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;
+	__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