On 26/01/08 12:27PM, Jonathan Cameron wrote:
> 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.
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
