mm/damon/sysfs: move sysfs_lock to common module

DAMON sysfs interface is implemented in a single file, sysfs.c, which has
about 2,800 lines of code.  As the interface is hierarchical and some of
the code can be reused by different hierarchies, it would make more sense
to split out the implementation into common parts and different parts in
multiple files.  As the beginning of the work, create files for common
code and move the global mutex for directories modifications protection
into the new file.

Link: https://lkml.kernel.org/r/20221026225943.100429-8-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/Makefile

@@ -3,7 +3,7 @@
 obj-y				:= core.o
 obj-$(CONFIG_DAMON_VADDR)	+= ops-common.o vaddr.o
 obj-$(CONFIG_DAMON_PADDR)	+= ops-common.o paddr.o
-obj-$(CONFIG_DAMON_SYSFS)	+= sysfs.o
+obj-$(CONFIG_DAMON_SYSFS)	+= sysfs-common.o sysfs.o
 obj-$(CONFIG_DAMON_DBGFS)	+= dbgfs.o
 obj-$(CONFIG_DAMON_RECLAIM)	+= reclaim.o
 obj-$(CONFIG_DAMON_LRU_SORT)	+= lru_sort.o

mm/damon/sysfs-common.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Common Primitives for DAMON Sysfs Interface
+ *
+ * Author: SeongJae Park <sj@kernel.org>
+ */
+
+#include "sysfs-common.h"
+
+DEFINE_MUTEX(damon_sysfs_lock);
+

mm/damon/sysfs-common.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Common Primitives for DAMON Sysfs Interface
+ *
+ * Author: SeongJae Park <sj@kernel.org>
+ */
+
+#include <linux/damon.h>
+#include <linux/kobject.h>
+
+extern struct mutex damon_sysfs_lock;

mm/damon/sysfs.c

@@ -5,13 +5,11 @@
  * Copyright (c) 2022 SeongJae Park <sj@kernel.org>
  */
 
-#include <linux/damon.h>
-#include <linux/kobject.h>
 #include <linux/pid.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
 
-static DEFINE_MUTEX(damon_sysfs_lock);
+#include "sysfs-common.h"
 
 /*
  * unsigned long range directory