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Wong" , Randy Dunlap , Jeff Layton , Amir Goldstein , Jonathan Cameron , Stefan Hajnoczi , Joanne Koong , Josef Bacik , Bagas Sanjaya , Chen Linxuan , James Morse , Fuad Tabba , Sean Christopherson , Shivank Garg , Ackerley Tng , Gregory Price , Aravind Ramesh , Ajay Joshi , venkataravis@micron.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, John Groves Subject: [PATCH V3 06/21] dax: Add fs_dax_get() func to prepare dax for fs-dax usage Date: Wed, 7 Jan 2026 09:33:15 -0600 Message-ID: <20260107153332.64727-7-john@groves.net> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260107153332.64727-1-john@groves.net> References: <20260107153244.64703-1-john@groves.net> <20260107153332.64727-1-john@groves.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 --- 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, struc= t kobj_uevent_env *env) return add_uevent_var(env, "MODALIAS=3D" DAX_DEVICE_MODALIAS_FMT, 0); } =20 -#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driv= er, drv) - static struct dax_id *__dax_match_id(const struct dax_device_driver *dax_d= rv, 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); }; =20 +#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driv= er, 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 #include #include "dax-private.h" +#include "bus.h" =20 /** * struct dax_device - anchor object for dax services @@ -121,6 +122,59 @@ void fs_put_dax(struct dax_device *dax_dev, void *hold= er) EXPORT_SYMBOL_GPL(fs_put_dax); #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */ =20 +#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 devic= e for + * fsdax. This is like fs_dax_get_by_bdev(), but the caller already has st= ruct + * 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 =3D 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 =3D dax_get_private(dax_dev); + if (!dev_dax || !dev_dax->dev.driver) { + iput(&dax_dev->inode); + return -ENODEV; + } + + dax_drv =3D to_dax_drv(dev_dax->dev.driver); + if (dax_drv->type !=3D 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 =3D hops; + + return 0; +} +EXPORT_SYMBOL_GPL(fs_dax_get); +#endif /* DEV_DAX_FS */ + enum dax_device_flags { /* !alive + rcu grace period =3D=3D 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 *o= ps); =20 #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); --=20 2.49.0