dax: define a unified inode/address_space for device-dax mappings

In support of enabling resize / truncate of device-dax instances, define
a pseudo-fs to provide a unified inode/address space for vm operations.

Cc: Al Viro <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/dax/dax.c

···
 #include <linux/pagemap.h>
 #include <linux/module.h>
 #include <linux/device.h>
+#include <linux/mount.h>
 #include <linux/pfn_t.h>
+#include <linux/hash.h>
 #include <linux/cdev.h>
 #include <linux/slab.h>
 #include <linux/dax.h>
···
 static DEFINE_IDA(dax_minor_ida);
 static int nr_dax = CONFIG_NR_DEV_DAX;
 module_param(nr_dax, int, S_IRUGO);
+static struct vfsmount *dax_mnt;
+static struct kmem_cache *dax_cache __read_mostly;
+static struct super_block *dax_superblock __read_mostly;
 MODULE_PARM_DESC(nr_dax, "max number of device-dax instances");
 
 /**
···
  */
 struct dax_dev {
 	struct dax_region *region;
+	struct inode *inode;
 	struct device dev;
 	struct cdev cdev;
 	bool alive;
···
 	int num_resources;
 	struct resource res[0];
 };
+
+static struct inode *dax_alloc_inode(struct super_block *sb)
+{
+	return kmem_cache_alloc(dax_cache, GFP_KERNEL);
+}
+
+static void dax_i_callback(struct rcu_head *head)
+{
+	struct inode *inode = container_of(head, struct inode, i_rcu);
+
+	kmem_cache_free(dax_cache, inode);
+}
+
+static void dax_destroy_inode(struct inode *inode)
+{
+	call_rcu(&inode->i_rcu, dax_i_callback);
+}
+
+static const struct super_operations dax_sops = {
+	.statfs = simple_statfs,
+	.alloc_inode = dax_alloc_inode,
+	.destroy_inode = dax_destroy_inode,
+	.drop_inode = generic_delete_inode,
+};
+
+static struct dentry *dax_mount(struct file_system_type *fs_type,
+		int flags, const char *dev_name, void *data)
+{
+	return mount_pseudo(fs_type, "dax:", &dax_sops, NULL, DAXFS_MAGIC);
+}
+
+static struct file_system_type dax_type = {
+	.name = "dax",
+	.mount = dax_mount,
+	.kill_sb = kill_anon_super,
+};
+
+static int dax_test(struct inode *inode, void *data)
+{
+	return inode->i_cdev == data;
+}
+
+static int dax_set(struct inode *inode, void *data)
+{
+	inode->i_cdev = data;
+	return 0;
+}
+
+static struct inode *dax_inode_get(struct cdev *cdev, dev_t devt)
+{
+	struct inode *inode;
+
+	inode = iget5_locked(dax_superblock, hash_32(devt + DAXFS_MAGIC, 31),
+			dax_test, dax_set, cdev);
+
+	if (!inode)
+		return NULL;
+
+	if (inode->i_state & I_NEW) {
+		inode->i_mode = S_IFCHR;
+		inode->i_flags = S_DAX;
+		inode->i_rdev = devt;
+		mapping_set_gfp_mask(&inode->i_data, GFP_USER);
+		unlock_new_inode(inode);
+	}
+	return inode;
+}
+
+static void init_once(void *inode)
+{
+	inode_init_once(inode);
+}
+
+static int dax_inode_init(void)
+{
+	int rc;
+
+	dax_cache = kmem_cache_create("dax_cache", sizeof(struct inode), 0,
+			(SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|
+			 SLAB_MEM_SPREAD|SLAB_ACCOUNT),
+			init_once);
+	if (!dax_cache)
+		return -ENOMEM;
+
+	rc = register_filesystem(&dax_type);
+	if (rc)
+		goto err_register_fs;
+
+	dax_mnt = kern_mount(&dax_type);
+	if (IS_ERR(dax_mnt)) {
+		rc = PTR_ERR(dax_mnt);
+		goto err_mount;
+	}
+	dax_superblock = dax_mnt->mnt_sb;
+
+	return 0;
+
+ err_mount:
+	unregister_filesystem(&dax_type);
+ err_register_fs:
+	kmem_cache_destroy(dax_cache);
+
+	return rc;
+}
+
+static void dax_inode_exit(void)
+{
+	kern_unmount(dax_mnt);
+	unregister_filesystem(&dax_type);
+	kmem_cache_destroy(dax_cache);
+}
 
 static void dax_region_free(struct kref *kref)
 {
···
 
 	dax_dev = container_of(inode->i_cdev, struct dax_dev, cdev);
 	dev_dbg(&dax_dev->dev, "%s\n", __func__);
+	inode->i_mapping = dax_dev->inode->i_mapping;
+	inode->i_mapping->host = dax_dev->inode;
+	filp->f_mapping = inode->i_mapping;
 	filp->private_data = dax_dev;
 	inode->i_flags = S_DAX;
 
···
 	ida_simple_remove(&dax_region->ida, dax_dev->id);
 	ida_simple_remove(&dax_minor_ida, MINOR(dev->devt));
 	dax_region_put(dax_region);
+	iput(dax_dev->inode);
 	kfree(dax_dev);
 }
 
···
 		goto err_minor;
 	}
 
+	dax_dev->inode = dax_inode_get(&dax_dev->cdev, dev_t);
+	if (!dax_dev->inode) {
+		rc = -ENOMEM;
+		goto err_inode;
+	}
+
 	/* device_initialize() so cdev can reference kobj parent */
 	dev_t = MKDEV(MAJOR(dax_devt), minor);
 	dev = &dax_dev->dev;
···
 	return devm_add_action_or_reset(dax_region->dev, unregister_dax_dev, dev);
 
  err_cdev:
+	iput(dax_dev->inode);
+ err_inode:
 	ida_simple_remove(&dax_minor_ida, minor);
  err_minor:
 	ida_simple_remove(&dax_region->ida, dax_dev->id);
···
 {
 	int rc;
 
-	nr_dax = max(nr_dax, 256);
-	rc = alloc_chrdev_region(&dax_devt, 0, nr_dax, "dax");
+	rc = dax_inode_init();
 	if (rc)
 		return rc;
 
-	dax_class = class_create(THIS_MODULE, "dax");
-	if (IS_ERR(dax_class))
-		unregister_chrdev_region(dax_devt, nr_dax);
+	nr_dax = max(nr_dax, 256);
+	rc = alloc_chrdev_region(&dax_devt, 0, nr_dax, "dax");
+	if (rc)
+		goto err_chrdev;
 
-	return PTR_ERR_OR_ZERO(dax_class);
+	dax_class = class_create(THIS_MODULE, "dax");
+	if (IS_ERR(dax_class)) {
+		rc = PTR_ERR(dax_class);
+		goto err_class;
+	}
+
+	return 0;
+
+ err_class:
+	unregister_chrdev_region(dax_devt, nr_dax);
+ err_chrdev:
+	dax_inode_exit();
+	return rc;
 }
 
 static void __exit dax_exit(void)
···
 	class_destroy(dax_class);
 	unregister_chrdev_region(dax_devt, nr_dax);
 	ida_destroy(&dax_minor_ida);
+	dax_inode_exit();
 }
 
 MODULE_AUTHOR("Intel Corporation");

fs/char_dev.c

···
 	spin_lock(&cdev_lock);
 	list_del_init(&inode->i_devices);
 	inode->i_cdev = NULL;
+	inode->i_mapping = &inode->i_data;
 	spin_unlock(&cdev_lock);
 }
 

include/uapi/linux/magic.h

···
 #define V9FS_MAGIC		0x01021997
 
 #define BDEVFS_MAGIC            0x62646576
+#define DAXFS_MAGIC             0x64646178
 #define BINFMTFS_MAGIC          0x42494e4d
 #define DEVPTS_SUPER_MAGIC	0x1cd1
 #define FUTEXFS_SUPER_MAGIC	0xBAD1DEA