zsmalloc: replace get_cpu_var with local_lock

The usage of get_cpu_var() in zs_map_object() is problematic because it
disables preemption and makes it impossible to acquire any sleeping lock
on PREEMPT_RT such as a spinlock_t.

Replace the get_cpu_var() usage with a local_lock_t which is embedded
struct mapping_area.  It ensures that the access the struct is
synchronized against all users on the same CPU.

[minchan: remove the bit_spin_lock part and change the title]

Link: https://lkml.kernel.org/r/20211115185909.3949505-10-minchan@kernel.orgSigned-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -65,6 +65,7 @@
 #include <linux/wait.h>
 #include <linux/pagemap.h>
 #include <linux/fs.h>
+#include <linux/local_lock.h>
 
 #define ZSPAGE_MAGIC	0x58
 
@@ -276,6 +277,7 @@ struct zspage {
 };
 
 struct mapping_area {
+	local_lock_t lock;
 	char *vm_buf; /* copy buffer for objects that span pages */
 	char *vm_addr; /* address of kmap_atomic()'ed pages */
 	enum zs_mapmode vm_mm; /* mapping mode */
@@ -451,7 +453,9 @@ MODULE_ALIAS("zpool-zsmalloc");
 #endif /* CONFIG_ZPOOL */
 
 /* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
-static DEFINE_PER_CPU(struct mapping_area, zs_map_area);
+static DEFINE_PER_CPU(struct mapping_area, zs_map_area) = {
+	.lock	= INIT_LOCAL_LOCK(lock),
+};
 
 static __maybe_unused int is_first_page(struct page *page)
 {
@@ -1269,7 +1273,8 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 	class = zspage_class(pool, zspage);
 	off = (class->size * obj_idx) & ~PAGE_MASK;
 
-	area = &get_cpu_var(zs_map_area);
+	local_lock(&zs_map_area.lock);
+	area = this_cpu_ptr(&zs_map_area);
 	area->vm_mm = mm;
 	if (off + class->size <= PAGE_SIZE) {
 		/* this object is contained entirely within a page */
@@ -1320,7 +1325,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
 
 		__zs_unmap_object(area, pages, off, class->size);
 	}
-	put_cpu_var(zs_map_area);
+	local_unlock(&zs_map_area.lock);
 
 	migrate_read_unlock(zspage);
 }