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Message-ID: <20260107153332.64727-7-john@groves.net>
Date: Wed, 7 Jan 2026 09:33:15 -0600
From: John Groves <John@...ves.net>
To: John Groves <John@...ves.net>,
Miklos Szeredi <miklos@...redi.hu>,
Dan Williams <dan.j.williams@...el.com>,
Bernd Schubert <bschubert@....com>,
Alison Schofield <alison.schofield@...el.com>
Cc: John Groves <jgroves@...ron.com>,
Jonathan Corbet <corbet@....net>,
Vishal Verma <vishal.l.verma@...el.com>,
Dave Jiang <dave.jiang@...el.com>,
Matthew Wilcox <willy@...radead.org>,
Jan Kara <jack@...e.cz>,
Alexander Viro <viro@...iv.linux.org.uk>,
David Hildenbrand <david@...nel.org>,
Christian Brauner <brauner@...nel.org>,
"Darrick J . Wong" <djwong@...nel.org>,
Randy Dunlap <rdunlap@...radead.org>,
Jeff Layton <jlayton@...nel.org>,
Amir Goldstein <amir73il@...il.com>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Stefan Hajnoczi <shajnocz@...hat.com>,
Joanne Koong <joannelkoong@...il.com>,
Josef Bacik <josef@...icpanda.com>,
Bagas Sanjaya <bagasdotme@...il.com>,
Chen Linxuan <chenlinxuan@...ontech.com>,
James Morse <james.morse@....com>,
Fuad Tabba <tabba@...gle.com>,
Sean Christopherson <seanjc@...gle.com>,
Shivank Garg <shivankg@....com>,
Ackerley Tng <ackerleytng@...gle.com>,
Gregory Price <gourry@...rry.net>,
Aravind Ramesh <arramesh@...ron.com>,
Ajay Joshi <ajayjoshi@...ron.com>,
venkataravis@...ron.com,
linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org,
nvdimm@...ts.linux.dev,
linux-cxl@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
John Groves <john@...ves.net>
Subject: [PATCH V3 06/21] dax: Add fs_dax_get() func to prepare dax for fs-dax usage
The fs_dax_get() function should be called by fs-dax file systems after
opening a fsdev dax device. This adds holder_operations, which provides
a memory failure callback path and effects exclusivity between callers
of fs_dax_get().
fs_dax_get() is specific to fsdev_dax, so it checks the driver type
(which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
not bound to the memory.
This function serves the same role as fs_dax_get_by_bdev(), which dax
file systems call after opening the pmem block device.
This can't be located in fsdev.c because struct dax_device is opaque
there.
This will be called by fs/fuse/famfs.c in a subsequent commit.
Signed-off-by: John Groves <john@...ves.net>
---
drivers/dax/bus.c | 2 --
drivers/dax/bus.h | 2 ++
drivers/dax/super.c | 54 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/dax.h | 1 +
4 files changed, 57 insertions(+), 2 deletions(-)
diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 0d7228acb913..6e0e28116edc 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -42,8 +42,6 @@ static int dax_bus_uevent(const struct device *dev, struct kobj_uevent_env *env)
return add_uevent_var(env, "MODALIAS=" DAX_DEVICE_MODALIAS_FMT, 0);
}
-#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driver, drv)
-
static struct dax_id *__dax_match_id(const struct dax_device_driver *dax_drv,
const char *dev_name)
{
diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
index 880bdf7e72d7..dc6f112ac4a4 100644
--- a/drivers/dax/bus.h
+++ b/drivers/dax/bus.h
@@ -42,6 +42,8 @@ struct dax_device_driver {
void (*remove)(struct dev_dax *dev);
};
+#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driver, drv)
+
int __dax_driver_register(struct dax_device_driver *dax_drv,
struct module *module, const char *mod_name);
#define dax_driver_register(driver) \
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index ba0b4cd18a77..68c45b918cff 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -14,6 +14,7 @@
#include <linux/fs.h>
#include <linux/cacheinfo.h>
#include "dax-private.h"
+#include "bus.h"
/**
* struct dax_device - anchor object for dax services
@@ -121,6 +122,59 @@ 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_FS)
+/**
+ * fs_dax_get() - get ownership of a devdax via holder/holder_ops
+ *
+ * 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)
+{
+ struct dev_dax *dev_dax;
+ struct dax_device_driver *dax_drv;
+ int id;
+
+ id = dax_read_lock();
+ if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode)) {
+ dax_read_unlock(id);
+ return -ENODEV;
+ }
+ dax_read_unlock(id);
+
+ /* Verify the device is bound to fsdev_dax driver */
+ dev_dax = dax_get_private(dax_dev);
+ if (!dev_dax || !dev_dax->dev.driver) {
+ iput(&dax_dev->inode);
+ return -ENODEV;
+ }
+
+ dax_drv = to_dax_drv(dev_dax->dev.driver);
+ if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
+ iput(&dax_dev->inode);
+ return -EOPNOTSUPP;
+ }
+
+ if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
+ iput(&dax_dev->inode);
+ return -EBUSY;
+ }
+
+ dax_dev->holder_ops = hops;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(fs_dax_get);
+#endif /* DEV_DAX_FS */
+
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 3fcd8562b72b..76f2a75f3144 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -53,6 +53,7 @@ struct dax_holder_operations {
struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
#if IS_ENABLED(CONFIG_DEV_DAX_FS)
+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);
--
2.49.0
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