On Wed, 7 Jan 2026 09:33:15 -0600 John Groves <[email protected]> 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 <[email protected]> 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);
