dax: refactor dax-fs into a generic provider of 'struct dax_device' instances
We want dax capable drivers to be able to publish a set of dax
operations [1]. However, we do not want to further abuse block_devices
to advertise these operations. Instead we will attach these operations
to a dax device and add a lookup mechanism to go from block device path
to a dax device. A dax capable driver like pmem or brd is responsible
for registering a dax device, alongside a block device, and then a dax
capable filesystem is responsible for retrieving the dax device by path
name if it wants to call dax_operations.
For now, we refactor the dax pseudo-fs to be a generic facility, rather
than an implementation detail, of the device-dax use case. Where a "dax
device" is just an inode + dax infrastructure, and "Device DAX" is a
mapping service layered on top of that base 'struct dax_device'.
"Filesystem DAX" is then a mapping service that layers a filesystem on
top of that same base device. Filesystem DAX is associated with a
block_device for now, but perhaps directly to a dax device in the
future, or for new pmem-only filesystems.
[1]: https://lkml.org/lkml/2017/1/19/880
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
new file mode 100644
index 0000000..c9f85f1
--- /dev/null
+++ b/drivers/dax/super.c
@@ -0,0 +1,303 @@
+/*
+ * Copyright(c) 2017 Intel Corporation. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ */
+#include <linux/pagemap.h>
+#include <linux/module.h>
+#include <linux/mount.h>
+#include <linux/magic.h>
+#include <linux/cdev.h>
+#include <linux/hash.h>
+#include <linux/slab.h>
+#include <linux/fs.h>
+
+static int nr_dax = CONFIG_NR_DEV_DAX;
+module_param(nr_dax, int, S_IRUGO);
+MODULE_PARM_DESC(nr_dax, "max number of dax device instances");
+
+static dev_t dax_devt;
+DEFINE_STATIC_SRCU(dax_srcu);
+static struct vfsmount *dax_mnt;
+static DEFINE_IDA(dax_minor_ida);
+static struct kmem_cache *dax_cache __read_mostly;
+static struct super_block *dax_superblock __read_mostly;
+
+int dax_read_lock(void)
+{
+ return srcu_read_lock(&dax_srcu);
+}
+EXPORT_SYMBOL_GPL(dax_read_lock);
+
+void dax_read_unlock(int id)
+{
+ srcu_read_unlock(&dax_srcu, id);
+}
+EXPORT_SYMBOL_GPL(dax_read_unlock);
+
+/**
+ * struct dax_device - anchor object for dax services
+ * @inode: core vfs
+ * @cdev: optional character interface for "device dax"
+ * @private: dax driver private data
+ * @alive: !alive + rcu grace period == no new operations / mappings
+ */
+struct dax_device {
+ struct inode inode;
+ struct cdev cdev;
+ void *private;
+ bool alive;
+};
+
+bool dax_alive(struct dax_device *dax_dev)
+{
+ lockdep_assert_held(&dax_srcu);
+ return dax_dev->alive;
+}
+EXPORT_SYMBOL_GPL(dax_alive);
+
+/*
+ * Note, rcu is not protecting the liveness of dax_dev, rcu is ensuring
+ * that any fault handlers or operations that might have seen
+ * dax_alive(), have completed. Any operations that start after
+ * synchronize_srcu() has run will abort upon seeing !dax_alive().
+ */
+void kill_dax(struct dax_device *dax_dev)
+{
+ if (!dax_dev)
+ return;
+
+ dax_dev->alive = false;
+ synchronize_srcu(&dax_srcu);
+ dax_dev->private = NULL;
+}
+EXPORT_SYMBOL_GPL(kill_dax);
+
+static struct inode *dax_alloc_inode(struct super_block *sb)
+{
+ struct dax_device *dax_dev;
+
+ dax_dev = kmem_cache_alloc(dax_cache, GFP_KERNEL);
+ return &dax_dev->inode;
+}
+
+static struct dax_device *to_dax_dev(struct inode *inode)
+{
+ return container_of(inode, struct dax_device, inode);
+}
+
+static void dax_i_callback(struct rcu_head *head)
+{
+ struct inode *inode = container_of(head, struct inode, i_rcu);
+ struct dax_device *dax_dev = to_dax_dev(inode);
+
+ ida_simple_remove(&dax_minor_ida, MINOR(inode->i_rdev));
+ kmem_cache_free(dax_cache, dax_dev);
+}
+
+static void dax_destroy_inode(struct inode *inode)
+{
+ struct dax_device *dax_dev = to_dax_dev(inode);
+
+ WARN_ONCE(dax_dev->alive,
+ "kill_dax() must be called before final iput()\n");
+ 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_fs_type = {
+ .name = "dax",
+ .mount = dax_mount,
+ .kill_sb = kill_anon_super,
+};
+
+static int dax_test(struct inode *inode, void *data)
+{
+ dev_t devt = *(dev_t *) data;
+
+ return inode->i_rdev == devt;
+}
+
+static int dax_set(struct inode *inode, void *data)
+{
+ dev_t devt = *(dev_t *) data;
+
+ inode->i_rdev = devt;
+ return 0;
+}
+
+static struct dax_device *dax_dev_get(dev_t devt)
+{
+ struct dax_device *dax_dev;
+ struct inode *inode;
+
+ inode = iget5_locked(dax_superblock, hash_32(devt + DAXFS_MAGIC, 31),
+ dax_test, dax_set, &devt);
+
+ if (!inode)
+ return NULL;
+
+ dax_dev = to_dax_dev(inode);
+ if (inode->i_state & I_NEW) {
+ dax_dev->alive = true;
+ inode->i_cdev = &dax_dev->cdev;
+ inode->i_mode = S_IFCHR;
+ inode->i_flags = S_DAX;
+ mapping_set_gfp_mask(&inode->i_data, GFP_USER);
+ unlock_new_inode(inode);
+ }
+
+ return dax_dev;
+}
+
+struct dax_device *alloc_dax(void *private)
+{
+ struct dax_device *dax_dev;
+ dev_t devt;
+ int minor;
+
+ minor = ida_simple_get(&dax_minor_ida, 0, nr_dax, GFP_KERNEL);
+ if (minor < 0)
+ return NULL;
+
+ devt = MKDEV(MAJOR(dax_devt), minor);
+ dax_dev = dax_dev_get(devt);
+ if (!dax_dev)
+ goto err_inode;
+
+ dax_dev->private = private;
+ return dax_dev;
+
+ err_inode:
+ ida_simple_remove(&dax_minor_ida, minor);
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(alloc_dax);
+
+void put_dax(struct dax_device *dax_dev)
+{
+ if (!dax_dev)
+ return;
+ iput(&dax_dev->inode);
+}
+EXPORT_SYMBOL_GPL(put_dax);
+
+/**
+ * inode_dax: convert a public inode into its dax_dev
+ * @inode: An inode with i_cdev pointing to a dax_dev
+ *
+ * Note this is not equivalent to to_dax_dev() which is for private
+ * internal use where we know the inode filesystem type == dax_fs_type.
+ */
+struct dax_device *inode_dax(struct inode *inode)
+{
+ struct cdev *cdev = inode->i_cdev;
+
+ return container_of(cdev, struct dax_device, cdev);
+}
+EXPORT_SYMBOL_GPL(inode_dax);
+
+struct inode *dax_inode(struct dax_device *dax_dev)
+{
+ return &dax_dev->inode;
+}
+EXPORT_SYMBOL_GPL(dax_inode);
+
+void *dax_get_private(struct dax_device *dax_dev)
+{
+ return dax_dev->private;
+}
+EXPORT_SYMBOL_GPL(dax_get_private);
+
+static void init_once(void *_dax_dev)
+{
+ struct dax_device *dax_dev = _dax_dev;
+ struct inode *inode = &dax_dev->inode;
+
+ inode_init_once(inode);
+}
+
+static int __dax_fs_init(void)
+{
+ int rc;
+
+ dax_cache = kmem_cache_create("dax_cache", sizeof(struct dax_device), 0,
+ (SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|
+ SLAB_MEM_SPREAD|SLAB_ACCOUNT),
+ init_once);
+ if (!dax_cache)
+ return -ENOMEM;
+
+ rc = register_filesystem(&dax_fs_type);
+ if (rc)
+ goto err_register_fs;
+
+ dax_mnt = kern_mount(&dax_fs_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_fs_type);
+ err_register_fs:
+ kmem_cache_destroy(dax_cache);
+
+ return rc;
+}
+
+static void __dax_fs_exit(void)
+{
+ kern_unmount(dax_mnt);
+ unregister_filesystem(&dax_fs_type);
+ kmem_cache_destroy(dax_cache);
+}
+
+static int __init dax_fs_init(void)
+{
+ int rc;
+
+ rc = __dax_fs_init();
+ if (rc)
+ return rc;
+
+ nr_dax = max(nr_dax, 256);
+ rc = alloc_chrdev_region(&dax_devt, 0, nr_dax, "dax");
+ if (rc)
+ __dax_fs_exit();
+ return rc;
+}
+
+static void __exit dax_fs_exit(void)
+{
+ unregister_chrdev_region(dax_devt, nr_dax);
+ ida_destroy(&dax_minor_ida);
+ __dax_fs_exit();
+}
+
+MODULE_AUTHOR("Intel Corporation");
+MODULE_LICENSE("GPL v2");
+subsys_initcall(dax_fs_init);
+module_exit(dax_fs_exit);