selinux: Use a kmem_cache for allocation struct file_security_struct

The size of struct file_security_struct is 16byte at my setup.
But, the real allocation size for per each file_security_struct
is 64bytes in my setup that kmalloc min size is 64bytes
because ARCH_DMA_MINALIGN is 64.

This allocation is called every times at file allocation(alloc_file()).
So, the total slack memory size(allocated size - request size)
is increased exponentially.

E.g) Min Kmalloc Size : 64bytes, Unit : bytes
      Allocated Size | Request Size | Slack Size | Allocation Count
    ---------------------------------------------------------------
         770048      |    192512    |   577536   |      12032

At the result, this change reduce memory usage 42bytes per each
file_security_struct

Signed-off-by: Sangwoo <sangwoo2.park@lge.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: removed extra subject prefix]
Signed-off-by: Paul Moore <pmoore@redhat.com>

security/selinux/hooks.c

@@ -126,6 +126,7 @@ int selinux_enabled = 1;
 #endif
 
 static struct kmem_cache *sel_inode_cache;
+static struct kmem_cache *file_security_cache;
 
 /**
  * selinux_secmark_enabled - Check to see if SECMARK is currently enabled
@@ -287,7 +288,7 @@ static int file_alloc_security(struct file *file)
 	struct file_security_struct *fsec;
 	u32 sid = current_sid();
 
-	fsec = kzalloc(sizeof(struct file_security_struct), GFP_KERNEL);
+	fsec = kmem_cache_zalloc(file_security_cache, GFP_KERNEL);
 	if (!fsec)
 		return -ENOMEM;
 
@@ -302,7 +303,7 @@ static void file_free_security(struct file *file)
 {
 	struct file_security_struct *fsec = file->f_security;
 	file->f_security = NULL;
-	kfree(fsec);
+	kmem_cache_free(file_security_cache, fsec);
 }
 
 static int superblock_alloc_security(struct super_block *sb)
@@ -6086,6 +6087,9 @@ static __init int selinux_init(void)
 	sel_inode_cache = kmem_cache_create("selinux_inode_security",
 					    sizeof(struct inode_security_struct),
 					    0, SLAB_PANIC, NULL);
+	file_security_cache = kmem_cache_create("selinux_file_security",
+					    sizeof(struct file_security_struct),
+					    0, SLAB_PANIC, NULL);
 	avc_init();
 
 	security_add_hooks(selinux_hooks, ARRAY_SIZE(selinux_hooks));