cachefiles: Register a miscdev and parse commands over it

Register a misc device with which to talk to the daemon.  The misc device
holds a cache set up through it around and closing the device kills the
cache.

cachefilesd communicates with the kernel by passing it single-line text
commands.  Parse these and use them to parameterise the cache state.  This
does not implement the command to actually bring a cache online.  That's
left for later.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819628388.215744.17712097043607299608.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906929128.143852.14065207858943654011.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967139085.1823006.3514846391807454287.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021538400.640689.9172006906288062041.stgit@warthog.procyon.org.uk/ # v4

fs/cachefiles/Makefile

@@ -4,6 +4,7 @@
 #
 
 cachefiles-y := \
+	daemon.o \
 	main.o \
 	security.o
 

fs/cachefiles/internal.h

@@ -65,6 +65,20 @@ struct cachefiles_cache {
 
 #include <trace/events/cachefiles.h>
 
+/*
+ * note change of state for daemon
+ */
+static inline void cachefiles_state_changed(struct cachefiles_cache *cache)
+{
+	set_bit(CACHEFILES_STATE_CHANGED, &cache->flags);
+	wake_up_all(&cache->daemon_pollwq);
+}
+
+/*
+ * daemon.c
+ */
+extern const struct file_operations cachefiles_daemon_fops;
+
 /*
  * error_inject.c
  */

fs/cachefiles/main.c

@@ -31,6 +31,12 @@ MODULE_DESCRIPTION("Mounted-filesystem based cache");
 MODULE_AUTHOR("Red Hat, Inc.");
 MODULE_LICENSE("GPL");
 
+static struct miscdevice cachefiles_dev = {
+	.minor	= MISC_DYNAMIC_MINOR,
+	.name	= "cachefiles",
+	.fops	= &cachefiles_daemon_fops,
+};
+
 /*
  * initialise the fs caching module
  */
@@ -41,10 +47,15 @@ static int __init cachefiles_init(void)
 	ret = cachefiles_register_error_injection();
 	if (ret < 0)
 		goto error_einj;
+	ret = misc_register(&cachefiles_dev);
+	if (ret < 0)
+		goto error_dev;
 
 	pr_info("Loaded\n");
 	return 0;
 
+error_dev:
+	cachefiles_unregister_error_injection();
 error_einj:
 	pr_err("failed to register: %d\n", ret);
 	return ret;
@@ -59,6 +70,7 @@ static void __exit cachefiles_exit(void)
 {
 	pr_info("Unloading\n");
 
+	misc_deregister(&cachefiles_dev);
 	cachefiles_unregister_error_injection();
 }