This function should be called by fs-dax file systems after opening the
devdax device. This adds holder_operations, which effects exclusivity
between callers of fs_dax_get().

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

This also adds the CONFIG_DEV_DAX_IOMAP Kconfig parameter

Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/Kconfig |  6 ++++++
 drivers/dax/super.c | 30 ++++++++++++++++++++++++++++++
 include/linux/dax.h |  5 +++++
 3 files changed, 41 insertions(+)

diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index d656e4c0eb84..ad19fa966b8b 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -78,4 +78,10 @@ config DEV_DAX_KMEM
 
 	  Say N if unsure.
 
+config DEV_DAX_IOMAP
+       depends on DEV_DAX && DAX
+       def_bool y
+       help
+         Support iomap mapping of devdax devices (for FS-DAX file
+         systems that reside on character /dev/dax devices)
 endif
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e16d1d40d773..48bab9b5f341 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -122,6 +122,36 @@ 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, <0 on failure
+ */
+int fs_dax_get(struct dax_device *dax_dev, void *holder,
+	const struct dax_holder_operations *hops)
+{
+	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode))
+		return -ENODEV;
+
+	if (cmpxchg(&dax_dev->holder_data, NULL, holder))
+		return -EBUSY;
+
+	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 df41a0017b31..86bf5922f1b0 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -51,6 +51,11 @@ 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);
+struct dax_device *inode_dax(struct inode *inode);
+#endif
 void *dax_holder(struct dax_device *dax_dev);
 void put_dax(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
-- 
2.49.0