dm: add clone target

Add the dm-clone target, which allows cloning of arbitrary block
devices.

dm-clone produces a one-to-one copy of an existing, read-only source
device into a writable destination device: It presents a virtual block
device which makes all data appear immediately, and redirects reads and
writes accordingly.

The main use case of dm-clone is to clone a potentially remote,
high-latency, read-only, archival-type block device into a writable,
fast, primary-type device for fast, low-latency I/O. The cloned device
is visible/mountable immediately and the copy of the source device to
the destination device happens in the background, in parallel with user
I/O.

When the cloning completes, the dm-clone table can be removed altogether
and be replaced, e.g., by a linear table, mapping directly to the
destination device.

For further information and examples of how to use dm-clone, please read
Documentation/admin-guide/device-mapper/dm-clone.rst
Suggested-by: Vangelis Koukis <vkoukis@arrikto.com>
Co-developed-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/Kconfig

@@ -347,6 +347,20 @@ config DM_ERA
          over time.  Useful for maintaining cache coherency when using
          vendor snapshots.
 
+config DM_CLONE
+       tristate "Clone target (EXPERIMENTAL)"
+       depends on BLK_DEV_DM
+       default n
+       select DM_PERSISTENT_DATA
+       ---help---
+         dm-clone produces a one-to-one copy of an existing, read-only source
+         device into a writable destination device. The cloned device is
+         visible/mountable immediately and the copy of the source device to the
+         destination device happens in the background, in parallel with user
+         I/O.
+
+         If unsure, say N.
+
 config DM_MIRROR
        tristate "Mirror target"
        depends on BLK_DEV_DM

drivers/md/Makefile

@@ -18,6 +18,7 @@ dm-cache-y	+= dm-cache-target.o dm-cache-metadata.o dm-cache-policy.o \
 		    dm-cache-background-tracker.o
 dm-cache-smq-y   += dm-cache-policy-smq.o
 dm-era-y	+= dm-era-target.o
+dm-clone-y	+= dm-clone-target.o dm-clone-metadata.o
 dm-verity-y	+= dm-verity-target.o
 md-mod-y	+= md.o md-bitmap.o
 raid456-y	+= raid5.o raid5-cache.o raid5-ppl.o
@@ -65,6 +66,7 @@ obj-$(CONFIG_DM_VERITY)		+= dm-verity.o
 obj-$(CONFIG_DM_CACHE)		+= dm-cache.o
 obj-$(CONFIG_DM_CACHE_SMQ)	+= dm-cache-smq.o
 obj-$(CONFIG_DM_ERA)		+= dm-era.o
+obj-$(CONFIG_DM_CLONE)		+= dm-clone.o
 obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o
 obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o
 obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o

drivers/md/dm-clone-metadata.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2019 Arrikto, Inc. All Rights Reserved.
+ */
+
+#ifndef DM_CLONE_METADATA_H
+#define DM_CLONE_METADATA_H
+
+#include "persistent-data/dm-block-manager.h"
+#include "persistent-data/dm-space-map-metadata.h"
+
+#define DM_CLONE_METADATA_BLOCK_SIZE DM_SM_METADATA_BLOCK_SIZE
+
+/*
+ * The metadata device is currently limited in size.
+ */
+#define DM_CLONE_METADATA_MAX_SECTORS DM_SM_METADATA_MAX_SECTORS
+
+/*
+ * A metadata device larger than 16GB triggers a warning.
+ */
+#define DM_CLONE_METADATA_MAX_SECTORS_WARNING (16 * (1024 * 1024 * 1024 >> SECTOR_SHIFT))
+
+#define SPACE_MAP_ROOT_SIZE 128
+
+/* dm-clone metadata */
+struct dm_clone_metadata;
+
+/*
+ * Set region status to hydrated.
+ *
+ * @cmd: The dm-clone metadata
+ * @region_nr: The region number
+ *
+ * This function doesn't block, so it's safe to call it from interrupt context.
+ */
+int dm_clone_set_region_hydrated(struct dm_clone_metadata *cmd, unsigned long region_nr);
+
+/*
+ * Set status of all regions in the provided range to hydrated, if not already
+ * hydrated.
+ *
+ * @cmd: The dm-clone metadata
+ * @start: Starting region number
+ * @nr_regions: Number of regions in the range
+ *
+ * This function doesn't block, so it's safe to call it from interrupt context.
+ */
+int dm_clone_cond_set_range(struct dm_clone_metadata *cmd, unsigned long start,
+			    unsigned long nr_regions);
+
+/*
+ * Read existing or create fresh metadata.
+ *
+ * @bdev: The device storing the metadata
+ * @target_size: The target size
+ * @region_size: The region size
+ *
+ * @returns: The dm-clone metadata
+ *
+ * This function reads the superblock of @bdev and checks if it's all zeroes.
+ * If it is, it formats @bdev and creates fresh metadata. If it isn't, it
+ * validates the metadata stored in @bdev.
+ */
+struct dm_clone_metadata *dm_clone_metadata_open(struct block_device *bdev,
+						 sector_t target_size,
+						 sector_t region_size);
+
+/*
+ * Free the resources related to metadata management.
+ */
+void dm_clone_metadata_close(struct dm_clone_metadata *cmd);
+
+/*
+ * Commit dm-clone metadata to disk.
+ */
+int dm_clone_metadata_commit(struct dm_clone_metadata *cmd);
+
+/*
+ * Reload the in core copy of the on-disk bitmap.
+ *
+ * This should be used after aborting a metadata transaction and setting the
+ * metadata to read-only, to invalidate the in-core cache and make it match the
+ * on-disk metadata.
+ *
+ * WARNING: It must not be called concurrently with either
+ * dm_clone_set_region_hydrated() or dm_clone_cond_set_range(), as it updates
+ * the region bitmap without taking the relevant spinlock. We don't take the
+ * spinlock because dm_clone_reload_in_core_bitset() does I/O, so it may block.
+ *
+ * But, it's safe to use it after calling dm_clone_metadata_set_read_only(),
+ * because the latter sets the metadata to read-only mode. Both
+ * dm_clone_set_region_hydrated() and dm_clone_cond_set_range() refuse to touch
+ * the region bitmap, after calling dm_clone_metadata_set_read_only().
+ */
+int dm_clone_reload_in_core_bitset(struct dm_clone_metadata *cmd);
+
+/*
+ * Check whether dm-clone's metadata changed this transaction.
+ */
+bool dm_clone_changed_this_transaction(struct dm_clone_metadata *cmd);
+
+/*
+ * Abort current metadata transaction and rollback metadata to the last
+ * committed transaction.
+ */
+int dm_clone_metadata_abort(struct dm_clone_metadata *cmd);
+
+/*
+ * Switches metadata to a read only mode. Once read-only mode has been entered
+ * the following functions will return -EPERM:
+ *
+ *   dm_clone_metadata_commit()
+ *   dm_clone_set_region_hydrated()
+ *   dm_clone_cond_set_range()
+ *   dm_clone_metadata_abort()
+ */
+void dm_clone_metadata_set_read_only(struct dm_clone_metadata *cmd);
+void dm_clone_metadata_set_read_write(struct dm_clone_metadata *cmd);
+
+/*
+ * Returns true if the hydration of the destination device is finished.
+ */
+bool dm_clone_is_hydration_done(struct dm_clone_metadata *cmd);
+
+/*
+ * Returns true if region @region_nr is hydrated.
+ */
+bool dm_clone_is_region_hydrated(struct dm_clone_metadata *cmd, unsigned long region_nr);
+
+/*
+ * Returns true if all the regions in the range are hydrated.
+ */
+bool dm_clone_is_range_hydrated(struct dm_clone_metadata *cmd,
+				unsigned long start, unsigned long nr_regions);
+
+/*
+ * Returns the number of hydrated regions.
+ */
+unsigned long dm_clone_nr_of_hydrated_regions(struct dm_clone_metadata *cmd);
+
+/*
+ * Returns the first unhydrated region with region_nr >= @start
+ */
+unsigned long dm_clone_find_next_unhydrated_region(struct dm_clone_metadata *cmd,
+						   unsigned long start);
+
+/*
+ * Get the number of free metadata blocks.
+ */
+int dm_clone_get_free_metadata_block_count(struct dm_clone_metadata *cmd, dm_block_t *result);
+
+/*
+ * Get the total number of metadata blocks.
+ */
+int dm_clone_get_metadata_dev_size(struct dm_clone_metadata *cmd, dm_block_t *result);
+
+#endif /* DM_CLONE_METADATA_H */