mm: cleancache: lazy initialization to allow tmem backends to build/run as modules

With the goal of allowing tmem backends (zcache, ramster, Xen tmem) to
be built/loaded as modules rather than built-in and enabled by a boot
parameter, this patch provides "lazy initialization", allowing backends
to register to cleancache even after filesystems were mounted.  Calls to
init_fs and init_shared_fs are remembered as fake poolids but no real
tmem_pools created.  On backend registration the fake poolids are mapped
to real poolids and respective tmem_pools.

Signed-off-by: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Florian Schmaus <fschmaus@gmail.com>
Signed-off-by: Andor Daam <andor.daam@googlemail.com>
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
[v1: Minor fixes: used #define for some values and bools]
[v2: Removed CLEANCACHE_HAS_LAZY_INIT]
[v3: Added more comments, added a lock for [shared_|]fs_poolid_map]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/cleancache.c

@@ -45,15 +45,99 @@ static u64 cleancache_puts;
 static u64 cleancache_invalidates;
 
 /*
- * register operations for cleancache, returning previous thus allowing
- * detection of multiple backends and possible nesting
+ * When no backend is registered all calls to init_fs and init_shared_fs
+ * are registered and fake poolids (FAKE_FS_POOLID_OFFSET or
+ * FAKE_SHARED_FS_POOLID_OFFSET, plus offset in the respective array
+ * [shared_|]fs_poolid_map) are given to the respective super block
+ * (sb->cleancache_poolid) and no tmem_pools are created. When a backend
+ * registers with cleancache the previous calls to init_fs and init_shared_fs
+ * are executed to create tmem_pools and set the respective poolids. While no
+ * backend is registered all "puts", "gets" and "flushes" are ignored or failed.
+ */
+#define MAX_INITIALIZABLE_FS 32
+#define FAKE_FS_POOLID_OFFSET 1000
+#define FAKE_SHARED_FS_POOLID_OFFSET 2000
+
+#define FS_NO_BACKEND (-1)
+#define FS_UNKNOWN (-2)
+static int fs_poolid_map[MAX_INITIALIZABLE_FS];
+static int shared_fs_poolid_map[MAX_INITIALIZABLE_FS];
+static char *uuids[MAX_INITIALIZABLE_FS];
+/*
+ * Mutex for the [shared_|]fs_poolid_map to guard against multiple threads
+ * invoking umount (and ending in __cleancache_invalidate_fs) and also multiple
+ * threads calling mount (and ending up in __cleancache_init_[shared|]fs).
+ */
+static DEFINE_MUTEX(poolid_mutex);
+/*
+ * When set to false (default) all calls to the cleancache functions, except
+ * the __cleancache_invalidate_fs and __cleancache_init_[shared|]fs are guarded
+ * by the if (!backend_registered) return. This means multiple threads (from
+ * different filesystems) will be checking backend_registered. The usage of a
+ * bool instead of a atomic_t or a bool guarded by a spinlock is OK - we are
+ * OK if the time between the backend's have been initialized (and
+ * backend_registered has been set to true) and when the filesystems start
+ * actually calling the backends. The inverse (when unloading) is obviously
+ * not good - but this shim does not do that (yet).
+ */
+static bool backend_registered __read_mostly;
+
+/*
+ * The backends and filesystems work all asynchronously. This is b/c the
+ * backends can be built as modules.
+ * The usual sequence of events is:
+ *	a) mount /	-> __cleancache_init_fs is called. We set the
+ *		[shared_|]fs_poolid_map and uuids for.
+ *
+ *	b). user does I/Os -> we call the rest of __cleancache_* functions
+ *		which return immediately as backend_registered is false.
+ *
+ *	c). modprobe zcache -> cleancache_register_ops. We init the backend
+ *		and set backend_registered to true, and for any fs_poolid_map
+ *		(which is set by __cleancache_init_fs) we initialize the poolid.
+ *
+ *	d). user does I/Os -> now that backend_registered is true all the
+ *		__cleancache_* functions can call the backend. They all check
+ *		that fs_poolid_map is valid and if so invoke the backend.
+ *
+ *	e). umount /	-> __cleancache_invalidate_fs, the fs_poolid_map is
+ *		reset (which is the second check in the __cleancache_* ops
+ *		to call the backend).
+ *
+ * The sequence of event could also be c), followed by a), and d). and e). The
+ * c) would not happen anymore. There is also the chance of c), and one thread
+ * doing a) + d), and another doing e). For that case we depend on the
+ * filesystem calling __cleancache_invalidate_fs in the proper sequence (so
+ * that it handles all I/Os before it invalidates the fs (which is last part
+ * of unmounting process).
+ *
+ * Note: The acute reader will notice that there is no "rmmod zcache" case.
+ * This is b/c the functionality for that is not yet implemented and when
+ * done, will require some extra locking not yet devised.
+ */
+
+/*
+ * Register operations for cleancache, returning previous thus allowing
+ * detection of multiple backends and possible nesting.
  */
 struct cleancache_ops cleancache_register_ops(struct cleancache_ops *ops)
 {
 	struct cleancache_ops old = cleancache_ops;
+	int i;
 
+	mutex_lock(&poolid_mutex);
 	cleancache_ops = *ops;
-	cleancache_enabled = 1;
+
+	backend_registered = true;
+	for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
+		if (fs_poolid_map[i] == FS_NO_BACKEND)
+			fs_poolid_map[i] = (*cleancache_ops.init_fs)(PAGE_SIZE);
+		if (shared_fs_poolid_map[i] == FS_NO_BACKEND)
+			shared_fs_poolid_map[i] = (*cleancache_ops.init_shared_fs)
+					(uuids[i], PAGE_SIZE);
+	}
+out:
+	mutex_unlock(&poolid_mutex);
 	return old;
 }
 EXPORT_SYMBOL(cleancache_register_ops);
@@ -61,15 +145,42 @@ EXPORT_SYMBOL(cleancache_register_ops);
 /* Called by a cleancache-enabled filesystem at time of mount */
 void __cleancache_init_fs(struct super_block *sb)
 {
-	sb->cleancache_poolid = (*cleancache_ops.init_fs)(PAGE_SIZE);
+	int i;
+
+	mutex_lock(&poolid_mutex);
+	for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
+		if (fs_poolid_map[i] == FS_UNKNOWN) {
+			sb->cleancache_poolid = i + FAKE_FS_POOLID_OFFSET;
+			if (backend_registered)
+				fs_poolid_map[i] = (*cleancache_ops.init_fs)(PAGE_SIZE);
+			else
+				fs_poolid_map[i] = FS_NO_BACKEND;
+			break;
+		}
+	}
+	mutex_unlock(&poolid_mutex);
 }
 EXPORT_SYMBOL(__cleancache_init_fs);
 
 /* Called by a cleancache-enabled clustered filesystem at time of mount */
 void __cleancache_init_shared_fs(char *uuid, struct super_block *sb)
 {
-	sb->cleancache_poolid =
-		(*cleancache_ops.init_shared_fs)(uuid, PAGE_SIZE);
+	int i;
+
+	mutex_lock(&poolid_mutex);
+	for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
+		if (shared_fs_poolid_map[i] == FS_UNKNOWN) {
+			sb->cleancache_poolid = i + FAKE_SHARED_FS_POOLID_OFFSET;
+			uuids[i] = uuid;
+			if (backend_registered)
+				shared_fs_poolid_map[i] = (*cleancache_ops.init_shared_fs)
+						(uuid, PAGE_SIZE);
+			else
+				shared_fs_poolid_map[i] = FS_NO_BACKEND;
+			break;
+		}
+	}
+	mutex_unlock(&poolid_mutex);
 }
 EXPORT_SYMBOL(__cleancache_init_shared_fs);
 
@@ -98,28 +209,54 @@ static int cleancache_get_key(struct inode *inode,
 	return 0;
 }
 
+/*
+ * Returns a pool_id that is associated with a given fake poolid.
+ */
+static int get_poolid_from_fake(int fake_pool_id)
+{
+	if (fake_pool_id >= FAKE_SHARED_FS_POOLID_OFFSET)
+		return shared_fs_poolid_map[fake_pool_id -
+			FAKE_SHARED_FS_POOLID_OFFSET];
+	else if (fake_pool_id >= FAKE_FS_POOLID_OFFSET)
+		return fs_poolid_map[fake_pool_id - FAKE_FS_POOLID_OFFSET];
+	return FS_NO_BACKEND;
+}
+
 /*
  * "Get" data from cleancache associated with the poolid/inode/index
  * that were specified when the data was put to cleanache and, if
  * successful, use it to fill the specified page with data and return 0.
  * The pageframe is unchanged and returns -1 if the get fails.
  * Page must be locked by caller.
+ *
+ * The function has two checks before any action is taken - whether
+ * a backend is registered and whether the sb->cleancache_poolid
+ * is correct.
  */
 int __cleancache_get_page(struct page *page)
 {
 	int ret = -1;
 	int pool_id;
+	int fake_pool_id;
 	struct cleancache_filekey key = { .u.key = { 0 } };
 
-	VM_BUG_ON(!PageLocked(page));
-	pool_id = page->mapping->host->i_sb->cleancache_poolid;
-	if (pool_id < 0)
+	if (!backend_registered) {
+		cleancache_failed_gets++;
 		goto out;
+	}
+
+	VM_BUG_ON(!PageLocked(page));
+	fake_pool_id = page->mapping->host->i_sb->cleancache_poolid;
+	if (fake_pool_id < 0)
+		goto out;
+	pool_id = get_poolid_from_fake(fake_pool_id);
 
 	if (cleancache_get_key(page->mapping->host, &key) < 0)
 		goto out;
 
-	ret = (*cleancache_ops.get_page)(pool_id, key, page->index, page);
+	if (pool_id >= 0)
+		ret = (*cleancache_ops.get_page)(pool_id,
+				key, page->index, page);
 	if (ret == 0)
 		cleancache_succ_gets++;
 	else
@@ -134,16 +271,31 @@ EXPORT_SYMBOL(__cleancache_get_page);
  * (previously-obtained per-filesystem) poolid and the page's,
  * inode and page index.  Page must be locked.  Note that a put_page
  * always "succeeds", though a subsequent get_page may succeed or fail.
+ *
+ * The function has two checks before any action is taken - whether
+ * a backend is registered and whether the sb->cleancache_poolid
+ * is correct.
  */
 void __cleancache_put_page(struct page *page)
 {
 	int pool_id;
+	int fake_pool_id;
 	struct cleancache_filekey key = { .u.key = { 0 } };
 
+	if (!backend_registered) {
+		cleancache_puts++;
+		return;
+	}
+
 	VM_BUG_ON(!PageLocked(page));
-	pool_id = page->mapping->host->i_sb->cleancache_poolid;
+	fake_pool_id = page->mapping->host->i_sb->cleancache_poolid;
+	if (fake_pool_id < 0)
+		return;
+
+	pool_id = get_poolid_from_fake(fake_pool_id);
+
 	if (pool_id >= 0 &&
-	      cleancache_get_key(page->mapping->host, &key) >= 0) {
+		cleancache_get_key(page->mapping->host, &key) >= 0) {
 		(*cleancache_ops.put_page)(pool_id, key, page->index, page);
 		cleancache_puts++;
 	}
@@ -153,19 +305,31 @@ EXPORT_SYMBOL(__cleancache_put_page);
 /*
  * Invalidate any data from cleancache associated with the poolid and the
  * page's inode and page index so that a subsequent "get" will fail.
+ *
+ * The function has two checks before any action is taken - whether
+ * a backend is registered and whether the sb->cleancache_poolid
+ * is correct.
  */
 void __cleancache_invalidate_page(struct address_space *mapping,
 					struct page *page)
 {
 	/* careful... page->mapping is NULL sometimes when this is called */
-	int pool_id = mapping->host->i_sb->cleancache_poolid;
+	int pool_id;
+	int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
 	struct cleancache_filekey key = { .u.key = { 0 } };
 
-	if (pool_id >= 0) {
+	if (!backend_registered)
+		return;
+
+	if (fake_pool_id >= 0) {
+		pool_id = get_poolid_from_fake(fake_pool_id);
+		if (pool_id < 0)
+			return;
+
 		VM_BUG_ON(!PageLocked(page));
 		if (cleancache_get_key(mapping->host, &key) >= 0) {
 			(*cleancache_ops.invalidate_page)(pool_id,
-							  key, page->index);
+					key, page->index);
 			cleancache_invalidates++;
 		}
 	}
@@ -176,12 +340,25 @@ EXPORT_SYMBOL(__cleancache_invalidate_page);
  * Invalidate all data from cleancache associated with the poolid and the
  * mappings's inode so that all subsequent gets to this poolid/inode
  * will fail.
+ *
+ * The function has two checks before any action is taken - whether
+ * a backend is registered and whether the sb->cleancache_poolid
+ * is correct.
  */
 void __cleancache_invalidate_inode(struct address_space *mapping)
 {
-	int pool_id = mapping->host->i_sb->cleancache_poolid;
+	int pool_id;
+	int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
 	struct cleancache_filekey key = { .u.key = { 0 } };
 
+	if (!backend_registered)
+		return;
+
+	if (fake_pool_id < 0)
+		return;
+
+	pool_id = get_poolid_from_fake(fake_pool_id);
+
 	if (pool_id >= 0 && cleancache_get_key(mapping->host, &key) >= 0)
 		(*cleancache_ops.invalidate_inode)(pool_id, key);
 }
@@ -189,21 +366,37 @@ EXPORT_SYMBOL(__cleancache_invalidate_inode);
 
 /*
  * Called by any cleancache-enabled filesystem at time of unmount;
- * note that pool_id is surrendered and may be reutrned by a subsequent
- * cleancache_init_fs or cleancache_init_shared_fs
+ * note that pool_id is surrendered and may be returned by a subsequent
+ * cleancache_init_fs or cleancache_init_shared_fs.
  */
 void __cleancache_invalidate_fs(struct super_block *sb)
 {
-	if (sb->cleancache_poolid >= 0) {
-		int old_poolid = sb->cleancache_poolid;
-		sb->cleancache_poolid = -1;
-		(*cleancache_ops.invalidate_fs)(old_poolid);
+	int index;
+	int fake_pool_id = sb->cleancache_poolid;
+	int old_poolid = fake_pool_id;
+
+	mutex_lock(&poolid_mutex);
+	if (fake_pool_id >= FAKE_SHARED_FS_POOLID_OFFSET) {
+		index = fake_pool_id - FAKE_SHARED_FS_POOLID_OFFSET;
+		old_poolid = shared_fs_poolid_map[index];
+		shared_fs_poolid_map[index] = FS_UNKNOWN;
+		uuids[index] = NULL;
+	} else if (fake_pool_id >= FAKE_FS_POOLID_OFFSET) {
+		index = fake_pool_id - FAKE_FS_POOLID_OFFSET;
+		old_poolid = fs_poolid_map[index];
+		fs_poolid_map[index] = FS_UNKNOWN;
 	}
+	sb->cleancache_poolid = -1;
+	if (backend_registered)
+		(*cleancache_ops.invalidate_fs)(old_poolid);
+	mutex_unlock(&poolid_mutex);
 }
 EXPORT_SYMBOL(__cleancache_invalidate_fs);
 
 static int __init init_cleancache(void)
 {
+	int i;
+
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *root = debugfs_create_dir("cleancache", NULL);
 	if (root == NULL)
@@ -215,6 +408,11 @@ static int __init init_cleancache(void)
 	debugfs_create_u64("invalidates", S_IRUGO,
 				root, &cleancache_invalidates);
 #endif
+	for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
+		fs_poolid_map[i] = FS_UNKNOWN;
+		shared_fs_poolid_map[i] = FS_UNKNOWN;
+	}
+	cleancache_enabled = 1;
 	return 0;
 }
 module_init(init_cleancache)