Merging upstream version 6.9.7.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2063 insertions, 0 deletions

diff --git a/drivers/md/dm-vdo/data-vio.c b/drivers/md/dm-vdo/data-vio.c
new file mode 100644
index 0000000000..94f6f1ccfb
--- /dev/null
+++ b/drivers/md/dm-vdo/data-vio.c
@@ -0,0 +1,2063 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2023 Red Hat
+ */
+
+#include "data-vio.h"
+
+#include <linux/atomic.h>
+#include <linux/bio.h>
+#include <linux/blkdev.h>
+#include <linux/delay.h>
+#include <linux/device-mapper.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/lz4.h>
+#include <linux/minmax.h>
+#include <linux/sched.h>
+#include <linux/spinlock.h>
+#include <linux/wait.h>
+
+#include "logger.h"
+#include "memory-alloc.h"
+#include "murmurhash3.h"
+#include "permassert.h"
+
+#include "block-map.h"
+#include "dump.h"
+#include "encodings.h"
+#include "int-map.h"
+#include "io-submitter.h"
+#include "logical-zone.h"
+#include "packer.h"
+#include "recovery-journal.h"
+#include "slab-depot.h"
+#include "status-codes.h"
+#include "types.h"
+#include "vdo.h"
+#include "vio.h"
+#include "wait-queue.h"
+
+/**
+ * DOC: Bio flags.
+ *
+ * For certain flags set on user bios, if the user bio has not yet been acknowledged, setting those
+ * flags on our own bio(s) for that request may help underlying layers better fulfill the user
+ * bio's needs. This constant contains the aggregate of those flags; VDO strips all the other
+ * flags, as they convey incorrect information.
+ *
+ * These flags are always irrelevant if we have already finished the user bio as they are only
+ * hints on IO importance. If VDO has finished the user bio, any remaining IO done doesn't care how
+ * important finishing the finished bio was.
+ *
+ * Note that bio.c contains the complete list of flags we believe may be set; the following list
+ * explains the action taken with each of those flags VDO could receive:
+ *
+ * * REQ_SYNC: Passed down if the user bio is not yet completed, since it indicates the user bio
+ *   completion is required for further work to be done by the issuer.
+ * * REQ_META: Passed down if the user bio is not yet completed, since it may mean the lower layer
+ *   treats it as more urgent, similar to REQ_SYNC.
+ * * REQ_PRIO: Passed down if the user bio is not yet completed, since it indicates the user bio is
+ *   important.
+ * * REQ_NOMERGE: Set only if the incoming bio was split; irrelevant to VDO IO.
+ * * REQ_IDLE: Set if the incoming bio had more IO quickly following; VDO's IO pattern doesn't
+ *   match incoming IO, so this flag is incorrect for it.
+ * * REQ_FUA: Handled separately, and irrelevant to VDO IO otherwise.
+ * * REQ_RAHEAD: Passed down, as, for reads, it indicates trivial importance.
+ * * REQ_BACKGROUND: Not passed down, as VIOs are a limited resource and VDO needs them recycled
+ *   ASAP to service heavy load, which is the only place where REQ_BACKGROUND might aid in load
+ *   prioritization.
+ */
+static blk_opf_t PASSTHROUGH_FLAGS = (REQ_PRIO | REQ_META | REQ_SYNC | REQ_RAHEAD);
+
+/**
+ * DOC:
+ *
+ * The data_vio_pool maintains the pool of data_vios which a vdo uses to service incoming bios. For
+ * correctness, and in order to avoid potentially expensive or blocking memory allocations during
+ * normal operation, the number of concurrently active data_vios is capped. Furthermore, in order
+ * to avoid starvation of reads and writes, at most 75% of the data_vios may be used for
+ * discards. The data_vio_pool is responsible for enforcing these limits. Threads submitting bios
+ * for which a data_vio or discard permit are not available will block until the necessary
+ * resources are available. The pool is also responsible for distributing resources to blocked
+ * threads and waking them. Finally, the pool attempts to batch the work of recycling data_vios by
+ * performing the work of actually assigning resources to blocked threads or placing data_vios back
+ * into the pool on a single cpu at a time.
+ *
+ * The pool contains two "limiters", one for tracking data_vios and one for tracking discard
+ * permits. The limiters also provide safe cross-thread access to pool statistics without the need
+ * to take the pool's lock. When a thread submits a bio to a vdo device, it will first attempt to
+ * get a discard permit if it is a discard, and then to get a data_vio. If the necessary resources
+ * are available, the incoming bio will be assigned to the acquired data_vio, and it will be
+ * launched. However, if either of these are unavailable, the arrival time of the bio is recorded
+ * in the bio's bi_private field, the bio and its submitter are both queued on the appropriate
+ * limiter and the submitting thread will then put itself to sleep. (note that this mechanism will
+ * break if jiffies are only 32 bits.)
+ *
+ * Whenever a data_vio has completed processing for the bio it was servicing, release_data_vio()
+ * will be called on it. This function will add the data_vio to a funnel queue, and then check the
+ * state of the pool. If the pool is not currently processing released data_vios, the pool's
+ * completion will be enqueued on a cpu queue. This obviates the need for the releasing threads to
+ * hold the pool's lock, and also batches release work while avoiding starvation of the cpu
+ * threads.
+ *
+ * Whenever the pool's completion is run on a cpu thread, it calls process_release_callback() which
+ * processes a batch of returned data_vios (currently at most 32) from the pool's funnel queue. For
+ * each data_vio, it first checks whether that data_vio was processing a discard. If so, and there
+ * is a blocked bio waiting for a discard permit, that permit is notionally transferred to the
+ * eldest discard waiter, and that waiter is moved to the end of the list of discard bios waiting
+ * for a data_vio. If there are no discard waiters, the discard permit is returned to the pool.
+ * Next, the data_vio is assigned to the oldest blocked bio which either has a discard permit, or
+ * doesn't need one and relaunched. If neither of these exist, the data_vio is returned to the
+ * pool. Finally, if any waiting bios were launched, the threads which blocked trying to submit
+ * them are awakened.
+ */
+
+#define DATA_VIO_RELEASE_BATCH_SIZE 128
+
+static const unsigned int VDO_SECTORS_PER_BLOCK_MASK = VDO_SECTORS_PER_BLOCK - 1;
+static const u32 COMPRESSION_STATUS_MASK = 0xff;
+static const u32 MAY_NOT_COMPRESS_MASK = 0x80000000;
+
+struct limiter;
+typedef void (*assigner_fn)(struct limiter *limiter);
+
+/* Bookkeeping structure for a single type of resource. */
+struct limiter {
+	/* The data_vio_pool to which this limiter belongs */
+	struct data_vio_pool *pool;
+	/* The maximum number of data_vios available */
+	data_vio_count_t limit;
+	/* The number of resources in use */
+	data_vio_count_t busy;
+	/* The maximum number of resources ever simultaneously in use */
+	data_vio_count_t max_busy;
+	/* The number of resources to release */
+	data_vio_count_t release_count;
+	/* The number of waiters to wake */
+	data_vio_count_t wake_count;
+	/* The list of waiting bios which are known to process_release_callback() */
+	struct bio_list waiters;
+	/* The list of waiting bios which are not yet known to process_release_callback() */
+	struct bio_list new_waiters;
+	/* The list of waiters which have their permits */
+	struct bio_list *permitted_waiters;
+	/* The function for assigning a resource to a waiter */
+	assigner_fn assigner;
+	/* The queue of blocked threads */
+	wait_queue_head_t blocked_threads;
+	/* The arrival time of the eldest waiter */
+	u64 arrival;
+};
+
+/*
+ * A data_vio_pool is a collection of preallocated data_vios which may be acquired from any thread,
+ * and are released in batches.
+ */
+struct data_vio_pool {
+	/* Completion for scheduling releases */
+	struct vdo_completion completion;
+	/* The administrative state of the pool */
+	struct admin_state state;
+	/* Lock protecting the pool */
+	spinlock_t lock;
+	/* The main limiter controlling the total data_vios in the pool. */
+	struct limiter limiter;
+	/* The limiter controlling data_vios for discard */
+	struct limiter discard_limiter;
+	/* The list of bios which have discard permits but still need a data_vio */
+	struct bio_list permitted_discards;
+	/* The list of available data_vios */
+	struct list_head available;
+	/* The queue of data_vios waiting to be returned to the pool */
+	struct funnel_queue *queue;
+	/* Whether the pool is processing, or scheduled to process releases */
+	atomic_t processing;
+	/* The data vios in the pool */
+	struct data_vio data_vios[];
+};
+
+static const char * const ASYNC_OPERATION_NAMES[] = {
+	"launch",
+	"acknowledge_write",
+	"acquire_hash_lock",
+	"attempt_logical_block_lock",
+	"lock_duplicate_pbn",
+	"check_for_duplication",
+	"cleanup",
+	"compress_data_vio",
+	"find_block_map_slot",
+	"get_mapped_block_for_read",
+	"get_mapped_block_for_write",
+	"hash_data_vio",
+	"journal_remapping",
+	"vdo_attempt_packing",
+	"put_mapped_block",
+	"read_data_vio",
+	"update_dedupe_index",
+	"update_reference_counts",
+	"verify_duplication",
+	"write_data_vio",
+};
+
+/* The steps taken cleaning up a VIO, in the order they are performed. */
+enum data_vio_cleanup_stage {
+	VIO_CLEANUP_START,
+	VIO_RELEASE_HASH_LOCK = VIO_CLEANUP_START,
+	VIO_RELEASE_ALLOCATED,
+	VIO_RELEASE_RECOVERY_LOCKS,
+	VIO_RELEASE_LOGICAL,
+	VIO_CLEANUP_DONE
+};
+
+static inline struct data_vio_pool * __must_check
+as_data_vio_pool(struct vdo_completion *completion)
+{
+	vdo_assert_completion_type(completion, VDO_DATA_VIO_POOL_COMPLETION);
+	return container_of(completion, struct data_vio_pool, completion);
+}
+
+static inline u64 get_arrival_time(struct bio *bio)
+{
+	return (u64) bio->bi_private;
+}
+
+/**
+ * check_for_drain_complete_locked() - Check whether a data_vio_pool has no outstanding data_vios
+ *				       or waiters while holding the pool's lock.
+ */
+static bool check_for_drain_complete_locked(struct data_vio_pool *pool)
+{
+	if (pool->limiter.busy > 0)
+		return false;
+
+	VDO_ASSERT_LOG_ONLY((pool->discard_limiter.busy == 0),
+			    "no outstanding discard permits");
+
+	return (bio_list_empty(&pool->limiter.new_waiters) &&
+		bio_list_empty(&pool->discard_limiter.new_waiters));
+}
+
+static void initialize_lbn_lock(struct data_vio *data_vio, logical_block_number_t lbn)
+{
+	struct vdo *vdo = vdo_from_data_vio(data_vio);
+	zone_count_t zone_number;
+	struct lbn_lock *lock = &data_vio->logical;
+
+	lock->lbn = lbn;
+	lock->locked = false;
+	vdo_waitq_init(&lock->waiters);
+	zone_number = vdo_compute_logical_zone(data_vio);
+	lock->zone = &vdo->logical_zones->zones[zone_number];
+}
+
+static void launch_locked_request(struct data_vio *data_vio)
+{
+	data_vio->logical.locked = true;
+	if (data_vio->write) {
+		struct vdo *vdo = vdo_from_data_vio(data_vio);
+
+		if (vdo_is_read_only(vdo)) {
+			continue_data_vio_with_error(data_vio, VDO_READ_ONLY);
+			return;
+		}
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_FIND_BLOCK_MAP_SLOT;
+	vdo_find_block_map_slot(data_vio);
+}
+
+static void acknowledge_data_vio(struct data_vio *data_vio)
+{
+	struct vdo *vdo = vdo_from_data_vio(data_vio);
+	struct bio *bio = data_vio->user_bio;
+	int error = vdo_status_to_errno(data_vio->vio.completion.result);
+
+	if (bio == NULL)
+		return;
+
+	VDO_ASSERT_LOG_ONLY((data_vio->remaining_discard <=
+			     (u32) (VDO_BLOCK_SIZE - data_vio->offset)),
+			    "data_vio to acknowledge is not an incomplete discard");
+
+	data_vio->user_bio = NULL;
+	vdo_count_bios(&vdo->stats.bios_acknowledged, bio);
+	if (data_vio->is_partial)
+		vdo_count_bios(&vdo->stats.bios_acknowledged_partial, bio);
+
+	bio->bi_status = errno_to_blk_status(error);
+	bio_endio(bio);
+}
+
+static void copy_to_bio(struct bio *bio, char *data_ptr)
+{
+	struct bio_vec biovec;
+	struct bvec_iter iter;
+
+	bio_for_each_segment(biovec, bio, iter) {
+		memcpy_to_bvec(&biovec, data_ptr);
+		data_ptr += biovec.bv_len;
+	}
+}
+
+struct data_vio_compression_status get_data_vio_compression_status(struct data_vio *data_vio)
+{
+	u32 packed = atomic_read(&data_vio->compression.status);
+
+	/* pairs with cmpxchg in set_data_vio_compression_status */
+	smp_rmb();
+	return (struct data_vio_compression_status) {
+		.stage = packed & COMPRESSION_STATUS_MASK,
+		.may_not_compress = ((packed & MAY_NOT_COMPRESS_MASK) != 0),
+	};
+}
+
+/**
+ * pack_status() - Convert a data_vio_compression_status into a u32 which may be stored
+ *                 atomically.
+ * @status: The state to convert.
+ *
+ * Return: The compression state packed into a u32.
+ */
+static u32 __must_check pack_status(struct data_vio_compression_status status)
+{
+	return status.stage | (status.may_not_compress ? MAY_NOT_COMPRESS_MASK : 0);
+}
+
+/**
+ * set_data_vio_compression_status() - Set the compression status of a data_vio.
+ * @state: The expected current status of the data_vio.
+ * @new_state: The status to set.
+ *
+ * Return: true if the new status was set, false if the data_vio's compression status did not
+ *         match the expected state, and so was left unchanged.
+ */
+static bool __must_check
+set_data_vio_compression_status(struct data_vio *data_vio,
+				struct data_vio_compression_status status,
+				struct data_vio_compression_status new_status)
+{
+	u32 actual;
+	u32 expected = pack_status(status);
+	u32 replacement = pack_status(new_status);
+
+	/*
+	 * Extra barriers because this was original developed using a CAS operation that implicitly
+	 * had them.
+	 */
+	smp_mb__before_atomic();
+	actual = atomic_cmpxchg(&data_vio->compression.status, expected, replacement);
+	/* same as before_atomic */
+	smp_mb__after_atomic();
+	return (expected == actual);
+}
+
+struct data_vio_compression_status advance_data_vio_compression_stage(struct data_vio *data_vio)
+{
+	for (;;) {
+		struct data_vio_compression_status status =
+			get_data_vio_compression_status(data_vio);
+		struct data_vio_compression_status new_status = status;
+
+		if (status.stage == DATA_VIO_POST_PACKER) {
+			/* We're already in the last stage. */
+			return status;
+		}
+
+		if (status.may_not_compress) {
+			/*
+			 * Compression has been dis-allowed for this VIO, so skip the rest of the
+			 * path and go to the end.
+			 */
+			new_status.stage = DATA_VIO_POST_PACKER;
+		} else {
+			/* Go to the next state. */
+			new_status.stage++;
+		}
+
+		if (set_data_vio_compression_status(data_vio, status, new_status))
+			return new_status;
+
+		/* Another thread changed the status out from under us so try again. */
+	}
+}
+
+/**
+ * cancel_data_vio_compression() - Prevent this data_vio from being compressed or packed.
+ *
+ * Return: true if the data_vio is in the packer and the caller was the first caller to cancel it.
+ */
+bool cancel_data_vio_compression(struct data_vio *data_vio)
+{
+	struct data_vio_compression_status status, new_status;
+
+	for (;;) {
+		status = get_data_vio_compression_status(data_vio);
+		if (status.may_not_compress || (status.stage == DATA_VIO_POST_PACKER)) {
+			/* This data_vio is already set up to not block in the packer. */
+			break;
+		}
+
+		new_status.stage = status.stage;
+		new_status.may_not_compress = true;
+
+		if (set_data_vio_compression_status(data_vio, status, new_status))
+			break;
+	}
+
+	return ((status.stage == DATA_VIO_PACKING) && !status.may_not_compress);
+}
+
+/**
+ * attempt_logical_block_lock() - Attempt to acquire the lock on a logical block.
+ * @completion: The data_vio for an external data request as a completion.
+ *
+ * This is the start of the path for all external requests. It is registered in launch_data_vio().
+ */
+static void attempt_logical_block_lock(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	struct lbn_lock *lock = &data_vio->logical;
+	struct vdo *vdo = vdo_from_data_vio(data_vio);
+	struct data_vio *lock_holder;
+	int result;
+
+	assert_data_vio_in_logical_zone(data_vio);
+
+	if (data_vio->logical.lbn >= vdo->states.vdo.config.logical_blocks) {
+		continue_data_vio_with_error(data_vio, VDO_OUT_OF_RANGE);
+		return;
+	}
+
+	result = vdo_int_map_put(lock->zone->lbn_operations, lock->lbn,
+				 data_vio, false, (void **) &lock_holder);
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(data_vio, result);
+		return;
+	}
+
+	if (lock_holder == NULL) {
+		/* We got the lock */
+		launch_locked_request(data_vio);
+		return;
+	}
+
+	result = VDO_ASSERT(lock_holder->logical.locked, "logical block lock held");
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(data_vio, result);
+		return;
+	}
+
+	/*
+	 * If the new request is a pure read request (not read-modify-write) and the lock_holder is
+	 * writing and has received an allocation, service the read request immediately by copying
+	 * data from the lock_holder to avoid having to flush the write out of the packer just to
+	 * prevent the read from waiting indefinitely. If the lock_holder does not yet have an
+	 * allocation, prevent it from blocking in the packer and wait on it. This is necessary in
+	 * order to prevent returning data that may not have actually been written.
+	 */
+	if (!data_vio->write && READ_ONCE(lock_holder->allocation_succeeded)) {
+		copy_to_bio(data_vio->user_bio, lock_holder->vio.data + data_vio->offset);
+		acknowledge_data_vio(data_vio);
+		complete_data_vio(completion);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_ATTEMPT_LOGICAL_BLOCK_LOCK;
+	vdo_waitq_enqueue_waiter(&lock_holder->logical.waiters, &data_vio->waiter);
+
+	/*
+	 * Prevent writes and read-modify-writes from blocking indefinitely on lock holders in the
+	 * packer.
+	 */
+	if (lock_holder->write && cancel_data_vio_compression(lock_holder)) {
+		data_vio->compression.lock_holder = lock_holder;
+		launch_data_vio_packer_callback(data_vio,
+						vdo_remove_lock_holder_from_packer);
+	}
+}
+
+/**
+ * launch_data_vio() - (Re)initialize a data_vio to have a new logical block number, keeping the
+ *		       same parent and other state and send it on its way.
+ */
+static void launch_data_vio(struct data_vio *data_vio, logical_block_number_t lbn)
+{
+	struct vdo_completion *completion = &data_vio->vio.completion;
+
+	/*
+	 * Clearing the tree lock must happen before initializing the LBN lock, which also adds
+	 * information to the tree lock.
+	 */
+	memset(&data_vio->tree_lock, 0, sizeof(data_vio->tree_lock));
+	initialize_lbn_lock(data_vio, lbn);
+	INIT_LIST_HEAD(&data_vio->hash_lock_entry);
+	INIT_LIST_HEAD(&data_vio->write_entry);
+
+	memset(&data_vio->allocation, 0, sizeof(data_vio->allocation));
+
+	data_vio->is_duplicate = false;
+
+	memset(&data_vio->record_name, 0, sizeof(data_vio->record_name));
+	memset(&data_vio->duplicate, 0, sizeof(data_vio->duplicate));
+	vdo_reset_completion(completion);
+	completion->error_handler = handle_data_vio_error;
+	set_data_vio_logical_callback(data_vio, attempt_logical_block_lock);
+	vdo_enqueue_completion(completion, VDO_DEFAULT_Q_MAP_BIO_PRIORITY);
+}
+
+static bool is_zero_block(char *block)
+{
+	int i;
+
+	for (i = 0; i < VDO_BLOCK_SIZE; i += sizeof(u64)) {
+		if (*((u64 *) &block[i]))
+			return false;
+	}
+
+	return true;
+}
+
+static void copy_from_bio(struct bio *bio, char *data_ptr)
+{
+	struct bio_vec biovec;
+	struct bvec_iter iter;
+
+	bio_for_each_segment(biovec, bio, iter) {
+		memcpy_from_bvec(data_ptr, &biovec);
+		data_ptr += biovec.bv_len;
+	}
+}
+
+static void launch_bio(struct vdo *vdo, struct data_vio *data_vio, struct bio *bio)
+{
+	logical_block_number_t lbn;
+	/*
+	 * Zero out the fields which don't need to be preserved (i.e. which are not pointers to
+	 * separately allocated objects).
+	 */
+	memset(data_vio, 0, offsetof(struct data_vio, vio));
+	memset(&data_vio->compression, 0, offsetof(struct compression_state, block));
+
+	data_vio->user_bio = bio;
+	data_vio->offset = to_bytes(bio->bi_iter.bi_sector & VDO_SECTORS_PER_BLOCK_MASK);
+	data_vio->is_partial = (bio->bi_iter.bi_size < VDO_BLOCK_SIZE) || (data_vio->offset != 0);
+
+	/*
+	 * Discards behave very differently than other requests when coming in from device-mapper.
+	 * We have to be able to handle any size discards and various sector offsets within a
+	 * block.
+	 */
+	if (bio_op(bio) == REQ_OP_DISCARD) {
+		data_vio->remaining_discard = bio->bi_iter.bi_size;
+		data_vio->write = true;
+		data_vio->is_discard = true;
+		if (data_vio->is_partial) {
+			vdo_count_bios(&vdo->stats.bios_in_partial, bio);
+			data_vio->read = true;
+		}
+	} else if (data_vio->is_partial) {
+		vdo_count_bios(&vdo->stats.bios_in_partial, bio);
+		data_vio->read = true;
+		if (bio_data_dir(bio) == WRITE)
+			data_vio->write = true;
+	} else if (bio_data_dir(bio) == READ) {
+		data_vio->read = true;
+	} else {
+		/*
+		 * Copy the bio data to a char array so that we can continue to use the data after
+		 * we acknowledge the bio.
+		 */
+		copy_from_bio(bio, data_vio->vio.data);
+		data_vio->is_zero = is_zero_block(data_vio->vio.data);
+		data_vio->write = true;
+	}
+
+	if (data_vio->user_bio->bi_opf & REQ_FUA)
+		data_vio->fua = true;
+
+	lbn = (bio->bi_iter.bi_sector - vdo->starting_sector_offset) / VDO_SECTORS_PER_BLOCK;
+	launch_data_vio(data_vio, lbn);
+}
+
+static void assign_data_vio(struct limiter *limiter, struct data_vio *data_vio)
+{
+	struct bio *bio = bio_list_pop(limiter->permitted_waiters);
+
+	launch_bio(limiter->pool->completion.vdo, data_vio, bio);
+	limiter->wake_count++;
+
+	bio = bio_list_peek(limiter->permitted_waiters);
+	limiter->arrival = ((bio == NULL) ? U64_MAX : get_arrival_time(bio));
+}
+
+static void assign_discard_permit(struct limiter *limiter)
+{
+	struct bio *bio = bio_list_pop(&limiter->waiters);
+
+	if (limiter->arrival == U64_MAX)
+		limiter->arrival = get_arrival_time(bio);
+
+	bio_list_add(limiter->permitted_waiters, bio);
+}
+
+static void get_waiters(struct limiter *limiter)
+{
+	bio_list_merge(&limiter->waiters, &limiter->new_waiters);
+	bio_list_init(&limiter->new_waiters);
+}
+
+static inline struct data_vio *get_available_data_vio(struct data_vio_pool *pool)
+{
+	struct data_vio *data_vio =
+		list_first_entry(&pool->available, struct data_vio, pool_entry);
+
+	list_del_init(&data_vio->pool_entry);
+	return data_vio;
+}
+
+static void assign_data_vio_to_waiter(struct limiter *limiter)
+{
+	assign_data_vio(limiter, get_available_data_vio(limiter->pool));
+}
+
+static void update_limiter(struct limiter *limiter)
+{
+	struct bio_list *waiters = &limiter->waiters;
+	data_vio_count_t available = limiter->limit - limiter->busy;
+
+	VDO_ASSERT_LOG_ONLY((limiter->release_count <= limiter->busy),
+			    "Release count %u is not more than busy count %u",
+			    limiter->release_count, limiter->busy);
+
+	get_waiters(limiter);
+	for (; (limiter->release_count > 0) && !bio_list_empty(waiters); limiter->release_count--)
+		limiter->assigner(limiter);
+
+	if (limiter->release_count > 0) {
+		WRITE_ONCE(limiter->busy, limiter->busy - limiter->release_count);
+		limiter->release_count = 0;
+		return;
+	}
+
+	for (; (available > 0) && !bio_list_empty(waiters); available--)
+		limiter->assigner(limiter);
+
+	WRITE_ONCE(limiter->busy, limiter->limit - available);
+	if (limiter->max_busy < limiter->busy)
+		WRITE_ONCE(limiter->max_busy, limiter->busy);
+}
+
+/**
+ * schedule_releases() - Ensure that release processing is scheduled.
+ *
+ * If this call switches the state to processing, enqueue. Otherwise, some other thread has already
+ * done so.
+ */
+static void schedule_releases(struct data_vio_pool *pool)
+{
+	/* Pairs with the barrier in process_release_callback(). */
+	smp_mb__before_atomic();
+	if (atomic_cmpxchg(&pool->processing, false, true))
+		return;
+
+	pool->completion.requeue = true;
+	vdo_launch_completion_with_priority(&pool->completion,
+					    CPU_Q_COMPLETE_VIO_PRIORITY);
+}
+
+static void reuse_or_release_resources(struct data_vio_pool *pool,
+				       struct data_vio *data_vio,
+				       struct list_head *returned)
+{
+	if (data_vio->remaining_discard > 0) {
+		if (bio_list_empty(&pool->discard_limiter.waiters)) {
+			/* Return the data_vio's discard permit. */
+			pool->discard_limiter.release_count++;
+		} else {
+			assign_discard_permit(&pool->discard_limiter);
+		}
+	}
+
+	if (pool->limiter.arrival < pool->discard_limiter.arrival) {
+		assign_data_vio(&pool->limiter, data_vio);
+	} else if (pool->discard_limiter.arrival < U64_MAX) {
+		assign_data_vio(&pool->discard_limiter, data_vio);
+	} else {
+		list_add(&data_vio->pool_entry, returned);
+		pool->limiter.release_count++;
+	}
+}
+
+/**
+ * process_release_callback() - Process a batch of data_vio releases.
+ * @completion: The pool with data_vios to release.
+ */
+static void process_release_callback(struct vdo_completion *completion)
+{
+	struct data_vio_pool *pool = as_data_vio_pool(completion);
+	bool reschedule;
+	bool drained;
+	data_vio_count_t processed;
+	data_vio_count_t to_wake;
+	data_vio_count_t discards_to_wake;
+	LIST_HEAD(returned);
+
+	spin_lock(&pool->lock);
+	get_waiters(&pool->discard_limiter);
+	get_waiters(&pool->limiter);
+	spin_unlock(&pool->lock);
+
+	if (pool->limiter.arrival == U64_MAX) {
+		struct bio *bio = bio_list_peek(&pool->limiter.waiters);
+
+		if (bio != NULL)
+			pool->limiter.arrival = get_arrival_time(bio);
+	}
+
+	for (processed = 0; processed < DATA_VIO_RELEASE_BATCH_SIZE; processed++) {
+		struct data_vio *data_vio;
+		struct funnel_queue_entry *entry = vdo_funnel_queue_poll(pool->queue);
+
+		if (entry == NULL)
+			break;
+
+		data_vio = as_data_vio(container_of(entry, struct vdo_completion,
+						    work_queue_entry_link));
+		acknowledge_data_vio(data_vio);
+		reuse_or_release_resources(pool, data_vio, &returned);
+	}
+
+	spin_lock(&pool->lock);
+	/*
+	 * There is a race where waiters could be added while we are in the unlocked section above.
+	 * Those waiters could not see the resources we are now about to release, so we assign
+	 * those resources now as we have no guarantee of being rescheduled. This is handled in
+	 * update_limiter().
+	 */
+	update_limiter(&pool->discard_limiter);
+	list_splice(&returned, &pool->available);
+	update_limiter(&pool->limiter);
+	to_wake = pool->limiter.wake_count;
+	pool->limiter.wake_count = 0;
+	discards_to_wake = pool->discard_limiter.wake_count;
+	pool->discard_limiter.wake_count = 0;
+
+	atomic_set(&pool->processing, false);
+	/* Pairs with the barrier in schedule_releases(). */
+	smp_mb();
+
+	reschedule = !vdo_is_funnel_queue_empty(pool->queue);
+	drained = (!reschedule &&
+		   vdo_is_state_draining(&pool->state) &&
+		   check_for_drain_complete_locked(pool));
+	spin_unlock(&pool->lock);
+
+	if (to_wake > 0)
+		wake_up_nr(&pool->limiter.blocked_threads, to_wake);
+
+	if (discards_to_wake > 0)
+		wake_up_nr(&pool->discard_limiter.blocked_threads, discards_to_wake);
+
+	if (reschedule)
+		schedule_releases(pool);
+	else if (drained)
+		vdo_finish_draining(&pool->state);
+}
+
+static void initialize_limiter(struct limiter *limiter, struct data_vio_pool *pool,
+			       assigner_fn assigner, data_vio_count_t limit)
+{
+	limiter->pool = pool;
+	limiter->assigner = assigner;
+	limiter->limit = limit;
+	limiter->arrival = U64_MAX;
+	init_waitqueue_head(&limiter->blocked_threads);
+}
+
+/**
+ * initialize_data_vio() - Allocate the components of a data_vio.
+ *
+ * The caller is responsible for cleaning up the data_vio on error.
+ *
+ * Return: VDO_SUCCESS or an error.
+ */
+static int initialize_data_vio(struct data_vio *data_vio, struct vdo *vdo)
+{
+	struct bio *bio;
+	int result;
+
+	BUILD_BUG_ON(VDO_BLOCK_SIZE > PAGE_SIZE);
+	result = vdo_allocate_memory(VDO_BLOCK_SIZE, 0, "data_vio data",
+				     &data_vio->vio.data);
+	if (result != VDO_SUCCESS)
+		return vdo_log_error_strerror(result,
+					      "data_vio data allocation failure");
+
+	result = vdo_allocate_memory(VDO_BLOCK_SIZE, 0, "compressed block",
+				     &data_vio->compression.block);
+	if (result != VDO_SUCCESS) {
+		return vdo_log_error_strerror(result,
+					      "data_vio compressed block allocation failure");
+	}
+
+	result = vdo_allocate_memory(VDO_BLOCK_SIZE, 0, "vio scratch",
+				     &data_vio->scratch_block);
+	if (result != VDO_SUCCESS)
+		return vdo_log_error_strerror(result,
+					      "data_vio scratch allocation failure");
+
+	result = vdo_create_bio(&bio);
+	if (result != VDO_SUCCESS)
+		return vdo_log_error_strerror(result,
+					      "data_vio data bio allocation failure");
+
+	vdo_initialize_completion(&data_vio->decrement_completion, vdo,
+				  VDO_DECREMENT_COMPLETION);
+	initialize_vio(&data_vio->vio, bio, 1, VIO_TYPE_DATA, VIO_PRIORITY_DATA, vdo);
+
+	return VDO_SUCCESS;
+}
+
+static void destroy_data_vio(struct data_vio *data_vio)
+{
+	if (data_vio == NULL)
+		return;
+
+	vdo_free_bio(vdo_forget(data_vio->vio.bio));
+	vdo_free(vdo_forget(data_vio->vio.data));
+	vdo_free(vdo_forget(data_vio->compression.block));
+	vdo_free(vdo_forget(data_vio->scratch_block));
+}
+
+/**
+ * make_data_vio_pool() - Initialize a data_vio pool.
+ * @vdo: The vdo to which the pool will belong.
+ * @pool_size: The number of data_vios in the pool.
+ * @discard_limit: The maximum number of data_vios which may be used for discards.
+ * @pool: A pointer to hold the newly allocated pool.
+ */
+int make_data_vio_pool(struct vdo *vdo, data_vio_count_t pool_size,
+		       data_vio_count_t discard_limit, struct data_vio_pool **pool_ptr)
+{
+	int result;
+	struct data_vio_pool *pool;
+	data_vio_count_t i;
+
+	result = vdo_allocate_extended(struct data_vio_pool, pool_size, struct data_vio,
+				       __func__, &pool);
+	if (result != VDO_SUCCESS)
+		return result;
+
+	VDO_ASSERT_LOG_ONLY((discard_limit <= pool_size),
+			    "discard limit does not exceed pool size");
+	initialize_limiter(&pool->discard_limiter, pool, assign_discard_permit,
+			   discard_limit);
+	pool->discard_limiter.permitted_waiters = &pool->permitted_discards;
+	initialize_limiter(&pool->limiter, pool, assign_data_vio_to_waiter, pool_size);
+	pool->limiter.permitted_waiters = &pool->limiter.waiters;
+	INIT_LIST_HEAD(&pool->available);
+	spin_lock_init(&pool->lock);
+	vdo_set_admin_state_code(&pool->state, VDO_ADMIN_STATE_NORMAL_OPERATION);
+	vdo_initialize_completion(&pool->completion, vdo, VDO_DATA_VIO_POOL_COMPLETION);
+	vdo_prepare_completion(&pool->completion, process_release_callback,
+			       process_release_callback, vdo->thread_config.cpu_thread,
+			       NULL);
+
+	result = vdo_make_funnel_queue(&pool->queue);
+	if (result != VDO_SUCCESS) {
+		free_data_vio_pool(vdo_forget(pool));
+		return result;
+	}
+
+	for (i = 0; i < pool_size; i++) {
+		struct data_vio *data_vio = &pool->data_vios[i];
+
+		result = initialize_data_vio(data_vio, vdo);
+		if (result != VDO_SUCCESS) {
+			destroy_data_vio(data_vio);
+			free_data_vio_pool(pool);
+			return result;
+		}
+
+		list_add(&data_vio->pool_entry, &pool->available);
+	}
+
+	*pool_ptr = pool;
+	return VDO_SUCCESS;
+}
+
+/**
+ * free_data_vio_pool() - Free a data_vio_pool and the data_vios in it.
+ *
+ * All data_vios must be returned to the pool before calling this function.
+ */
+void free_data_vio_pool(struct data_vio_pool *pool)
+{
+	struct data_vio *data_vio, *tmp;
+
+	if (pool == NULL)
+		return;
+
+	/*
+	 * Pairs with the barrier in process_release_callback(). Possibly not needed since it
+	 * caters to an enqueue vs. free race.
+	 */
+	smp_mb();
+	BUG_ON(atomic_read(&pool->processing));
+
+	spin_lock(&pool->lock);
+	VDO_ASSERT_LOG_ONLY((pool->limiter.busy == 0),
+			    "data_vio pool must not have %u busy entries when being freed",
+			    pool->limiter.busy);
+	VDO_ASSERT_LOG_ONLY((bio_list_empty(&pool->limiter.waiters) &&
+			     bio_list_empty(&pool->limiter.new_waiters)),
+			    "data_vio pool must not have threads waiting to read or write when being freed");
+	VDO_ASSERT_LOG_ONLY((bio_list_empty(&pool->discard_limiter.waiters) &&
+			     bio_list_empty(&pool->discard_limiter.new_waiters)),
+			    "data_vio pool must not have threads waiting to discard when being freed");
+	spin_unlock(&pool->lock);
+
+	list_for_each_entry_safe(data_vio, tmp, &pool->available, pool_entry) {
+		list_del_init(&data_vio->pool_entry);
+		destroy_data_vio(data_vio);
+	}
+
+	vdo_free_funnel_queue(vdo_forget(pool->queue));
+	vdo_free(pool);
+}
+
+static bool acquire_permit(struct limiter *limiter)
+{
+	if (limiter->busy >= limiter->limit)
+		return false;
+
+	WRITE_ONCE(limiter->busy, limiter->busy + 1);
+	if (limiter->max_busy < limiter->busy)
+		WRITE_ONCE(limiter->max_busy, limiter->busy);
+	return true;
+}
+
+static void wait_permit(struct limiter *limiter, struct bio *bio)
+	__releases(&limiter->pool->lock)
+{
+	DEFINE_WAIT(wait);
+
+	bio_list_add(&limiter->new_waiters, bio);
+	prepare_to_wait_exclusive(&limiter->blocked_threads, &wait,
+				  TASK_UNINTERRUPTIBLE);
+	spin_unlock(&limiter->pool->lock);
+	io_schedule();
+	finish_wait(&limiter->blocked_threads, &wait);
+}
+
+/**
+ * vdo_launch_bio() - Acquire a data_vio from the pool, assign the bio to it, and launch it.
+ *
+ * This will block if data_vios or discard permits are not available.
+ */
+void vdo_launch_bio(struct data_vio_pool *pool, struct bio *bio)
+{
+	struct data_vio *data_vio;
+
+	VDO_ASSERT_LOG_ONLY(!vdo_is_state_quiescent(&pool->state),
+			    "data_vio_pool not quiescent on acquire");
+
+	bio->bi_private = (void *) jiffies;
+	spin_lock(&pool->lock);
+	if ((bio_op(bio) == REQ_OP_DISCARD) &&
+	    !acquire_permit(&pool->discard_limiter)) {
+		wait_permit(&pool->discard_limiter, bio);
+		return;
+	}
+
+	if (!acquire_permit(&pool->limiter)) {
+		wait_permit(&pool->limiter, bio);
+		return;
+	}
+
+	data_vio = get_available_data_vio(pool);
+	spin_unlock(&pool->lock);
+	launch_bio(pool->completion.vdo, data_vio, bio);
+}
+
+/* Implements vdo_admin_initiator_fn. */
+static void initiate_drain(struct admin_state *state)
+{
+	bool drained;
+	struct data_vio_pool *pool = container_of(state, struct data_vio_pool, state);
+
+	spin_lock(&pool->lock);
+	drained = check_for_drain_complete_locked(pool);
+	spin_unlock(&pool->lock);
+
+	if (drained)
+		vdo_finish_draining(state);
+}
+
+static void assert_on_vdo_cpu_thread(const struct vdo *vdo, const char *name)
+{
+	VDO_ASSERT_LOG_ONLY((vdo_get_callback_thread_id() == vdo->thread_config.cpu_thread),
+			    "%s called on cpu thread", name);
+}
+
+/**
+ * drain_data_vio_pool() - Wait asynchronously for all data_vios to be returned to the pool.
+ * @completion: The completion to notify when the pool has drained.
+ */
+void drain_data_vio_pool(struct data_vio_pool *pool, struct vdo_completion *completion)
+{
+	assert_on_vdo_cpu_thread(completion->vdo, __func__);
+	vdo_start_draining(&pool->state, VDO_ADMIN_STATE_SUSPENDING, completion,
+			   initiate_drain);
+}
+
+/**
+ * resume_data_vio_pool() - Resume a data_vio pool.
+ * @completion: The completion to notify when the pool has resumed.
+ */
+void resume_data_vio_pool(struct data_vio_pool *pool, struct vdo_completion *completion)
+{
+	assert_on_vdo_cpu_thread(completion->vdo, __func__);
+	vdo_continue_completion(completion, vdo_resume_if_quiescent(&pool->state));
+}
+
+static void dump_limiter(const char *name, struct limiter *limiter)
+{
+	vdo_log_info("%s: %u of %u busy (max %u), %s", name, limiter->busy,
+		     limiter->limit, limiter->max_busy,
+		     ((bio_list_empty(&limiter->waiters) &&
+		       bio_list_empty(&limiter->new_waiters)) ?
+		      "no waiters" : "has waiters"));
+}
+
+/**
+ * dump_data_vio_pool() - Dump a data_vio pool to the log.
+ * @dump_vios: Whether to dump the details of each busy data_vio as well.
+ */
+void dump_data_vio_pool(struct data_vio_pool *pool, bool dump_vios)
+{
+	/*
+	 * In order that syslog can empty its buffer, sleep after 35 elements for 4ms (till the
+	 * second clock tick).  These numbers were picked based on experiments with lab machines.
+	 */
+	static const int ELEMENTS_PER_BATCH = 35;
+	static const int SLEEP_FOR_SYSLOG = 4000;
+
+	if (pool == NULL)
+		return;
+
+	spin_lock(&pool->lock);
+	dump_limiter("data_vios", &pool->limiter);
+	dump_limiter("discard permits", &pool->discard_limiter);
+	if (dump_vios) {
+		int i;
+		int dumped = 0;
+
+		for (i = 0; i < pool->limiter.limit; i++) {
+			struct data_vio *data_vio = &pool->data_vios[i];
+
+			if (!list_empty(&data_vio->pool_entry))
+				continue;
+
+			dump_data_vio(data_vio);
+			if (++dumped >= ELEMENTS_PER_BATCH) {
+				spin_unlock(&pool->lock);
+				dumped = 0;
+				fsleep(SLEEP_FOR_SYSLOG);
+				spin_lock(&pool->lock);
+			}
+		}
+	}
+
+	spin_unlock(&pool->lock);
+}
+
+data_vio_count_t get_data_vio_pool_active_discards(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->discard_limiter.busy);
+}
+
+data_vio_count_t get_data_vio_pool_discard_limit(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->discard_limiter.limit);
+}
+
+data_vio_count_t get_data_vio_pool_maximum_discards(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->discard_limiter.max_busy);
+}
+
+int set_data_vio_pool_discard_limit(struct data_vio_pool *pool, data_vio_count_t limit)
+{
+	if (get_data_vio_pool_request_limit(pool) < limit) {
+		// The discard limit may not be higher than the data_vio limit.
+		return -EINVAL;
+	}
+
+	spin_lock(&pool->lock);
+	pool->discard_limiter.limit = limit;
+	spin_unlock(&pool->lock);
+
+	return VDO_SUCCESS;
+}
+
+data_vio_count_t get_data_vio_pool_active_requests(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->limiter.busy);
+}
+
+data_vio_count_t get_data_vio_pool_request_limit(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->limiter.limit);
+}
+
+data_vio_count_t get_data_vio_pool_maximum_requests(struct data_vio_pool *pool)
+{
+	return READ_ONCE(pool->limiter.max_busy);
+}
+
+static void update_data_vio_error_stats(struct data_vio *data_vio)
+{
+	u8 index = 0;
+	static const char * const operations[] = {
+		[0] = "empty",
+		[1] = "read",
+		[2] = "write",
+		[3] = "read-modify-write",
+		[5] = "read+fua",
+		[6] = "write+fua",
+		[7] = "read-modify-write+fua",
+	};
+
+	if (data_vio->read)
+		index = 1;
+
+	if (data_vio->write)
+		index += 2;
+
+	if (data_vio->fua)
+		index += 4;
+
+	update_vio_error_stats(&data_vio->vio,
+			       "Completing %s vio for LBN %llu with error after %s",
+			       operations[index],
+			       (unsigned long long) data_vio->logical.lbn,
+			       get_data_vio_operation_name(data_vio));
+}
+
+static void perform_cleanup_stage(struct data_vio *data_vio,
+				  enum data_vio_cleanup_stage stage);
+
+/**
+ * release_allocated_lock() - Release the PBN lock and/or the reference on the allocated block at
+ *			      the end of processing a data_vio.
+ */
+static void release_allocated_lock(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_allocated_zone(data_vio);
+	release_data_vio_allocation_lock(data_vio, false);
+	perform_cleanup_stage(data_vio, VIO_RELEASE_RECOVERY_LOCKS);
+}
+
+/** release_lock() - Release an uncontended LBN lock. */
+static void release_lock(struct data_vio *data_vio, struct lbn_lock *lock)
+{
+	struct int_map *lock_map = lock->zone->lbn_operations;
+	struct data_vio *lock_holder;
+
+	if (!lock->locked) {
+		/*  The lock is not locked, so it had better not be registered in the lock map. */
+		struct data_vio *lock_holder = vdo_int_map_get(lock_map, lock->lbn);
+
+		VDO_ASSERT_LOG_ONLY((data_vio != lock_holder),
+				    "no logical block lock held for block %llu",
+				    (unsigned long long) lock->lbn);
+		return;
+	}
+
+	/* Release the lock by removing the lock from the map. */
+	lock_holder = vdo_int_map_remove(lock_map, lock->lbn);
+	VDO_ASSERT_LOG_ONLY((data_vio == lock_holder),
+			    "logical block lock mismatch for block %llu",
+			    (unsigned long long) lock->lbn);
+	lock->locked = false;
+}
+
+/** transfer_lock() - Transfer a contended LBN lock to the eldest waiter. */
+static void transfer_lock(struct data_vio *data_vio, struct lbn_lock *lock)
+{
+	struct data_vio *lock_holder, *next_lock_holder;
+	int result;
+
+	VDO_ASSERT_LOG_ONLY(lock->locked, "lbn_lock with waiters is not locked");
+
+	/* Another data_vio is waiting for the lock, transfer it in a single lock map operation. */
+	next_lock_holder =
+		vdo_waiter_as_data_vio(vdo_waitq_dequeue_waiter(&lock->waiters));
+
+	/* Transfer the remaining lock waiters to the next lock holder. */
+	vdo_waitq_transfer_all_waiters(&lock->waiters,
+				       &next_lock_holder->logical.waiters);
+
+	result = vdo_int_map_put(lock->zone->lbn_operations, lock->lbn,
+				 next_lock_holder, true, (void **) &lock_holder);
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(next_lock_holder, result);
+		return;
+	}
+
+	VDO_ASSERT_LOG_ONLY((lock_holder == data_vio),
+			    "logical block lock mismatch for block %llu",
+			    (unsigned long long) lock->lbn);
+	lock->locked = false;
+
+	/*
+	 * If there are still waiters, other data_vios must be trying to get the lock we just
+	 * transferred. We must ensure that the new lock holder doesn't block in the packer.
+	 */
+	if (vdo_waitq_has_waiters(&next_lock_holder->logical.waiters))
+		cancel_data_vio_compression(next_lock_holder);
+
+	/*
+	 * Avoid stack overflow on lock transfer.
+	 * FIXME: this is only an issue in the 1 thread config.
+	 */
+	next_lock_holder->vio.completion.requeue = true;
+	launch_locked_request(next_lock_holder);
+}
+
+/**
+ * release_logical_lock() - Release the logical block lock and flush generation lock at the end of
+ *			    processing a data_vio.
+ */
+static void release_logical_lock(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	struct lbn_lock *lock = &data_vio->logical;
+
+	assert_data_vio_in_logical_zone(data_vio);
+
+	if (vdo_waitq_has_waiters(&lock->waiters))
+		transfer_lock(data_vio, lock);
+	else
+		release_lock(data_vio, lock);
+
+	vdo_release_flush_generation_lock(data_vio);
+	perform_cleanup_stage(data_vio, VIO_CLEANUP_DONE);
+}
+
+/** clean_hash_lock() - Release the hash lock at the end of processing a data_vio. */
+static void clean_hash_lock(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_hash_zone(data_vio);
+	if (completion->result != VDO_SUCCESS) {
+		vdo_clean_failed_hash_lock(data_vio);
+		return;
+	}
+
+	vdo_release_hash_lock(data_vio);
+	perform_cleanup_stage(data_vio, VIO_RELEASE_LOGICAL);
+}
+
+/**
+ * finish_cleanup() - Make some assertions about a data_vio which has finished cleaning up.
+ *
+ * If it is part of a multi-block discard, starts on the next block, otherwise, returns it to the
+ * pool.
+ */
+static void finish_cleanup(struct data_vio *data_vio)
+{
+	struct vdo_completion *completion = &data_vio->vio.completion;
+
+	VDO_ASSERT_LOG_ONLY(data_vio->allocation.lock == NULL,
+			    "complete data_vio has no allocation lock");
+	VDO_ASSERT_LOG_ONLY(data_vio->hash_lock == NULL,
+			    "complete data_vio has no hash lock");
+	if ((data_vio->remaining_discard <= VDO_BLOCK_SIZE) ||
+	    (completion->result != VDO_SUCCESS)) {
+		struct data_vio_pool *pool = completion->vdo->data_vio_pool;
+
+		vdo_funnel_queue_put(pool->queue, &completion->work_queue_entry_link);
+		schedule_releases(pool);
+		return;
+	}
+
+	data_vio->remaining_discard -= min_t(u32, data_vio->remaining_discard,
+					     VDO_BLOCK_SIZE - data_vio->offset);
+	data_vio->is_partial = (data_vio->remaining_discard < VDO_BLOCK_SIZE);
+	data_vio->read = data_vio->is_partial;
+	data_vio->offset = 0;
+	completion->requeue = true;
+	launch_data_vio(data_vio, data_vio->logical.lbn + 1);
+}
+
+/** perform_cleanup_stage() - Perform the next step in the process of cleaning up a data_vio. */
+static void perform_cleanup_stage(struct data_vio *data_vio,
+				  enum data_vio_cleanup_stage stage)
+{
+	struct vdo *vdo = vdo_from_data_vio(data_vio);
+
+	switch (stage) {
+	case VIO_RELEASE_HASH_LOCK:
+		if (data_vio->hash_lock != NULL) {
+			launch_data_vio_hash_zone_callback(data_vio, clean_hash_lock);
+			return;
+		}
+		fallthrough;
+
+	case VIO_RELEASE_ALLOCATED:
+		if (data_vio_has_allocation(data_vio)) {
+			launch_data_vio_allocated_zone_callback(data_vio,
+								release_allocated_lock);
+			return;
+		}
+		fallthrough;
+
+	case VIO_RELEASE_RECOVERY_LOCKS:
+		if ((data_vio->recovery_sequence_number > 0) &&
+		    (READ_ONCE(vdo->read_only_notifier.read_only_error) == VDO_SUCCESS) &&
+		    (data_vio->vio.completion.result != VDO_READ_ONLY))
+			vdo_log_warning("VDO not read-only when cleaning data_vio with RJ lock");
+		fallthrough;
+
+	case VIO_RELEASE_LOGICAL:
+		launch_data_vio_logical_callback(data_vio, release_logical_lock);
+		return;
+
+	default:
+		finish_cleanup(data_vio);
+	}
+}
+
+void complete_data_vio(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	completion->error_handler = NULL;
+	data_vio->last_async_operation = VIO_ASYNC_OP_CLEANUP;
+	perform_cleanup_stage(data_vio,
+			      (data_vio->write ? VIO_CLEANUP_START : VIO_RELEASE_LOGICAL));
+}
+
+static void enter_read_only_mode(struct vdo_completion *completion)
+{
+	if (vdo_is_read_only(completion->vdo))
+		return;
+
+	if (completion->result != VDO_READ_ONLY) {
+		struct data_vio *data_vio = as_data_vio(completion);
+
+		vdo_log_error_strerror(completion->result,
+				       "Preparing to enter read-only mode: data_vio for LBN %llu (becoming mapped to %llu, previously mapped to %llu, allocated %llu) is completing with a fatal error after operation %s",
+				       (unsigned long long) data_vio->logical.lbn,
+				       (unsigned long long) data_vio->new_mapped.pbn,
+				       (unsigned long long) data_vio->mapped.pbn,
+				       (unsigned long long) data_vio->allocation.pbn,
+				       get_data_vio_operation_name(data_vio));
+	}
+
+	vdo_enter_read_only_mode(completion->vdo, completion->result);
+}
+
+void handle_data_vio_error(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	if ((completion->result == VDO_READ_ONLY) || (data_vio->user_bio == NULL))
+		enter_read_only_mode(completion);
+
+	update_data_vio_error_stats(data_vio);
+	complete_data_vio(completion);
+}
+
+/**
+ * get_data_vio_operation_name() - Get the name of the last asynchronous operation performed on a
+ *				   data_vio.
+ */
+const char *get_data_vio_operation_name(struct data_vio *data_vio)
+{
+	BUILD_BUG_ON((MAX_VIO_ASYNC_OPERATION_NUMBER - MIN_VIO_ASYNC_OPERATION_NUMBER) !=
+		     ARRAY_SIZE(ASYNC_OPERATION_NAMES));
+
+	return ((data_vio->last_async_operation < MAX_VIO_ASYNC_OPERATION_NUMBER) ?
+		ASYNC_OPERATION_NAMES[data_vio->last_async_operation] :
+		"unknown async operation");
+}
+
+/**
+ * data_vio_allocate_data_block() - Allocate a data block.
+ *
+ * @write_lock_type: The type of write lock to obtain on the block.
+ * @callback: The callback which will attempt an allocation in the current zone and continue if it
+ *	      succeeds.
+ * @error_handler: The handler for errors while allocating.
+ */
+void data_vio_allocate_data_block(struct data_vio *data_vio,
+				  enum pbn_lock_type write_lock_type,
+				  vdo_action_fn callback, vdo_action_fn error_handler)
+{
+	struct allocation *allocation = &data_vio->allocation;
+
+	VDO_ASSERT_LOG_ONLY((allocation->pbn == VDO_ZERO_BLOCK),
+			    "data_vio does not have an allocation");
+	allocation->write_lock_type = write_lock_type;
+	allocation->zone = vdo_get_next_allocation_zone(data_vio->logical.zone);
+	allocation->first_allocation_zone = allocation->zone->zone_number;
+
+	data_vio->vio.completion.error_handler = error_handler;
+	launch_data_vio_allocated_zone_callback(data_vio, callback);
+}
+
+/**
+ * release_data_vio_allocation_lock() - Release the PBN lock on a data_vio's allocated block.
+ * @reset: If true, the allocation will be reset (i.e. any allocated pbn will be forgotten).
+ *
+ * If the reference to the locked block is still provisional, it will be released as well.
+ */
+void release_data_vio_allocation_lock(struct data_vio *data_vio, bool reset)
+{
+	struct allocation *allocation = &data_vio->allocation;
+	physical_block_number_t locked_pbn = allocation->pbn;
+
+	assert_data_vio_in_allocated_zone(data_vio);
+
+	if (reset || vdo_pbn_lock_has_provisional_reference(allocation->lock))
+		allocation->pbn = VDO_ZERO_BLOCK;
+
+	vdo_release_physical_zone_pbn_lock(allocation->zone, locked_pbn,
+					   vdo_forget(allocation->lock));
+}
+
+/**
+ * uncompress_data_vio() - Uncompress the data a data_vio has just read.
+ * @mapping_state: The mapping state indicating which fragment to decompress.
+ * @buffer: The buffer to receive the uncompressed data.
+ */
+int uncompress_data_vio(struct data_vio *data_vio,
+			enum block_mapping_state mapping_state, char *buffer)
+{
+	int size;
+	u16 fragment_offset, fragment_size;
+	struct compressed_block *block = data_vio->compression.block;
+	int result = vdo_get_compressed_block_fragment(mapping_state, block,
+						       &fragment_offset, &fragment_size);
+
+	if (result != VDO_SUCCESS) {
+		vdo_log_debug("%s: compressed fragment error %d", __func__, result);
+		return result;
+	}
+
+	size = LZ4_decompress_safe((block->data + fragment_offset), buffer,
+				   fragment_size, VDO_BLOCK_SIZE);
+	if (size != VDO_BLOCK_SIZE) {
+		vdo_log_debug("%s: lz4 error", __func__);
+		return VDO_INVALID_FRAGMENT;
+	}
+
+	return VDO_SUCCESS;
+}
+
+/**
+ * modify_for_partial_write() - Do the modify-write part of a read-modify-write cycle.
+ * @completion: The data_vio which has just finished its read.
+ *
+ * This callback is registered in read_block().
+ */
+static void modify_for_partial_write(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	char *data = data_vio->vio.data;
+	struct bio *bio = data_vio->user_bio;
+
+	assert_data_vio_on_cpu_thread(data_vio);
+
+	if (bio_op(bio) == REQ_OP_DISCARD) {
+		memset(data + data_vio->offset, '\0', min_t(u32,
+							    data_vio->remaining_discard,
+							    VDO_BLOCK_SIZE - data_vio->offset));
+	} else {
+		copy_from_bio(bio, data + data_vio->offset);
+	}
+
+	data_vio->is_zero = is_zero_block(data);
+	data_vio->read = false;
+	launch_data_vio_logical_callback(data_vio,
+					 continue_data_vio_with_block_map_slot);
+}
+
+static void complete_read(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	char *data = data_vio->vio.data;
+	bool compressed = vdo_is_state_compressed(data_vio->mapped.state);
+
+	assert_data_vio_on_cpu_thread(data_vio);
+
+	if (compressed) {
+		int result = uncompress_data_vio(data_vio, data_vio->mapped.state, data);
+
+		if (result != VDO_SUCCESS) {
+			continue_data_vio_with_error(data_vio, result);
+			return;
+		}
+	}
+
+	if (data_vio->write) {
+		modify_for_partial_write(completion);
+		return;
+	}
+
+	if (compressed || data_vio->is_partial)
+		copy_to_bio(data_vio->user_bio, data + data_vio->offset);
+
+	acknowledge_data_vio(data_vio);
+	complete_data_vio(completion);
+}
+
+static void read_endio(struct bio *bio)
+{
+	struct data_vio *data_vio = vio_as_data_vio(bio->bi_private);
+	int result = blk_status_to_errno(bio->bi_status);
+
+	vdo_count_completed_bios(bio);
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(data_vio, result);
+		return;
+	}
+
+	launch_data_vio_cpu_callback(data_vio, complete_read,
+				     CPU_Q_COMPLETE_READ_PRIORITY);
+}
+
+static void complete_zero_read(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_on_cpu_thread(data_vio);
+
+	if (data_vio->is_partial) {
+		memset(data_vio->vio.data, 0, VDO_BLOCK_SIZE);
+		if (data_vio->write) {
+			modify_for_partial_write(completion);
+			return;
+		}
+	} else {
+		zero_fill_bio(data_vio->user_bio);
+	}
+
+	complete_read(completion);
+}
+
+/**
+ * read_block() - Read a block asynchronously.
+ *
+ * This is the callback registered in read_block_mapping().
+ */
+static void read_block(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	struct vio *vio = as_vio(completion);
+	int result = VDO_SUCCESS;
+
+	if (data_vio->mapped.pbn == VDO_ZERO_BLOCK) {
+		launch_data_vio_cpu_callback(data_vio, complete_zero_read,
+					     CPU_Q_COMPLETE_VIO_PRIORITY);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_READ_DATA_VIO;
+	if (vdo_is_state_compressed(data_vio->mapped.state)) {
+		result = vio_reset_bio(vio, (char *) data_vio->compression.block,
+				       read_endio, REQ_OP_READ, data_vio->mapped.pbn);
+	} else {
+		blk_opf_t opf = ((data_vio->user_bio->bi_opf & PASSTHROUGH_FLAGS) | REQ_OP_READ);
+
+		if (data_vio->is_partial) {
+			result = vio_reset_bio(vio, vio->data, read_endio, opf,
+					       data_vio->mapped.pbn);
+		} else {
+			/* A full 4k read. Use the incoming bio to avoid having to copy the data */
+			bio_reset(vio->bio, vio->bio->bi_bdev, opf);
+			bio_init_clone(data_vio->user_bio->bi_bdev, vio->bio,
+				       data_vio->user_bio, GFP_KERNEL);
+
+			/* Copy over the original bio iovec and opflags. */
+			vdo_set_bio_properties(vio->bio, vio, read_endio, opf,
+					       data_vio->mapped.pbn);
+		}
+	}
+
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(data_vio, result);
+		return;
+	}
+
+	vdo_submit_data_vio(data_vio);
+}
+
+static inline struct data_vio *
+reference_count_update_completion_as_data_vio(struct vdo_completion *completion)
+{
+	if (completion->type == VIO_COMPLETION)
+		return as_data_vio(completion);
+
+	return container_of(completion, struct data_vio, decrement_completion);
+}
+
+/**
+ * update_block_map() - Rendezvous of the data_vio and decrement completions after each has
+ *                      made its reference updates. Handle any error from either, or proceed
+ *                      to updating the block map.
+ * @completion: The completion of the write in progress.
+ */
+static void update_block_map(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = reference_count_update_completion_as_data_vio(completion);
+
+	assert_data_vio_in_logical_zone(data_vio);
+
+	if (!data_vio->first_reference_operation_complete) {
+		/* Rendezvous, we're first */
+		data_vio->first_reference_operation_complete = true;
+		return;
+	}
+
+	completion = &data_vio->vio.completion;
+	vdo_set_completion_result(completion, data_vio->decrement_completion.result);
+	if (completion->result != VDO_SUCCESS) {
+		handle_data_vio_error(completion);
+		return;
+	}
+
+	completion->error_handler = handle_data_vio_error;
+	if (data_vio->hash_lock != NULL)
+		set_data_vio_hash_zone_callback(data_vio, vdo_continue_hash_lock);
+	else
+		completion->callback = complete_data_vio;
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_PUT_MAPPED_BLOCK;
+	vdo_put_mapped_block(data_vio);
+}
+
+static void decrement_reference_count(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = container_of(completion, struct data_vio,
+						 decrement_completion);
+
+	assert_data_vio_in_mapped_zone(data_vio);
+
+	vdo_set_completion_callback(completion, update_block_map,
+				    data_vio->logical.zone->thread_id);
+	completion->error_handler = update_block_map;
+	vdo_modify_reference_count(completion, &data_vio->decrement_updater);
+}
+
+static void increment_reference_count(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_new_mapped_zone(data_vio);
+
+	if (data_vio->downgrade_allocation_lock) {
+		/*
+		 * Now that the data has been written, it's safe to deduplicate against the
+		 * block. Downgrade the allocation lock to a read lock so it can be used later by
+		 * the hash lock. This is done here since it needs to happen sometime before we
+		 * return to the hash zone, and we are currently on the correct thread. For
+		 * compressed blocks, the downgrade will have already been done.
+		 */
+		vdo_downgrade_pbn_write_lock(data_vio->allocation.lock, false);
+	}
+
+	set_data_vio_logical_callback(data_vio, update_block_map);
+	completion->error_handler = update_block_map;
+	vdo_modify_reference_count(completion, &data_vio->increment_updater);
+}
+
+/** journal_remapping() - Add a recovery journal entry for a data remapping. */
+static void journal_remapping(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_journal_zone(data_vio);
+
+	data_vio->decrement_updater.operation = VDO_JOURNAL_DATA_REMAPPING;
+	data_vio->decrement_updater.zpbn = data_vio->mapped;
+	if (data_vio->new_mapped.pbn == VDO_ZERO_BLOCK) {
+		data_vio->first_reference_operation_complete = true;
+		if (data_vio->mapped.pbn == VDO_ZERO_BLOCK)
+			set_data_vio_logical_callback(data_vio, update_block_map);
+	} else {
+		set_data_vio_new_mapped_zone_callback(data_vio,
+						      increment_reference_count);
+	}
+
+	if (data_vio->mapped.pbn == VDO_ZERO_BLOCK) {
+		data_vio->first_reference_operation_complete = true;
+	} else {
+		vdo_set_completion_callback(&data_vio->decrement_completion,
+					    decrement_reference_count,
+					    data_vio->mapped.zone->thread_id);
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_JOURNAL_REMAPPING;
+	vdo_add_recovery_journal_entry(completion->vdo->recovery_journal, data_vio);
+}
+
+/**
+ * read_old_block_mapping() - Get the previous PBN/LBN mapping of an in-progress write.
+ *
+ * Gets the previous PBN mapped to this LBN from the block map, so as to make an appropriate
+ * journal entry referencing the removal of this LBN->PBN mapping.
+ */
+static void read_old_block_mapping(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_logical_zone(data_vio);
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_GET_MAPPED_BLOCK_FOR_WRITE;
+	set_data_vio_journal_callback(data_vio, journal_remapping);
+	vdo_get_mapped_block(data_vio);
+}
+
+void update_metadata_for_data_vio_write(struct data_vio *data_vio, struct pbn_lock *lock)
+{
+	data_vio->increment_updater = (struct reference_updater) {
+		.operation = VDO_JOURNAL_DATA_REMAPPING,
+		.increment = true,
+		.zpbn = data_vio->new_mapped,
+		.lock = lock,
+	};
+
+	launch_data_vio_logical_callback(data_vio, read_old_block_mapping);
+}
+
+/**
+ * pack_compressed_data() - Attempt to pack the compressed data_vio into a block.
+ *
+ * This is the callback registered in launch_compress_data_vio().
+ */
+static void pack_compressed_data(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_packer_zone(data_vio);
+
+	if (!vdo_get_compressing(vdo_from_data_vio(data_vio)) ||
+	    get_data_vio_compression_status(data_vio).may_not_compress) {
+		write_data_vio(data_vio);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_ATTEMPT_PACKING;
+	vdo_attempt_packing(data_vio);
+}
+
+/**
+ * compress_data_vio() - Do the actual work of compressing the data on a CPU queue.
+ *
+ * This callback is registered in launch_compress_data_vio().
+ */
+static void compress_data_vio(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	int size;
+
+	assert_data_vio_on_cpu_thread(data_vio);
+
+	/*
+	 * By putting the compressed data at the start of the compressed block data field, we won't
+	 * need to copy it if this data_vio becomes a compressed write agent.
+	 */
+	size = LZ4_compress_default(data_vio->vio.data,
+				    data_vio->compression.block->data, VDO_BLOCK_SIZE,
+				    VDO_MAX_COMPRESSED_FRAGMENT_SIZE,
+				    (char *) vdo_get_work_queue_private_data());
+	if ((size > 0) && (size < VDO_COMPRESSED_BLOCK_DATA_SIZE)) {
+		data_vio->compression.size = size;
+		launch_data_vio_packer_callback(data_vio, pack_compressed_data);
+		return;
+	}
+
+	write_data_vio(data_vio);
+}
+
+/**
+ * launch_compress_data_vio() - Continue a write by attempting to compress the data.
+ *
+ * This is a re-entry point to vio_write used by hash locks.
+ */
+void launch_compress_data_vio(struct data_vio *data_vio)
+{
+	VDO_ASSERT_LOG_ONLY(!data_vio->is_duplicate, "compressing a non-duplicate block");
+	VDO_ASSERT_LOG_ONLY(data_vio->hash_lock != NULL,
+			    "data_vio to compress has a hash_lock");
+	VDO_ASSERT_LOG_ONLY(data_vio_has_allocation(data_vio),
+			    "data_vio to compress has an allocation");
+
+	/*
+	 * There are 4 reasons why a data_vio which has reached this point will not be eligible for
+	 * compression:
+	 *
+	 * 1) Since data_vios can block indefinitely in the packer, it would be bad to do so if the
+	 * write request also requests FUA.
+	 *
+	 * 2) A data_vio should not be compressed when compression is disabled for the vdo.
+	 *
+	 * 3) A data_vio could be doing a partial write on behalf of a larger discard which has not
+	 * yet been acknowledged and hence blocking in the packer would be bad.
+	 *
+	 * 4) Some other data_vio may be waiting on this data_vio in which case blocking in the
+	 * packer would also be bad.
+	 */
+	if (data_vio->fua ||
+	    !vdo_get_compressing(vdo_from_data_vio(data_vio)) ||
+	    ((data_vio->user_bio != NULL) && (bio_op(data_vio->user_bio) == REQ_OP_DISCARD)) ||
+	    (advance_data_vio_compression_stage(data_vio).stage != DATA_VIO_COMPRESSING)) {
+		write_data_vio(data_vio);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_COMPRESS_DATA_VIO;
+	launch_data_vio_cpu_callback(data_vio, compress_data_vio,
+				     CPU_Q_COMPRESS_BLOCK_PRIORITY);
+}
+
+/**
+ * hash_data_vio() - Hash the data in a data_vio and set the hash zone (which also flags the record
+ *		     name as set).
+
+ * This callback is registered in prepare_for_dedupe().
+ */
+static void hash_data_vio(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_on_cpu_thread(data_vio);
+	VDO_ASSERT_LOG_ONLY(!data_vio->is_zero, "zero blocks should not be hashed");
+
+	murmurhash3_128(data_vio->vio.data, VDO_BLOCK_SIZE, 0x62ea60be,
+			&data_vio->record_name);
+
+	data_vio->hash_zone = vdo_select_hash_zone(vdo_from_data_vio(data_vio)->hash_zones,
+						   &data_vio->record_name);
+	data_vio->last_async_operation = VIO_ASYNC_OP_ACQUIRE_VDO_HASH_LOCK;
+	launch_data_vio_hash_zone_callback(data_vio, vdo_acquire_hash_lock);
+}
+
+/** prepare_for_dedupe() - Prepare for the dedupe path after attempting to get an allocation. */
+static void prepare_for_dedupe(struct data_vio *data_vio)
+{
+	/* We don't care what thread we are on. */
+	VDO_ASSERT_LOG_ONLY(!data_vio->is_zero, "must not prepare to dedupe zero blocks");
+
+	/*
+	 * Before we can dedupe, we need to know the record name, so the first
+	 * step is to hash the block data.
+	 */
+	data_vio->last_async_operation = VIO_ASYNC_OP_HASH_DATA_VIO;
+	launch_data_vio_cpu_callback(data_vio, hash_data_vio, CPU_Q_HASH_BLOCK_PRIORITY);
+}
+
+/**
+ * write_bio_finished() - This is the bio_end_io function registered in write_block() to be called
+ *			  when a data_vio's write to the underlying storage has completed.
+ */
+static void write_bio_finished(struct bio *bio)
+{
+	struct data_vio *data_vio = vio_as_data_vio((struct vio *) bio->bi_private);
+
+	vdo_count_completed_bios(bio);
+	vdo_set_completion_result(&data_vio->vio.completion,
+				  blk_status_to_errno(bio->bi_status));
+	data_vio->downgrade_allocation_lock = true;
+	update_metadata_for_data_vio_write(data_vio, data_vio->allocation.lock);
+}
+
+/** write_data_vio() - Write a data block to storage without compression. */
+void write_data_vio(struct data_vio *data_vio)
+{
+	struct data_vio_compression_status status, new_status;
+	int result;
+
+	if (!data_vio_has_allocation(data_vio)) {
+		/*
+		 * There was no space to write this block and we failed to deduplicate or compress
+		 * it.
+		 */
+		continue_data_vio_with_error(data_vio, VDO_NO_SPACE);
+		return;
+	}
+
+	new_status = (struct data_vio_compression_status) {
+		.stage = DATA_VIO_POST_PACKER,
+		.may_not_compress = true,
+	};
+
+	do {
+		status = get_data_vio_compression_status(data_vio);
+	} while ((status.stage != DATA_VIO_POST_PACKER) &&
+		 !set_data_vio_compression_status(data_vio, status, new_status));
+
+	/* Write the data from the data block buffer. */
+	result = vio_reset_bio(&data_vio->vio, data_vio->vio.data,
+			       write_bio_finished, REQ_OP_WRITE,
+			       data_vio->allocation.pbn);
+	if (result != VDO_SUCCESS) {
+		continue_data_vio_with_error(data_vio, result);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_WRITE_DATA_VIO;
+	vdo_submit_data_vio(data_vio);
+}
+
+/**
+ * acknowledge_write_callback() - Acknowledge a write to the requestor.
+ *
+ * This callback is registered in allocate_block() and continue_write_with_block_map_slot().
+ */
+static void acknowledge_write_callback(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+	struct vdo *vdo = completion->vdo;
+
+	VDO_ASSERT_LOG_ONLY((!vdo_uses_bio_ack_queue(vdo) ||
+			     (vdo_get_callback_thread_id() == vdo->thread_config.bio_ack_thread)),
+			    "%s() called on bio ack queue", __func__);
+	VDO_ASSERT_LOG_ONLY(data_vio_has_flush_generation_lock(data_vio),
+			    "write VIO to be acknowledged has a flush generation lock");
+	acknowledge_data_vio(data_vio);
+	if (data_vio->new_mapped.pbn == VDO_ZERO_BLOCK) {
+		/* This is a zero write or discard */
+		update_metadata_for_data_vio_write(data_vio, NULL);
+		return;
+	}
+
+	prepare_for_dedupe(data_vio);
+}
+
+/**
+ * allocate_block() - Attempt to allocate a block in the current allocation zone.
+ *
+ * This callback is registered in continue_write_with_block_map_slot().
+ */
+static void allocate_block(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_allocated_zone(data_vio);
+
+	if (!vdo_allocate_block_in_zone(data_vio))
+		return;
+
+	completion->error_handler = handle_data_vio_error;
+	WRITE_ONCE(data_vio->allocation_succeeded, true);
+	data_vio->new_mapped = (struct zoned_pbn) {
+		.zone = data_vio->allocation.zone,
+		.pbn = data_vio->allocation.pbn,
+		.state = VDO_MAPPING_STATE_UNCOMPRESSED,
+	};
+
+	if (data_vio->fua) {
+		prepare_for_dedupe(data_vio);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_ACKNOWLEDGE_WRITE;
+	launch_data_vio_on_bio_ack_queue(data_vio, acknowledge_write_callback);
+}
+
+/**
+ * handle_allocation_error() - Handle an error attempting to allocate a block.
+ *
+ * This error handler is registered in continue_write_with_block_map_slot().
+ */
+static void handle_allocation_error(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	if (completion->result == VDO_NO_SPACE) {
+		/* We failed to get an allocation, but we can try to dedupe. */
+		vdo_reset_completion(completion);
+		completion->error_handler = handle_data_vio_error;
+		prepare_for_dedupe(data_vio);
+		return;
+	}
+
+	/* We got a "real" error, not just a failure to allocate, so fail the request. */
+	handle_data_vio_error(completion);
+}
+
+static int assert_is_discard(struct data_vio *data_vio)
+{
+	int result = VDO_ASSERT(data_vio->is_discard,
+				"data_vio with no block map page is a discard");
+
+	return ((result == VDO_SUCCESS) ? result : VDO_READ_ONLY);
+}
+
+/**
+ * continue_data_vio_with_block_map_slot() - Read the data_vio's mapping from the block map.
+ *
+ * This callback is registered in launch_read_data_vio().
+ */
+void continue_data_vio_with_block_map_slot(struct vdo_completion *completion)
+{
+	struct data_vio *data_vio = as_data_vio(completion);
+
+	assert_data_vio_in_logical_zone(data_vio);
+	if (data_vio->read) {
+		set_data_vio_logical_callback(data_vio, read_block);
+		data_vio->last_async_operation = VIO_ASYNC_OP_GET_MAPPED_BLOCK_FOR_READ;
+		vdo_get_mapped_block(data_vio);
+		return;
+	}
+
+	vdo_acquire_flush_generation_lock(data_vio);
+
+	if (data_vio->tree_lock.tree_slots[0].block_map_slot.pbn == VDO_ZERO_BLOCK) {
+		/*
+		 * This is a discard for a block on a block map page which has not been allocated, so
+		 * there's nothing more we need to do.
+		 */
+		completion->callback = complete_data_vio;
+		continue_data_vio_with_error(data_vio, assert_is_discard(data_vio));
+		return;
+	}
+
+	/*
+	 * We need an allocation if this is neither a full-block discard nor a
+	 * full-block zero write.
+	 */
+	if (!data_vio->is_zero && (!data_vio->is_discard || data_vio->is_partial)) {
+		data_vio_allocate_data_block(data_vio, VIO_WRITE_LOCK, allocate_block,
+					     handle_allocation_error);
+		return;
+	}
+
+
+	/*
+	 * We don't need to write any data, so skip allocation and just update the block map and
+	 * reference counts (via the journal).
+	 */
+	data_vio->new_mapped.pbn = VDO_ZERO_BLOCK;
+	if (data_vio->is_zero)
+		data_vio->new_mapped.state = VDO_MAPPING_STATE_UNCOMPRESSED;
+
+	if (data_vio->remaining_discard > VDO_BLOCK_SIZE) {
+		/* This is not the final block of a discard so we can't acknowledge it yet. */
+		update_metadata_for_data_vio_write(data_vio, NULL);
+		return;
+	}
+
+	data_vio->last_async_operation = VIO_ASYNC_OP_ACKNOWLEDGE_WRITE;
+	launch_data_vio_on_bio_ack_queue(data_vio, acknowledge_write_callback);
+}