On Mon, Aug 03, 2026 at 02:29:47AM +0000, John Groves wrote:
> From: John Groves <[email protected]>
>
> 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 <[email protected]>
> ---
> 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
>
>
>
