[PATCH v4 1/9] dax: add fsdev_dax_from_file()

[Miklos Szeredi <mszeredi@redhat.com>]

From: John Groves <John@Groves.net>

If the file is a fsdev_dax open, take the dax_dev from the device inode's
i_cdev.  Return with a reference grabbed on the private dax inode.

Make dax_dev_get() static again (internal to super.c for alloc_dax), export
fsdev_dax_from_file() instead.

This fix is in response to a Sashiko review, and some subsequent analysis.

About the 'fixes' tag: this removes the export of dax_dev_get(), which was
flawed, and replaces it with fsdev_dax_from_file(). It feels like the fixes
tag makes sense for correcting an ABI error.

Fixes: 2ae624d5a555d ("dax: export dax_dev_get()")
Originally-by: John Groves <john@groves.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 drivers/dax/fsdev.c | 24 ++++++++++++++++++++++++
 drivers/dax/super.c |  3 +--
 include/linux/dax.h |  3 ++-
 3 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index 598604bf5ac5..04b04d0d3da5 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -234,6 +234,30 @@ static const struct file_operations fsdev_fops = {
 	.release = fsdev_release,
 };
 
+/**
+ * fsdev_dax_from_file - get dax_device from an open file
+ * @file: open device file
+ *
+ * Returns a dax_device pointer if @file refers to a fsdev_dax device.
+ * Otherwise return NULL.
+ *
+ * Caller must put_dax() the returned device when done.
+ */
+struct dax_device *fsdev_dax_from_file(struct file *file)
+{
+	struct dax_device *dax_dev;
+
+	if (file->f_op != &fsdev_fops)
+		return NULL;
+
+	dax_dev = inode_dax(file_inode(file));
+	WARN_ON(!dax_alive(dax_dev));
+	ihold(dax_inode(dax_dev));
+
+	return dax_dev;
+}
+EXPORT_SYMBOL_GPL(fsdev_dax_from_file);
+
 /*
  * Acquire the dev_pagemap for probe: the static (pre-populated) one if
  * present, or a devm-allocated one for the dynamic case. Note that
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 45f84b0eb909..25836f389bde 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -565,7 +565,7 @@ static int dax_set(struct inode *inode, void *data)
 	return 0;
 }
 
-struct dax_device *dax_dev_get(dev_t devt)
+static struct dax_device *dax_dev_get(dev_t devt)
 {
 	struct dax_device *dax_dev;
 	struct inode *inode;
@@ -588,7 +588,6 @@ struct dax_device *dax_dev_get(dev_t devt)
 
 	return dax_dev;
 }
-EXPORT_SYMBOL_GPL(dax_dev_get);
 
 struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
 {
diff --git a/include/linux/dax.h b/include/linux/dax.h
index fe6c3ded1b50..2c9fc81f031b 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -54,7 +54,6 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
 void *dax_holder(struct dax_device *dax_dev);
 void put_dax(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
-struct dax_device *dax_dev_get(dev_t devt);
 void dax_write_cache(struct dax_device *dax_dev, bool wc);
 bool dax_write_cache_enabled(struct dax_device *dax_dev);
 bool dax_synchronous(struct dax_device *dax_dev);
@@ -323,3 +322,5 @@ typedef int (*walk_hmem_fn)(struct device *dev, int target_nid,
 			    const struct resource *res);
 int walk_hmem_resources(struct device *dev, walk_hmem_fn fn);
 #endif
+
+struct dax_device *fsdev_dax_from_file(struct file *file);
-- 
2.54.0