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@groves.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);
--
2.49.0On Wed, 7 Jan 2026 09:33:15 -0600
John Groves <John@Groves.net> wrote:
> 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@groves.net>
Hi John,
A few passing comments on this one.
Jonathan
> ---
> #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();
Given this is an srcu_read_lock under the hood you could do similar
to the DEFINE_LOCK_GUARD_1 for the srcu (srcu.h) (though here it's a
DEFINE_LOCK_GUARD_0 given the lock itself isn't a parameter and then
use scoped_guard() here. Might not be worth the hassle and would need
a wrapper macro to poke &dax_srcu in which means exposing that at least
a little in a header.
DEFINE_LOCK_GUARD_0(_T->idx = dax_read_lock, dax_read_lock(_T->idx), idx);
Based loosely on the irqflags.h irqsave one.
> + 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);
I'd wrap this. It's rather long and there isn't a huge readability benefit in keeping
it on one line.
> struct dax_device *inode_dax(struct inode *inode);
> #endif
> void *dax_holder(struct dax_device *dax_dev);
On 26/01/08 12:27PM, Jonathan Cameron wrote:
> On Wed, 7 Jan 2026 09:33:15 -0600
> John Groves <John@Groves.net> wrote:
>
> > 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@groves.net>
> Hi John,
>
> A few passing comments on this one.
>
> Jonathan
>
> > ---
>
> > #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();
>
> Given this is an srcu_read_lock under the hood you could do similar
> to the DEFINE_LOCK_GUARD_1 for the srcu (srcu.h) (though here it's a
> DEFINE_LOCK_GUARD_0 given the lock itself isn't a parameter and then
> use scoped_guard() here. Might not be worth the hassle and would need
> a wrapper macro to poke &dax_srcu in which means exposing that at least
> a little in a header.
>
> DEFINE_LOCK_GUARD_0(_T->idx = dax_read_lock, dax_read_lock(_T->idx), idx);
> Based loosely on the irqflags.h irqsave one.
I'm getting more comfortable with scoped_guard(), but this feels like
a good leanup patch addressing all call sites of dax_read_lock() - after
the famfs dust settles.
If feelings are strong about this I'm open...
>
> > + 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);
> I'd wrap this. It's rather long and there isn't a huge readability benefit in keeping
> it on one line.
Done, thanks!
John
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