Re: [PATCH V12 09/12] famfs: Register secondary daxdevs by path (FAMFSIOC_DAXDEV_OPEN)

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Aug 03, 2026 at 02:29:47AM +0000, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> Famfs file maps (fmaps) may reference multiple daxdevs. Before passing
> an fmap that references a new daxdev, the daxdev is pushed into the
> kernel via FAMFSIOC_DAXDEV_OPEN). This adds daxdevs to daxdev_table for
> index-based resolution from famfs extents to daxdevs.
> 
> Signed-off-by: John Groves <john@groves.net>
> ---
>  fs/famfs/famfs_file.c            | 74 ++++++++++++++++++++++++++++++++
>  include/uapi/linux/famfs_ioctl.h | 24 +++++++++++
>  2 files changed, 98 insertions(+)
> 
> diff --git a/fs/famfs/famfs_file.c b/fs/famfs/famfs_file.c
> index e7f271ce6d03..e11a55ecf8d7 100644
> --- a/fs/famfs/famfs_file.c
> +++ b/fs/famfs/famfs_file.c
> @@ -287,6 +287,76 @@ famfs_file_init_dax(struct file *file, void __user *arg)
>  	return rc;
>  }
>  
> +/**
> + * famfs_daxdev_open() - FAMFSIOC_DAXDEV_OPEN ioctl handler
> + * @file: any file in the famfs mount (the table is per-superblock)
> + * @arg:  ptr to struct famfs_ioc_daxdev in user space
> + *
> + * Register a devdax device (identified by path) into the mount's daxdev table
> + * at the caller-specified index, so files whose extents reference that index
> + * can be mapped. The path is resolved by lookup_daxdev() - the same helper the
> + * mount uses for the primary daxdev - so every slot is resolved identically.
> + * Registering exposes raw device memory, so it requires CAP_SYS_RAWIO.
> + */
> +static int
> +famfs_daxdev_open(struct file *file, void __user *arg)
> +{
> +	struct super_block *sb = file_inode(file)->i_sb;
> +	struct famfs_fs_info *fsi = sb->s_fs_info;
> +	struct famfs_ioc_daxdev dd;
> +	dev_t devno;
> +	char *path;
> +	int rc;
> +
> +	if (!capable(CAP_SYS_RAWIO))
> +		return -EPERM;
> +
> +	if (copy_from_user(&dd, arg, sizeof(dd)))
> +		return -EFAULT;
> +
> +	/* @flags is reserved; reject non-zero so it stays available */
> +	if (dd.flags)
> +		return -EINVAL;
> +
> +	/*
> +	 * If this daxdev index is already populated there is nothing to do.
> +	 * The index is cluster-invariant, so a valid slot already names this
> +	 * device; skip the path resolution entirely. install_daxdev() rechecks
> +	 * ->valid under the write lock, so this is purely an optimization.
> +	 */
> +	scoped_guard(rwsem_read, &fsi->devlist_sem) {
> +		if (dd.daxdev_index >= fsi->dax_devlist->nslots)
> +			return -EINVAL;
> +		if (fsi->dax_devlist->devlist[dd.daxdev_index].valid)
> +			return 0;
> +	}
> +
> +	if (dd.daxdev_path_len == 0 || dd.daxdev_path_len >= PATH_MAX)
> +		return -EINVAL;
> +
> +	/* +1 so the terminating NUL is included within the bound */
> +	path = strndup_user((const char __user *)(uintptr_t)dd.daxdev_path,
> +			    dd.daxdev_path_len + 1);
> +	if (IS_ERR(path))
> +		return PTR_ERR(path);
> +
> +	rc = lookup_daxdev(path, &devno);
> +	if (rc)
> +		goto out;
> +
> +	/*
> +	 * The daxdev table is allocated at mount time (for the slot-0 primary),
> +	 * so it is always present here; no need to allocate it.
> +	 */
> +	rc = famfs_install_daxdev(fsi, sb, dd.daxdev_index, devno, path);
> +	if (rc)
> +		pr_debug("%s: failed to install daxdev index %llu (%s)\n",
> +		       __func__, dd.daxdev_index, path);
> +out:
> +	kfree(path);
> +	return rc;
> +}
> +
>  /**
>   * famfs_file_ioctl() - Top-level famfs file ioctl handler
>   * @file: the file
> @@ -308,6 +378,10 @@ famfs_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  		rc = 0;
>  		break;
>  
> +	case FAMFSIOC_DAXDEV_OPEN:
> +		rc = famfs_daxdev_open(file, (void __user *)arg);
> +		break;
> +
>  	case FAMFSIOC_MAP_CREATE:
>  		rc = famfs_file_init_dax(file, (void __user *)arg);
>  		break;
> diff --git a/include/uapi/linux/famfs_ioctl.h b/include/uapi/linux/famfs_ioctl.h
> index b4eb373c1ade..751d8b033c2e 100644
> --- a/include/uapi/linux/famfs_ioctl.h
> +++ b/include/uapi/linux/famfs_ioctl.h
> @@ -77,6 +77,29 @@ struct famfs_ioc_fmap_header {
>  	__u64 reserved1;
>  };
>  
> +/**
> + * struct famfs_ioc_daxdev - register an additional backing daxdev by path
> + * @daxdev_index:    the (cluster-invariant) index this daxdev occupies in
> + *                   extent dev_index fields. Index 0 is the mount-time primary.
> + * @daxdev_path:     userspace pointer to the devdax device path (e.g.
> + *                   "/dev/dax0.0"); resolved in the kernel the same way the
> + *                   mount primary is.
> + * @daxdev_path_len: length of the path string, not counting the NUL.
> + * @flags:           reserved; must be zero.
> + *
> + * Standalone famfs registers every daxdev by path: the mount primary comes in
> + * as the mount device name, and slots 1..n come in here. (This deliberately
> + * differs from fuse's fd-based FUSE_DEV_IOC_DAXDEV_OPEN; each side is uniform
> + * within itself.) Passing the path by pointer keeps the struct fixed-size, so
> + * longer paths never require an ABI change.
> + */
> +struct famfs_ioc_daxdev {
> +	__u64 daxdev_index;
> +	__u64 daxdev_path;
> +	__u32 daxdev_path_len;

Can't we just pass an open fd to a dax device and have it use that
rather than doing its own path operations?

--D

> +	__u32 flags;
> +};
> +
>  #define FAMFSIOC_MAGIC 'u'
>  
>  /* famfs file ioctl opcodes */
> @@ -87,5 +110,6 @@ struct famfs_ioc_fmap_header {
>   * famfs_ioc_fmap_header followed by the extent list (see above).
>   */
>  #define FAMFSIOC_MAP_CREATE    _IOW(FAMFSIOC_MAGIC,  0x51, struct famfs_ioc_fmap_header)
> +#define FAMFSIOC_DAXDEV_OPEN   _IOW(FAMFSIOC_MAGIC,  0x52, struct famfs_ioc_daxdev)
>  
>  #endif /* FAMFS_IOCTL_H */
> -- 
> 2.53.0
> 
> 
>