mm/damon: improve damon_new_region strategy

Kdamond is implemented as a periodical split-merge pattern, which will
create and destroy regions possibly at high frequency (hundreds or even
thousands of per sec), depending on the number of regions and aggregation
period.  In that case, kmalloc and kfree could bring speed and space
overheads, which can be improved by using a private kmem cache.

[set_pte_at@outlook.com: creating kmem cache for damon regions by KMEM_CACHE()]
  Link: https://lkml.kernel.org/r/Message-ID:
Link: https://lkml.kernel.org/r/TYCP286MB2323DA1894FA55BB9CF90978CA449@TYCP286MB2323.JPNP286.PROD.OUTLOOK.COMSigned-off-by: Dawei Li <set_pte_at@outlook.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/core.c

@@ -29,6 +29,8 @@ static bool running_exclusive_ctxs;
 static DEFINE_MUTEX(damon_ops_lock);
 static struct damon_operations damon_registered_ops[NR_DAMON_OPS];
 
+static struct kmem_cache *damon_region_cache __ro_after_init;
+
 /* Should be called under damon_ops_lock with id smaller than NR_DAMON_OPS */
 static bool __damon_is_registered_ops(enum damon_ops_id id)
 {
@@ -119,7 +121,7 @@ struct damon_region *damon_new_region(unsigned long start, unsigned long end)
 {
 	struct damon_region *region;
 
-	region = kmalloc(sizeof(*region), GFP_KERNEL);
+	region = kmem_cache_alloc(damon_region_cache, GFP_KERNEL);
 	if (!region)
 		return NULL;
 
@@ -148,7 +150,7 @@ static void damon_del_region(struct damon_region *r, struct damon_target *t)
 
 static void damon_free_region(struct damon_region *r)
 {
-	kfree(r);
+	kmem_cache_free(damon_region_cache, r);
 }
 
 void damon_destroy_region(struct damon_region *r, struct damon_target *t)
@@ -1285,4 +1287,17 @@ bool damon_find_biggest_system_ram(unsigned long *start, unsigned long *end)
 	return true;
 }
 
+static int __init damon_init(void)
+{
+	damon_region_cache = KMEM_CACHE(damon_region, 0);
+	if (unlikely(!damon_region_cache)) {
+		pr_err("creating damon_region_cache fails\n");
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+subsys_initcall(damon_init);
+
 #include "core-test.h"