[RFC PATCH 02/20] dev_dax_iomap: Add fs_dax_get() func to prepare dax for fs-dax usage

[John Groves <John@Groves.net>]

This function should be called by fs-dax file systems after opening the
devdax device. This adds holder_operations.

This function serves the same role as fs_dax_get_by_bdev(), which dax
file systems call after opening the pmem block device.

Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/super.c | 38 ++++++++++++++++++++++++++++++++++++++
 include/linux/dax.h |  5 +++++
 2 files changed, 43 insertions(+)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index f4b635526345..fc96362de237 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -121,6 +121,44 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
 EXPORT_SYMBOL_GPL(fs_put_dax);
 #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
 
+#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
+
+/**
+ * fs_dax_get()
+ *
+ * fs-dax file systems call this function to prepare to use a devdax device for fsdax.
+ * This is like fs_dax_get_by_bdev(), but the caller already has struct dev_dax (and there
+ * is no bdev). The holder makes this exclusive.
+ *
+ * @dax_dev: dev to be prepared for fs-dax usage
+ * @holder: filesystem or mapped device inside the dax_device
+ * @hops: operations for the inner holder
+ *
+ * Returns: 0 on success, -1 on failure
+ */
+int fs_dax_get(
+	struct dax_device *dax_dev,
+	void *holder,
+	const struct dax_holder_operations *hops)
+{
+	/* dax_dev->ops should have been populated by devm_create_dev_dax() */
+	if (WARN_ON(!dax_dev->ops))
+		return -1;
+
+	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode))
+		return -1;
+
+	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
+		pr_warn("%s: holder_data already set\n", __func__);
+		return -1;
+	}
+	dax_dev->holder_ops = hops;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fs_dax_get);
+#endif /* DEV_DAX_IOMAP */
+
 enum dax_device_flags {
 	/* !alive + rcu grace period == no new operations / mappings */
 	DAXDEV_ALIVE,
diff --git a/include/linux/dax.h b/include/linux/dax.h
index b463502b16e1..e973289bfde3 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -57,7 +57,12 @@ struct dax_holder_operations {
 
 #if IS_ENABLED(CONFIG_DAX)
 struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
+
+#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
+int fs_dax_get(struct dax_device *dax_dev, void *holder, const struct dax_holder_operations *hops);
+#endif
 void *dax_holder(struct dax_device *dax_dev);
+struct dax_device *inode_dax(struct inode *inode);
 void put_dax(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
 void dax_write_cache(struct dax_device *dax_dev, bool wc);
-- 
2.43.0