[PATCH v2 04/54] fs: add an i_obj_count refcount to the inode

[Josef Bacik <josef@xxxxxxxxxxxxxx>]

Currently the inode's lifetime is controlled by it's main refcount,
i_count.  We overload the eviction of the inode (the final unlink) with
the deletion of the in-memory object, which leads to some hilarity when
we iput() in unfortunate places.

In order to make this less of a footgun, we want to separate the notion
of "is this inode in use by a user" and "is this inode object currently
in use", since deleting an inode is a much heavier operation that
deleting the object and cleaning up its memory.

To that end, introduce ->i_obj_count to the inode. This will be used to
control the lifetime of the inode object itself. We will continue to use
the ->i_count refcount as normal to reduce the churn of adding a new
refcount to inode. Subsequent patches will expand the usage of
->i_obj_count for internal uses, and then I will separate out the
tear down operations of the inode, and then finally adjust the refount
rules for ->i_count to be more consistent with all other refcounts.

Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 fs/inode.c         | 20 ++++++++++++++++++++
 include/linux/fs.h | 12 ++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index 13e80b434323..d426f54c05d9 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -235,6 +235,7 @@ int inode_init_always_gfp(struct super_block *sb, struct inode *inode, gfp_t gfp
 	inode->i_flags = 0;
 	inode->i_state = 0;
 	atomic64_set(&inode->i_sequence, 0);
+	refcount_set(&inode->i_obj_count, 1);
 	atomic_set(&inode->i_count, 1);
 	inode->i_op = &empty_iops;
 	inode->i_fop = &no_open_fops;
@@ -831,6 +832,11 @@ static void evict(struct inode *inode)
 	inode_wake_up_bit(inode, __I_NEW);
 	BUG_ON(inode->i_state != (I_FREEING | I_CLEAR));
 
+	/*
+	 * refcount_dec_and_test must be used here to avoid the underflow
+	 * warning.
+	 */
+	WARN_ON(!refcount_dec_and_test(&inode->i_obj_count));
 	destroy_inode(inode);
 }
 
@@ -1930,6 +1936,20 @@ void iput(struct inode *inode)
 }
 EXPORT_SYMBOL(iput);
 
+/**
+ *	iobj_put	- put a object reference on an inode
+ *	@inode: inode to put
+ *
+ *	Puts a object reference on an inode.
+ */
+void iobj_put(struct inode *inode)
+{
+	if (!inode)
+		return;
+	refcount_dec(&inode->i_obj_count);
+}
+EXPORT_SYMBOL(iobj_put);
+
 #ifdef CONFIG_BLOCK
 /**
  *	bmap	- find a block number in a file
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 56041d3387fe..84f5218755c3 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -871,6 +871,7 @@ struct inode {
 #if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)
 	atomic_t		i_readcount; /* struct files open RO */
 #endif
+	refcount_t		i_obj_count;
 	union {
 		const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
 		void (*free_inode)(struct inode *);
@@ -2809,6 +2810,7 @@ extern int current_umask(void);
 
 extern void ihold(struct inode * inode);
 extern void iput(struct inode *);
+extern void iobj_put(struct inode *inode);
 int inode_update_timestamps(struct inode *inode, int flags);
 int generic_update_time(struct inode *, int);
 
@@ -3364,6 +3366,16 @@ static inline bool is_zero_ino(ino_t ino)
 	return (u32)ino == 0;
 }
 
+static inline void iobj_get(struct inode *inode)
+{
+	refcount_inc(&inode->i_obj_count);
+}
+
+static inline unsigned int iobj_count_read(const struct inode *inode)
+{
+	return refcount_read(&inode->i_obj_count);
+}
+
 /*
  * inode->i_lock must be held
  */
-- 
2.49.0