new inode method: ->free_inode()

A lot of ->destroy_inode() instances end with call_rcu() of a callback
that does RCU-delayed part of freeing.  Introduce a new method for
doing just that, with saner signature.

Rules:
->destroy_inode		->free_inode
	f			g		immediate call of f(),
						RCU-delayed call of g()
	f			NULL		immediate call of f(),
						no RCU-delayed calls
	NULL			g		RCU-delayed call of g()
	NULL			NULL		RCU-delayed default freeing

IOW, NULL ->free_inode gives the same behaviour as now.

Note that NULL, NULL is equivalent to NULL, free_inode_nonrcu; we could
mandate the latter form, but that would have very little benefit beyond
making rules a bit more symmetric.  It would break backwards compatibility,
require extra boilerplate and expected semantics for (NULL, NULL) pair
would have no use whatsoever...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/Locking

@@ -118,6 +118,7 @@ set:		exclusive
 --------------------------- super_operations ---------------------------
 prototypes:
 	struct inode *(*alloc_inode)(struct super_block *sb);
+	void (*free_inode)(struct inode *);
 	void (*destroy_inode)(struct inode *);
 	void (*dirty_inode) (struct inode *, int flags);
 	int (*write_inode) (struct inode *, struct writeback_control *wbc);
@@ -139,6 +140,7 @@ locking rules:
 	All may block [not true, see below]
 			s_umount
 alloc_inode:
+free_inode:				called from RCU callback
 destroy_inode:
 dirty_inode:
 write_inode:

Documentation/filesystems/porting

@@ -638,3 +638,28 @@ in your dentry operations instead.
 	inode to d_splice_alias() will also do the right thing (equivalent of
 	d_add(dentry, NULL); return NULL;), so that kind of special cases
 	also doesn't need a separate treatment.
+--
+[strongly recommended]
+	take the RCU-delayed parts of ->destroy_inode() into a new method -
+	->free_inode().  If ->destroy_inode() becomes empty - all the better,
+	just get rid of it.  Synchronous work (e.g. the stuff that can't
+	be done from an RCU callback, or any WARN_ON() where we want the
+	stack trace) *might* be movable to ->evict_inode(); however,
+	that goes only for the things that are not needed to balance something
+	done by ->alloc_inode().  IOW, if it's cleaning up the stuff that
+	might have accumulated over the life of in-core inode, ->evict_inode()
+	might be a fit.
+
+	Rules for inode destruction:
+		* if ->destroy_inode() is non-NULL, it gets called
+		* if ->free_inode() is non-NULL, it gets scheduled by call_rcu()
+		* combination of NULL ->destroy_inode and NULL ->free_inode is
+		  treated as NULL/free_inode_nonrcu, to preserve the compatibility.
+
+	Note that the callback (be it via ->free_inode() or explicit call_rcu()
+	in ->destroy_inode()) is *NOT* ordered wrt superblock destruction;
+	as the matter of fact, the superblock and all associated structures
+	might be already gone.  The filesystem driver is guaranteed to be still
+	there, but that's it.  Freeing memory in the callback is fine; doing
+	more than that is possible, but requires a lot of care and is best
+	avoided.

fs/inode.c

@@ -202,12 +202,28 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 }
 EXPORT_SYMBOL(inode_init_always);
 
+void free_inode_nonrcu(struct inode *inode)
+{
+	kmem_cache_free(inode_cachep, inode);
+}
+EXPORT_SYMBOL(free_inode_nonrcu);
+
+static void i_callback(struct rcu_head *head)
+{
+	struct inode *inode = container_of(head, struct inode, i_rcu);
+	if (inode->free_inode)
+		inode->free_inode(inode);
+	else
+		free_inode_nonrcu(inode);
+}
+
 static struct inode *alloc_inode(struct super_block *sb)
 {
+	const struct super_operations *ops = sb->s_op;
 	struct inode *inode;
 
-	if (sb->s_op->alloc_inode)
+	if (ops->alloc_inode)
-		inode = sb->s_op->alloc_inode(sb);
+		inode = ops->alloc_inode(sb);
 	else
 		inode = kmem_cache_alloc(inode_cachep, GFP_KERNEL);
 
@@ -215,22 +231,19 @@ static struct inode *alloc_inode(struct super_block *sb)
 		return NULL;
 
 	if (unlikely(inode_init_always(sb, inode))) {
-		if (inode->i_sb->s_op->destroy_inode)
+		if (ops->destroy_inode) {
-			inode->i_sb->s_op->destroy_inode(inode);
+			ops->destroy_inode(inode);
-		else
+			if (!ops->free_inode)
-			kmem_cache_free(inode_cachep, inode);
+				return NULL;
+		}
+		inode->free_inode = ops->free_inode;
+		i_callback(&inode->i_rcu);
 		return NULL;
 	}
 
 	return inode;
 }
 
-void free_inode_nonrcu(struct inode *inode)
-{
-	kmem_cache_free(inode_cachep, inode);
-}
-EXPORT_SYMBOL(free_inode_nonrcu);
-
 void __destroy_inode(struct inode *inode)
 {
 	BUG_ON(inode_has_buffers(inode));
@@ -253,20 +266,19 @@ void __destroy_inode(struct inode *inode)
 }
 EXPORT_SYMBOL(__destroy_inode);
 
-static void i_callback(struct rcu_head *head)
-{
-	struct inode *inode = container_of(head, struct inode, i_rcu);
-	kmem_cache_free(inode_cachep, inode);
-}
-
 static void destroy_inode(struct inode *inode)
 {
+	const struct super_operations *ops = inode->i_sb->s_op;
+
 	BUG_ON(!list_empty(&inode->i_lru));
 	__destroy_inode(inode);
-	if (inode->i_sb->s_op->destroy_inode)
+	if (ops->destroy_inode) {
-		inode->i_sb->s_op->destroy_inode(inode);
+		ops->destroy_inode(inode);
-	else
+		if (!ops->free_inode)
-		call_rcu(&inode->i_rcu, i_callback);
+			return;
+	}
+	inode->free_inode = ops->free_inode;
+	call_rcu(&inode->i_rcu, i_callback);
 }
 
 /**

include/linux/fs.h

@@ -694,7 +694,10 @@ struct inode {
 #ifdef CONFIG_IMA
 	atomic_t		i_readcount; /* struct files open RO */
 #endif
-	const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
+	union {
+		const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
+		void (*free_inode)(struct inode *);
+	};
 	struct file_lock_context	*i_flctx;
 	struct address_space	i_data;
 	struct list_head	i_devices;
@@ -1903,6 +1906,7 @@ extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 struct super_operations {
    	struct inode *(*alloc_inode)(struct super_block *sb);
 	void (*destroy_inode)(struct inode *);
+	void (*free_inode)(struct inode *);
 
    	void (*dirty_inode) (struct inode *, int flags);
 	int (*write_inode) (struct inode *, struct writeback_control *wbc);