Re: [PATCH V7 07/19] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

John Groves
2026-02-26T23:21:03Z

On 26/02/19 09:07AM, Dave Jiang wrote:
>
>
> On 1/18/26 3:32 PM, John Groves wrote:
> > From: John Groves <john@groves.net>
> >
> > 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 | 58 ++++++++++++++++++++++++++++++++++++++++++++-
> > include/linux/dax.h | 20 ++++++++++------
> > 4 files changed, 72 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> > index e79daf825b52..01402d5103ef 100644
> > --- a/drivers/dax/bus.c
> > +++ b/drivers/dax/bus.c
> > @@ -39,8 +39,6 @@ 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..00c330ef437c 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
> > @@ -111,6 +112,10 @@ struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
> > }
> > EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
> >
> > +#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> > +
> > +#if IS_ENABLED(CONFIG_FS_DAX)
> > +
> > void fs_put_dax(struct dax_device *dax_dev, void *holder)
> > {
> > if (dax_dev && holder &&
> > @@ -119,7 +124,58 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
> > put_dax(dax_dev);
> > }
> > EXPORT_SYMBOL_GPL(fs_put_dax);
> > -#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> > +
> > +/**
> > + * 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) {
>
> Don't you need to hold the dev_dax->dev device lock in order to check the driver?
>
> DJ

Derp. Thanks for catching that Dave!

I believe it's fixed for v8, which is probably coming early next week.

John

[snip]