i, j;
+
+	for (count = 0, i = 0; i < nr_vmap_nodes; i++) {
+		vn = &vmap_nodes[i];
+
+		for (j = 0; j < MAX_VA_SIZE_PAGES; j++)
+			count += READ_ONCE(vn->pool[j].len);
+	}
+
+	return count ? count : SHRINK_EMPTY;
+}
+
+static unsigned long
+vmap_node_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
+{
+	int i;
+
+	for (i = 0; i < nr_vmap_nodes; i++)
+		decay_va_pool_node(&vmap_nodes[i], true);
+
+	return SHRINK_STOP;
+}
+
 void __init vmalloc_init(void)
 {
+	struct shrinker *vmap_node_shrinker;
 	struct vmap_area *va;
 	struct vmap_node *vn;
 	struct vm_struct *tmp;
@@ -4966,4 +4995,14 @@ void __init vmalloc_init(void)
 	 */
 	vmap_init_free_space();
 	vmap_initialized = true;
+
+	vmap_node_shrinker = shrinker_alloc(0, "vmap-node");
+	if (!vmap_node_shrinker) {
+		pr_err("Failed to allocate vmap-node shrinker!\n");
+		return;
+	}
+
+	vmap_node_shrinker->count_objects = vmap_node_shrink_count;
+	vmap_node_shrinker->scan_objects = vmap_node_shrink_scan;
+	shrinker_register(vmap_node_shrinker);
 }