Adding upstream version 6.1.76.upstream/6.1.76upstream

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

1 files changed, 1463 insertions, 0 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
new file mode 100644
index 000000000..a5cf7f1e8
--- /dev/null
+++ b/drivers/block/xen-blkback/blkback.c
@@ -0,0 +1,1463 @@
+/******************************************************************************
+ *
+ * Back-end of the driver for virtual block devices. This portion of the
+ * driver exports a 'unified' block-device interface that can be accessed
+ * by any operating system that implements a compatible front end. A
+ * reference front-end implementation can be found in:
+ *  drivers/block/xen-blkfront.c
+ *
+ * Copyright (c) 2003-2004, Keir Fraser & Steve Hand
+ * Copyright (c) 2005, Christopher Clark
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation; or, when distributed
+ * separately from the Linux kernel or incorporated into other
+ * software packages, subject to the following license:
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this source file (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy, modify,
+ * merge, publish, distribute, sublicense, and/or sell copies of the Software,
+ * and to permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#define pr_fmt(fmt) "xen-blkback: " fmt
+
+#include <linux/spinlock.h>
+#include <linux/kthread.h>
+#include <linux/list.h>
+#include <linux/delay.h>
+#include <linux/freezer.h>
+#include <linux/bitmap.h>
+
+#include <xen/events.h>
+#include <xen/page.h>
+#include <xen/xen.h>
+#include <asm/xen/hypervisor.h>
+#include <asm/xen/hypercall.h>
+#include <xen/balloon.h>
+#include <xen/grant_table.h>
+#include "common.h"
+
+/*
+ * Maximum number of unused free pages to keep in the internal buffer.
+ * Setting this to a value too low will reduce memory used in each backend,
+ * but can have a performance penalty.
+ *
+ * A sane value is xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST, but can
+ * be set to a lower value that might degrade performance on some intensive
+ * IO workloads.
+ */
+
+static int max_buffer_pages = 1024;
+module_param_named(max_buffer_pages, max_buffer_pages, int, 0644);
+MODULE_PARM_DESC(max_buffer_pages,
+"Maximum number of free pages to keep in each block backend buffer");
+
+/*
+ * Maximum number of grants to map persistently in blkback. For maximum
+ * performance this should be the total numbers of grants that can be used
+ * to fill the ring, but since this might become too high, specially with
+ * the use of indirect descriptors, we set it to a value that provides good
+ * performance without using too much memory.
+ *
+ * When the list of persistent grants is full we clean it up using a LRU
+ * algorithm.
+ */
+
+static int max_pgrants = 1056;
+module_param_named(max_persistent_grants, max_pgrants, int, 0644);
+MODULE_PARM_DESC(max_persistent_grants,
+                 "Maximum number of grants to map persistently");
+
+/*
+ * How long a persistent grant is allowed to remain allocated without being in
+ * use. The time is in seconds, 0 means indefinitely long.
+ */
+
+static unsigned int pgrant_timeout = 60;
+module_param_named(persistent_grant_unused_seconds, pgrant_timeout,
+		   uint, 0644);
+MODULE_PARM_DESC(persistent_grant_unused_seconds,
+		 "Time in seconds an unused persistent grant is allowed to "
+		 "remain allocated. Default is 60, 0 means unlimited.");
+
+/*
+ * Maximum number of rings/queues blkback supports, allow as many queues as there
+ * are CPUs if user has not specified a value.
+ */
+unsigned int xenblk_max_queues;
+module_param_named(max_queues, xenblk_max_queues, uint, 0644);
+MODULE_PARM_DESC(max_queues,
+		 "Maximum number of hardware queues per virtual disk." \
+		 "By default it is the number of online CPUs.");
+
+/*
+ * Maximum order of pages to be used for the shared ring between front and
+ * backend, 4KB page granularity is used.
+ */
+unsigned int xen_blkif_max_ring_order = XENBUS_MAX_RING_GRANT_ORDER;
+module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, 0444);
+MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
+/*
+ * The LRU mechanism to clean the lists of persistent grants needs to
+ * be executed periodically. The time interval between consecutive executions
+ * of the purge mechanism is set in ms.
+ */
+#define LRU_INTERVAL 100
+
+/*
+ * When the persistent grants list is full we will remove unused grants
+ * from the list. The percent number of grants to be removed at each LRU
+ * execution.
+ */
+#define LRU_PERCENT_CLEAN 5
+
+/* Run-time switchable: /sys/module/blkback/parameters/ */
+static unsigned int log_stats;
+module_param(log_stats, int, 0644);
+
+#define BLKBACK_INVALID_HANDLE (~0)
+
+static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
+{
+	return pgrant_timeout && (jiffies - persistent_gnt->last_used >=
+			HZ * pgrant_timeout);
+}
+
+#define vaddr(page) ((unsigned long)pfn_to_kaddr(page_to_pfn(page)))
+
+static int do_block_io_op(struct xen_blkif_ring *ring, unsigned int *eoi_flags);
+static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
+				struct blkif_request *req,
+				struct pending_req *pending_req);
+static void make_response(struct xen_blkif_ring *ring, u64 id,
+			  unsigned short op, int st);
+
+#define foreach_grant_safe(pos, n, rbtree, node) \
+	for ((pos) = container_of(rb_first((rbtree)), typeof(*(pos)), node), \
+	     (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL; \
+	     &(pos)->node != NULL; \
+	     (pos) = container_of(n, typeof(*(pos)), node), \
+	     (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL)
+
+
+/*
+ * We don't need locking around the persistent grant helpers
+ * because blkback uses a single-thread for each backend, so we
+ * can be sure that this functions will never be called recursively.
+ *
+ * The only exception to that is put_persistent_grant, that can be called
+ * from interrupt context (by xen_blkbk_unmap), so we have to use atomic
+ * bit operations to modify the flags of a persistent grant and to count
+ * the number of used grants.
+ */
+static int add_persistent_gnt(struct xen_blkif_ring *ring,
+			       struct persistent_gnt *persistent_gnt)
+{
+	struct rb_node **new = NULL, *parent = NULL;
+	struct persistent_gnt *this;
+	struct xen_blkif *blkif = ring->blkif;
+
+	if (ring->persistent_gnt_c >= max_pgrants) {
+		if (!blkif->vbd.overflow_max_grants)
+			blkif->vbd.overflow_max_grants = 1;
+		return -EBUSY;
+	}
+	/* Figure out where to put new node */
+	new = &ring->persistent_gnts.rb_node;
+	while (*new) {
+		this = container_of(*new, struct persistent_gnt, node);
+
+		parent = *new;
+		if (persistent_gnt->gnt < this->gnt)
+			new = &((*new)->rb_left);
+		else if (persistent_gnt->gnt > this->gnt)
+			new = &((*new)->rb_right);
+		else {
+			pr_alert_ratelimited("trying to add a gref that's already in the tree\n");
+			return -EINVAL;
+		}
+	}
+
+	persistent_gnt->active = true;
+	/* Add new node and rebalance tree. */
+	rb_link_node(&(persistent_gnt->node), parent, new);
+	rb_insert_color(&(persistent_gnt->node), &ring->persistent_gnts);
+	ring->persistent_gnt_c++;
+	atomic_inc(&ring->persistent_gnt_in_use);
+	return 0;
+}
+
+static struct persistent_gnt *get_persistent_gnt(struct xen_blkif_ring *ring,
+						 grant_ref_t gref)
+{
+	struct persistent_gnt *data;
+	struct rb_node *node = NULL;
+
+	node = ring->persistent_gnts.rb_node;
+	while (node) {
+		data = container_of(node, struct persistent_gnt, node);
+
+		if (gref < data->gnt)
+			node = node->rb_left;
+		else if (gref > data->gnt)
+			node = node->rb_right;
+		else {
+			if (data->active) {
+				pr_alert_ratelimited("requesting a grant already in use\n");
+				return NULL;
+			}
+			data->active = true;
+			atomic_inc(&ring->persistent_gnt_in_use);
+			return data;
+		}
+	}
+	return NULL;
+}
+
+static void put_persistent_gnt(struct xen_blkif_ring *ring,
+                               struct persistent_gnt *persistent_gnt)
+{
+	if (!persistent_gnt->active)
+		pr_alert_ratelimited("freeing a grant already unused\n");
+	persistent_gnt->last_used = jiffies;
+	persistent_gnt->active = false;
+	atomic_dec(&ring->persistent_gnt_in_use);
+}
+
+static void free_persistent_gnts(struct xen_blkif_ring *ring, struct rb_root *root,
+                                 unsigned int num)
+{
+	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct persistent_gnt *persistent_gnt;
+	struct rb_node *n;
+	int segs_to_unmap = 0;
+	struct gntab_unmap_queue_data unmap_data;
+
+	unmap_data.pages = pages;
+	unmap_data.unmap_ops = unmap;
+	unmap_data.kunmap_ops = NULL;
+
+	foreach_grant_safe(persistent_gnt, n, root, node) {
+		BUG_ON(persistent_gnt->handle ==
+			BLKBACK_INVALID_HANDLE);
+		gnttab_set_unmap_op(&unmap[segs_to_unmap],
+			(unsigned long) pfn_to_kaddr(page_to_pfn(
+				persistent_gnt->page)),
+			GNTMAP_host_map,
+			persistent_gnt->handle);
+
+		pages[segs_to_unmap] = persistent_gnt->page;
+
+		if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST ||
+			!rb_next(&persistent_gnt->node)) {
+
+			unmap_data.count = segs_to_unmap;
+			BUG_ON(gnttab_unmap_refs_sync(&unmap_data));
+
+			gnttab_page_cache_put(&ring->free_pages, pages,
+					      segs_to_unmap);
+			segs_to_unmap = 0;
+		}
+
+		rb_erase(&persistent_gnt->node, root);
+		kfree(persistent_gnt);
+		num--;
+	}
+	BUG_ON(num != 0);
+}
+
+void xen_blkbk_unmap_purged_grants(struct work_struct *work)
+{
+	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct persistent_gnt *persistent_gnt;
+	int segs_to_unmap = 0;
+	struct xen_blkif_ring *ring = container_of(work, typeof(*ring), persistent_purge_work);
+	struct gntab_unmap_queue_data unmap_data;
+
+	unmap_data.pages = pages;
+	unmap_data.unmap_ops = unmap;
+	unmap_data.kunmap_ops = NULL;
+
+	while(!list_empty(&ring->persistent_purge_list)) {
+		persistent_gnt = list_first_entry(&ring->persistent_purge_list,
+		                                  struct persistent_gnt,
+		                                  remove_node);
+		list_del(&persistent_gnt->remove_node);
+
+		gnttab_set_unmap_op(&unmap[segs_to_unmap],
+			vaddr(persistent_gnt->page),
+			GNTMAP_host_map,
+			persistent_gnt->handle);
+
+		pages[segs_to_unmap] = persistent_gnt->page;
+
+		if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST) {
+			unmap_data.count = segs_to_unmap;
+			BUG_ON(gnttab_unmap_refs_sync(&unmap_data));
+			gnttab_page_cache_put(&ring->free_pages, pages,
+					      segs_to_unmap);
+			segs_to_unmap = 0;
+		}
+		kfree(persistent_gnt);
+	}
+	if (segs_to_unmap > 0) {
+		unmap_data.count = segs_to_unmap;
+		BUG_ON(gnttab_unmap_refs_sync(&unmap_data));
+		gnttab_page_cache_put(&ring->free_pages, pages, segs_to_unmap);
+	}
+}
+
+static void purge_persistent_gnt(struct xen_blkif_ring *ring)
+{
+	struct persistent_gnt *persistent_gnt;
+	struct rb_node *n;
+	unsigned int num_clean, total;
+	bool scan_used = false;
+	struct rb_root *root;
+
+	if (work_busy(&ring->persistent_purge_work)) {
+		pr_alert_ratelimited("Scheduled work from previous purge is still busy, cannot purge list\n");
+		goto out;
+	}
+
+	if (ring->persistent_gnt_c < max_pgrants ||
+	    (ring->persistent_gnt_c == max_pgrants &&
+	    !ring->blkif->vbd.overflow_max_grants)) {
+		num_clean = 0;
+	} else {
+		num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
+		num_clean = ring->persistent_gnt_c - max_pgrants + num_clean;
+		num_clean = min(ring->persistent_gnt_c, num_clean);
+		pr_debug("Going to purge at least %u persistent grants\n",
+			 num_clean);
+	}
+
+	/*
+	 * At this point, we can assure that there will be no calls
+         * to get_persistent_grant (because we are executing this code from
+         * xen_blkif_schedule), there can only be calls to put_persistent_gnt,
+         * which means that the number of currently used grants will go down,
+         * but never up, so we will always be able to remove the requested
+         * number of grants.
+	 */
+
+	total = 0;
+
+	BUG_ON(!list_empty(&ring->persistent_purge_list));
+	root = &ring->persistent_gnts;
+purge_list:
+	foreach_grant_safe(persistent_gnt, n, root, node) {
+		BUG_ON(persistent_gnt->handle ==
+			BLKBACK_INVALID_HANDLE);
+
+		if (persistent_gnt->active)
+			continue;
+		if (!scan_used && !persistent_gnt_timeout(persistent_gnt))
+			continue;
+		if (scan_used && total >= num_clean)
+			continue;
+
+		rb_erase(&persistent_gnt->node, root);
+		list_add(&persistent_gnt->remove_node,
+			 &ring->persistent_purge_list);
+		total++;
+	}
+	/*
+	 * Check whether we also need to start cleaning
+	 * grants that were used since last purge in order to cope
+	 * with the requested num
+	 */
+	if (!scan_used && total < num_clean) {
+		pr_debug("Still missing %u purged frames\n", num_clean - total);
+		scan_used = true;
+		goto purge_list;
+	}
+
+	if (total) {
+		ring->persistent_gnt_c -= total;
+		ring->blkif->vbd.overflow_max_grants = 0;
+
+		/* We can defer this work */
+		schedule_work(&ring->persistent_purge_work);
+		pr_debug("Purged %u/%u\n", num_clean, total);
+	}
+
+out:
+	return;
+}
+
+/*
+ * Retrieve from the 'pending_reqs' a free pending_req structure to be used.
+ */
+static struct pending_req *alloc_req(struct xen_blkif_ring *ring)
+{
+	struct pending_req *req = NULL;
+	unsigned long flags;
+
+	spin_lock_irqsave(&ring->pending_free_lock, flags);
+	if (!list_empty(&ring->pending_free)) {
+		req = list_entry(ring->pending_free.next, struct pending_req,
+				 free_list);
+		list_del(&req->free_list);
+	}
+	spin_unlock_irqrestore(&ring->pending_free_lock, flags);
+	return req;
+}
+
+/*
+ * Return the 'pending_req' structure back to the freepool. We also
+ * wake up the thread if it was waiting for a free page.
+ */
+static void free_req(struct xen_blkif_ring *ring, struct pending_req *req)
+{
+	unsigned long flags;
+	int was_empty;
+
+	spin_lock_irqsave(&ring->pending_free_lock, flags);
+	was_empty = list_empty(&ring->pending_free);
+	list_add(&req->free_list, &ring->pending_free);
+	spin_unlock_irqrestore(&ring->pending_free_lock, flags);
+	if (was_empty)
+		wake_up(&ring->pending_free_wq);
+}
+
+/*
+ * Routines for managing virtual block devices (vbds).
+ */
+static int xen_vbd_translate(struct phys_req *req, struct xen_blkif *blkif,
+			     enum req_op operation)
+{
+	struct xen_vbd *vbd = &blkif->vbd;
+	int rc = -EACCES;
+
+	if ((operation != REQ_OP_READ) && vbd->readonly)
+		goto out;
+
+	if (likely(req->nr_sects)) {
+		blkif_sector_t end = req->sector_number + req->nr_sects;
+
+		if (unlikely(end < req->sector_number))
+			goto out;
+		if (unlikely(end > vbd_sz(vbd)))
+			goto out;
+	}
+
+	req->dev  = vbd->pdevice;
+	req->bdev = vbd->bdev;
+	rc = 0;
+
+ out:
+	return rc;
+}
+
+static void xen_vbd_resize(struct xen_blkif *blkif)
+{
+	struct xen_vbd *vbd = &blkif->vbd;
+	struct xenbus_transaction xbt;
+	int err;
+	struct xenbus_device *dev = xen_blkbk_xenbus(blkif->be);
+	unsigned long long new_size = vbd_sz(vbd);
+
+	pr_info("VBD Resize: Domid: %d, Device: (%d, %d)\n",
+		blkif->domid, MAJOR(vbd->pdevice), MINOR(vbd->pdevice));
+	pr_info("VBD Resize: new size %llu\n", new_size);
+	vbd->size = new_size;
+again:
+	err = xenbus_transaction_start(&xbt);
+	if (err) {
+		pr_warn("Error starting transaction\n");
+		return;
+	}
+	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
+			    (unsigned long long)vbd_sz(vbd));
+	if (err) {
+		pr_warn("Error writing new size\n");
+		goto abort;
+	}
+	/*
+	 * Write the current state; we will use this to synchronize
+	 * the front-end. If the current state is "connected" the
+	 * front-end will get the new size information online.
+	 */
+	err = xenbus_printf(xbt, dev->nodename, "state", "%d", dev->state);
+	if (err) {
+		pr_warn("Error writing the state\n");
+		goto abort;
+	}
+
+	err = xenbus_transaction_end(xbt, 0);
+	if (err == -EAGAIN)
+		goto again;
+	if (err)
+		pr_warn("Error ending transaction\n");
+	return;
+abort:
+	xenbus_transaction_end(xbt, 1);
+}
+
+/*
+ * Notification from the guest OS.
+ */
+static void blkif_notify_work(struct xen_blkif_ring *ring)
+{
+	ring->waiting_reqs = 1;
+	wake_up(&ring->wq);
+}
+
+irqreturn_t xen_blkif_be_int(int irq, void *dev_id)
+{
+	blkif_notify_work(dev_id);
+	return IRQ_HANDLED;
+}
+
+/*
+ * SCHEDULER FUNCTIONS
+ */
+
+static void print_stats(struct xen_blkif_ring *ring)
+{
+	pr_info("(%s): oo %3llu  |  rd %4llu  |  wr %4llu  |  f %4llu"
+		 "  |  ds %4llu | pg: %4u/%4d\n",
+		 current->comm, ring->st_oo_req,
+		 ring->st_rd_req, ring->st_wr_req,
+		 ring->st_f_req, ring->st_ds_req,
+		 ring->persistent_gnt_c, max_pgrants);
+	ring->st_print = jiffies + msecs_to_jiffies(10 * 1000);
+	ring->st_rd_req = 0;
+	ring->st_wr_req = 0;
+	ring->st_oo_req = 0;
+	ring->st_ds_req = 0;
+}
+
+int xen_blkif_schedule(void *arg)
+{
+	struct xen_blkif_ring *ring = arg;
+	struct xen_blkif *blkif = ring->blkif;
+	struct xen_vbd *vbd = &blkif->vbd;
+	unsigned long timeout;
+	int ret;
+	bool do_eoi;
+	unsigned int eoi_flags = XEN_EOI_FLAG_SPURIOUS;
+
+	set_freezable();
+	while (!kthread_should_stop()) {
+		if (try_to_freeze())
+			continue;
+		if (unlikely(vbd->size != vbd_sz(vbd)))
+			xen_vbd_resize(blkif);
+
+		timeout = msecs_to_jiffies(LRU_INTERVAL);
+
+		timeout = wait_event_interruptible_timeout(
+			ring->wq,
+			ring->waiting_reqs || kthread_should_stop(),
+			timeout);
+		if (timeout == 0)
+			goto purge_gnt_list;
+		timeout = wait_event_interruptible_timeout(
+			ring->pending_free_wq,
+			!list_empty(&ring->pending_free) ||
+			kthread_should_stop(),
+			timeout);
+		if (timeout == 0)
+			goto purge_gnt_list;
+
+		do_eoi = ring->waiting_reqs;
+
+		ring->waiting_reqs = 0;
+		smp_mb(); /* clear flag *before* checking for work */
+
+		ret = do_block_io_op(ring, &eoi_flags);
+		if (ret > 0)
+			ring->waiting_reqs = 1;
+		if (ret == -EACCES)
+			wait_event_interruptible(ring->shutdown_wq,
+						 kthread_should_stop());
+
+		if (do_eoi && !ring->waiting_reqs) {
+			xen_irq_lateeoi(ring->irq, eoi_flags);
+			eoi_flags |= XEN_EOI_FLAG_SPURIOUS;
+		}
+
+purge_gnt_list:
+		if (blkif->vbd.feature_gnt_persistent &&
+		    time_after(jiffies, ring->next_lru)) {
+			purge_persistent_gnt(ring);
+			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
+		}
+
+		/* Shrink the free pages pool if it is too large. */
+		if (time_before(jiffies, blkif->buffer_squeeze_end))
+			gnttab_page_cache_shrink(&ring->free_pages, 0);
+		else
+			gnttab_page_cache_shrink(&ring->free_pages,
+						 max_buffer_pages);
+
+		if (log_stats && time_after(jiffies, ring->st_print))
+			print_stats(ring);
+	}
+
+	/* Drain pending purge work */
+	flush_work(&ring->persistent_purge_work);
+
+	if (log_stats)
+		print_stats(ring);
+
+	ring->xenblkd = NULL;
+
+	return 0;
+}
+
+/*
+ * Remove persistent grants and empty the pool of free pages
+ */
+void xen_blkbk_free_caches(struct xen_blkif_ring *ring)
+{
+	/* Free all persistent grant pages */
+	if (!RB_EMPTY_ROOT(&ring->persistent_gnts))
+		free_persistent_gnts(ring, &ring->persistent_gnts,
+			ring->persistent_gnt_c);
+
+	BUG_ON(!RB_EMPTY_ROOT(&ring->persistent_gnts));
+	ring->persistent_gnt_c = 0;
+
+	/* Since we are shutting down remove all pages from the buffer */
+	gnttab_page_cache_shrink(&ring->free_pages, 0 /* All */);
+}
+
+static unsigned int xen_blkbk_unmap_prepare(
+	struct xen_blkif_ring *ring,
+	struct grant_page **pages,
+	unsigned int num,
+	struct gnttab_unmap_grant_ref *unmap_ops,
+	struct page **unmap_pages)
+{
+	unsigned int i, invcount = 0;
+
+	for (i = 0; i < num; i++) {
+		if (pages[i]->persistent_gnt != NULL) {
+			put_persistent_gnt(ring, pages[i]->persistent_gnt);
+			continue;
+		}
+		if (pages[i]->handle == BLKBACK_INVALID_HANDLE)
+			continue;
+		unmap_pages[invcount] = pages[i]->page;
+		gnttab_set_unmap_op(&unmap_ops[invcount], vaddr(pages[i]->page),
+				    GNTMAP_host_map, pages[i]->handle);
+		pages[i]->handle = BLKBACK_INVALID_HANDLE;
+		invcount++;
+	}
+
+	return invcount;
+}
+
+static void xen_blkbk_unmap_and_respond_callback(int result, struct gntab_unmap_queue_data *data)
+{
+	struct pending_req *pending_req = (struct pending_req *)(data->data);
+	struct xen_blkif_ring *ring = pending_req->ring;
+	struct xen_blkif *blkif = ring->blkif;
+
+	/* BUG_ON used to reproduce existing behaviour,
+	   but is this the best way to deal with this? */
+	BUG_ON(result);
+
+	gnttab_page_cache_put(&ring->free_pages, data->pages, data->count);
+	make_response(ring, pending_req->id,
+		      pending_req->operation, pending_req->status);
+	free_req(ring, pending_req);
+	/*
+	 * Make sure the request is freed before releasing blkif,
+	 * or there could be a race between free_req and the
+	 * cleanup done in xen_blkif_free during shutdown.
+	 *
+	 * NB: The fact that we might try to wake up pending_free_wq
+	 * before drain_complete (in case there's a drain going on)
+	 * it's not a problem with our current implementation
+	 * because we can assure there's no thread waiting on
+	 * pending_free_wq if there's a drain going on, but it has
+	 * to be taken into account if the current model is changed.
+	 */
+	if (atomic_dec_and_test(&ring->inflight) && atomic_read(&blkif->drain)) {
+		complete(&blkif->drain_complete);
+	}
+	xen_blkif_put(blkif);
+}
+
+static void xen_blkbk_unmap_and_respond(struct pending_req *req)
+{
+	struct gntab_unmap_queue_data* work = &req->gnttab_unmap_data;
+	struct xen_blkif_ring *ring = req->ring;
+	struct grant_page **pages = req->segments;
+	unsigned int invcount;
+
+	invcount = xen_blkbk_unmap_prepare(ring, pages, req->nr_segs,
+					   req->unmap, req->unmap_pages);
+
+	work->data = req;
+	work->done = xen_blkbk_unmap_and_respond_callback;
+	work->unmap_ops = req->unmap;
+	work->kunmap_ops = NULL;
+	work->pages = req->unmap_pages;
+	work->count = invcount;
+
+	gnttab_unmap_refs_async(&req->gnttab_unmap_data);
+}
+
+
+/*
+ * Unmap the grant references.
+ *
+ * This could accumulate ops up to the batch size to reduce the number
+ * of hypercalls, but since this is only used in error paths there's
+ * no real need.
+ */
+static void xen_blkbk_unmap(struct xen_blkif_ring *ring,
+                            struct grant_page *pages[],
+                            int num)
+{
+	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page *unmap_pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	unsigned int invcount = 0;
+	int ret;
+
+	while (num) {
+		unsigned int batch = min(num, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+
+		invcount = xen_blkbk_unmap_prepare(ring, pages, batch,
+						   unmap, unmap_pages);
+		if (invcount) {
+			ret = gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
+			BUG_ON(ret);
+			gnttab_page_cache_put(&ring->free_pages, unmap_pages,
+					      invcount);
+		}
+		pages += batch;
+		num -= batch;
+	}
+}
+
+static int xen_blkbk_map(struct xen_blkif_ring *ring,
+			 struct grant_page *pages[],
+			 int num, bool ro)
+{
+	struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page *pages_to_gnt[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct persistent_gnt *persistent_gnt = NULL;
+	phys_addr_t addr = 0;
+	int i, seg_idx, new_map_idx;
+	int segs_to_map = 0;
+	int ret = 0;
+	int last_map = 0, map_until = 0;
+	int use_persistent_gnts;
+	struct xen_blkif *blkif = ring->blkif;
+
+	use_persistent_gnts = (blkif->vbd.feature_gnt_persistent);
+
+	/*
+	 * Fill out preq.nr_sects with proper amount of sectors, and setup
+	 * assign map[..] with the PFN of the page in our domain with the
+	 * corresponding grant reference for each page.
+	 */
+again:
+	for (i = map_until; i < num; i++) {
+		uint32_t flags;
+
+		if (use_persistent_gnts) {
+			persistent_gnt = get_persistent_gnt(
+				ring,
+				pages[i]->gref);
+		}
+
+		if (persistent_gnt) {
+			/*
+			 * We are using persistent grants and
+			 * the grant is already mapped
+			 */
+			pages[i]->page = persistent_gnt->page;
+			pages[i]->persistent_gnt = persistent_gnt;
+		} else {
+			if (gnttab_page_cache_get(&ring->free_pages,
+						  &pages[i]->page)) {
+				gnttab_page_cache_put(&ring->free_pages,
+						      pages_to_gnt,
+						      segs_to_map);
+				ret = -ENOMEM;
+				goto out;
+			}
+			addr = vaddr(pages[i]->page);
+			pages_to_gnt[segs_to_map] = pages[i]->page;
+			pages[i]->persistent_gnt = NULL;
+			flags = GNTMAP_host_map;
+			if (!use_persistent_gnts && ro)
+				flags |= GNTMAP_readonly;
+			gnttab_set_map_op(&map[segs_to_map++], addr,
+					  flags, pages[i]->gref,
+					  blkif->domid);
+		}
+		map_until = i + 1;
+		if (segs_to_map == BLKIF_MAX_SEGMENTS_PER_REQUEST)
+			break;
+	}
+
+	if (segs_to_map)
+		ret = gnttab_map_refs(map, NULL, pages_to_gnt, segs_to_map);
+
+	/*
+	 * Now swizzle the MFN in our domain with the MFN from the other domain
+	 * so that when we access vaddr(pending_req,i) it has the contents of
+	 * the page from the other domain.
+	 */
+	for (seg_idx = last_map, new_map_idx = 0; seg_idx < map_until; seg_idx++) {
+		if (!pages[seg_idx]->persistent_gnt) {
+			/* This is a newly mapped grant */
+			BUG_ON(new_map_idx >= segs_to_map);
+			if (unlikely(map[new_map_idx].status != 0)) {
+				pr_debug("invalid buffer -- could not remap it\n");
+				gnttab_page_cache_put(&ring->free_pages,
+						      &pages[seg_idx]->page, 1);
+				pages[seg_idx]->handle = BLKBACK_INVALID_HANDLE;
+				ret |= !ret;
+				goto next;
+			}
+			pages[seg_idx]->handle = map[new_map_idx].handle;
+		} else {
+			continue;
+		}
+		if (use_persistent_gnts &&
+		    ring->persistent_gnt_c < max_pgrants) {
+			/*
+			 * We are using persistent grants, the grant is
+			 * not mapped but we might have room for it.
+			 */
+			persistent_gnt = kmalloc(sizeof(struct persistent_gnt),
+				                 GFP_KERNEL);
+			if (!persistent_gnt) {
+				/*
+				 * If we don't have enough memory to
+				 * allocate the persistent_gnt struct
+				 * map this grant non-persistenly
+				 */
+				goto next;
+			}
+			persistent_gnt->gnt = map[new_map_idx].ref;
+			persistent_gnt->handle = map[new_map_idx].handle;
+			persistent_gnt->page = pages[seg_idx]->page;
+			if (add_persistent_gnt(ring,
+			                       persistent_gnt)) {
+				kfree(persistent_gnt);
+				persistent_gnt = NULL;
+				goto next;
+			}
+			pages[seg_idx]->persistent_gnt = persistent_gnt;
+			pr_debug("grant %u added to the tree of persistent grants, using %u/%u\n",
+				 persistent_gnt->gnt, ring->persistent_gnt_c,
+				 max_pgrants);
+			goto next;
+		}
+		if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
+			blkif->vbd.overflow_max_grants = 1;
+			pr_debug("domain %u, device %#x is using maximum number of persistent grants\n",
+			         blkif->domid, blkif->vbd.handle);
+		}
+		/*
+		 * We could not map this grant persistently, so use it as
+		 * a non-persistent grant.
+		 */
+next:
+		new_map_idx++;
+	}
+	segs_to_map = 0;
+	last_map = map_until;
+	if (!ret && map_until != num)
+		goto again;
+
+out:
+	for (i = last_map; i < num; i++) {
+		/* Don't zap current batch's valid persistent grants. */
+		if(i >= map_until)
+			pages[i]->persistent_gnt = NULL;
+		pages[i]->handle = BLKBACK_INVALID_HANDLE;
+	}
+
+	return ret;
+}
+
+static int xen_blkbk_map_seg(struct pending_req *pending_req)
+{
+	int rc;
+
+	rc = xen_blkbk_map(pending_req->ring, pending_req->segments,
+			   pending_req->nr_segs,
+	                   (pending_req->operation != BLKIF_OP_READ));
+
+	return rc;
+}
+
+static int xen_blkbk_parse_indirect(struct blkif_request *req,
+				    struct pending_req *pending_req,
+				    struct seg_buf seg[],
+				    struct phys_req *preq)
+{
+	struct grant_page **pages = pending_req->indirect_pages;
+	struct xen_blkif_ring *ring = pending_req->ring;
+	int indirect_grefs, rc, n, nseg, i;
+	struct blkif_request_segment *segments = NULL;
+
+	nseg = pending_req->nr_segs;
+	indirect_grefs = INDIRECT_PAGES(nseg);
+	BUG_ON(indirect_grefs > BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST);
+
+	for (i = 0; i < indirect_grefs; i++)
+		pages[i]->gref = req->u.indirect.indirect_grefs[i];
+
+	rc = xen_blkbk_map(ring, pages, indirect_grefs, true);
+	if (rc)
+		goto unmap;
+
+	for (n = 0; n < nseg; n++) {
+		uint8_t first_sect, last_sect;
+
+		if ((n % SEGS_PER_INDIRECT_FRAME) == 0) {
+			/* Map indirect segments */
+			if (segments)
+				kunmap_atomic(segments);
+			segments = kmap_atomic(pages[n/SEGS_PER_INDIRECT_FRAME]->page);
+		}
+		i = n % SEGS_PER_INDIRECT_FRAME;
+
+		pending_req->segments[n]->gref = segments[i].gref;
+
+		first_sect = READ_ONCE(segments[i].first_sect);
+		last_sect = READ_ONCE(segments[i].last_sect);
+		if (last_sect >= (XEN_PAGE_SIZE >> 9) || last_sect < first_sect) {
+			rc = -EINVAL;
+			goto unmap;
+		}
+
+		seg[n].nsec = last_sect - first_sect + 1;
+		seg[n].offset = first_sect << 9;
+		preq->nr_sects += seg[n].nsec;
+	}
+
+unmap:
+	if (segments)
+		kunmap_atomic(segments);
+	xen_blkbk_unmap(ring, pages, indirect_grefs);
+	return rc;
+}
+
+static int dispatch_discard_io(struct xen_blkif_ring *ring,
+				struct blkif_request *req)
+{
+	int err = 0;
+	int status = BLKIF_RSP_OKAY;
+	struct xen_blkif *blkif = ring->blkif;
+	struct block_device *bdev = blkif->vbd.bdev;
+	struct phys_req preq;
+
+	xen_blkif_get(blkif);
+
+	preq.sector_number = req->u.discard.sector_number;
+	preq.nr_sects      = req->u.discard.nr_sectors;
+
+	err = xen_vbd_translate(&preq, blkif, REQ_OP_WRITE);
+	if (err) {
+		pr_warn("access denied: DISCARD [%llu->%llu] on dev=%04x\n",
+			preq.sector_number,
+			preq.sector_number + preq.nr_sects, blkif->vbd.pdevice);
+		goto fail_response;
+	}
+	ring->st_ds_req++;
+
+	if (blkif->vbd.discard_secure &&
+	    (req->u.discard.flag & BLKIF_DISCARD_SECURE))
+		err = blkdev_issue_secure_erase(bdev,
+				req->u.discard.sector_number,
+				req->u.discard.nr_sectors, GFP_KERNEL);
+	else
+		err = blkdev_issue_discard(bdev, req->u.discard.sector_number,
+				req->u.discard.nr_sectors, GFP_KERNEL);
+
+fail_response:
+	if (err == -EOPNOTSUPP) {
+		pr_debug("discard op failed, not supported\n");
+		status = BLKIF_RSP_EOPNOTSUPP;
+	} else if (err)
+		status = BLKIF_RSP_ERROR;
+
+	make_response(ring, req->u.discard.id, req->operation, status);
+	xen_blkif_put(blkif);
+	return err;
+}
+
+static int dispatch_other_io(struct xen_blkif_ring *ring,
+			     struct blkif_request *req,
+			     struct pending_req *pending_req)
+{
+	free_req(ring, pending_req);
+	make_response(ring, req->u.other.id, req->operation,
+		      BLKIF_RSP_EOPNOTSUPP);
+	return -EIO;
+}
+
+static void xen_blk_drain_io(struct xen_blkif_ring *ring)
+{
+	struct xen_blkif *blkif = ring->blkif;
+
+	atomic_set(&blkif->drain, 1);
+	do {
+		if (atomic_read(&ring->inflight) == 0)
+			break;
+		wait_for_completion_interruptible_timeout(
+				&blkif->drain_complete, HZ);
+
+		if (!atomic_read(&blkif->drain))
+			break;
+	} while (!kthread_should_stop());
+	atomic_set(&blkif->drain, 0);
+}
+
+static void __end_block_io_op(struct pending_req *pending_req,
+		blk_status_t error)
+{
+	/* An error fails the entire request. */
+	if (pending_req->operation == BLKIF_OP_FLUSH_DISKCACHE &&
+	    error == BLK_STS_NOTSUPP) {
+		pr_debug("flush diskcache op failed, not supported\n");
+		xen_blkbk_flush_diskcache(XBT_NIL, pending_req->ring->blkif->be, 0);
+		pending_req->status = BLKIF_RSP_EOPNOTSUPP;
+	} else if (pending_req->operation == BLKIF_OP_WRITE_BARRIER &&
+		   error == BLK_STS_NOTSUPP) {
+		pr_debug("write barrier op failed, not supported\n");
+		xen_blkbk_barrier(XBT_NIL, pending_req->ring->blkif->be, 0);
+		pending_req->status = BLKIF_RSP_EOPNOTSUPP;
+	} else if (error) {
+		pr_debug("Buffer not up-to-date at end of operation,"
+			 " error=%d\n", error);
+		pending_req->status = BLKIF_RSP_ERROR;
+	}
+
+	/*
+	 * If all of the bio's have completed it is time to unmap
+	 * the grant references associated with 'request' and provide
+	 * the proper response on the ring.
+	 */
+	if (atomic_dec_and_test(&pending_req->pendcnt))
+		xen_blkbk_unmap_and_respond(pending_req);
+}
+
+/*
+ * bio callback.
+ */
+static void end_block_io_op(struct bio *bio)
+{
+	__end_block_io_op(bio->bi_private, bio->bi_status);
+	bio_put(bio);
+}
+
+
+
+/*
+ * Function to copy the from the ring buffer the 'struct blkif_request'
+ * (which has the sectors we want, number of them, grant references, etc),
+ * and transmute  it to the block API to hand it over to the proper block disk.
+ */
+static int
+__do_block_io_op(struct xen_blkif_ring *ring, unsigned int *eoi_flags)
+{
+	union blkif_back_rings *blk_rings = &ring->blk_rings;
+	struct blkif_request req;
+	struct pending_req *pending_req;
+	RING_IDX rc, rp;
+	int more_to_do = 0;
+
+	rc = blk_rings->common.req_cons;
+	rp = blk_rings->common.sring->req_prod;
+	rmb(); /* Ensure we see queued requests up to 'rp'. */
+
+	if (RING_REQUEST_PROD_OVERFLOW(&blk_rings->common, rp)) {
+		rc = blk_rings->common.rsp_prod_pvt;
+		pr_warn("Frontend provided bogus ring requests (%d - %d = %d). Halting ring processing on dev=%04x\n",
+			rp, rc, rp - rc, ring->blkif->vbd.pdevice);
+		return -EACCES;
+	}
+	while (rc != rp) {
+
+		if (RING_REQUEST_CONS_OVERFLOW(&blk_rings->common, rc))
+			break;
+
+		/* We've seen a request, so clear spurious eoi flag. */
+		*eoi_flags &= ~XEN_EOI_FLAG_SPURIOUS;
+
+		if (kthread_should_stop()) {
+			more_to_do = 1;
+			break;
+		}
+
+		pending_req = alloc_req(ring);
+		if (NULL == pending_req) {
+			ring->st_oo_req++;
+			more_to_do = 1;
+			break;
+		}
+
+		switch (ring->blkif->blk_protocol) {
+		case BLKIF_PROTOCOL_NATIVE:
+			memcpy(&req, RING_GET_REQUEST(&blk_rings->native, rc), sizeof(req));
+			break;
+		case BLKIF_PROTOCOL_X86_32:
+			blkif_get_x86_32_req(&req, RING_GET_REQUEST(&blk_rings->x86_32, rc));
+			break;
+		case BLKIF_PROTOCOL_X86_64:
+			blkif_get_x86_64_req(&req, RING_GET_REQUEST(&blk_rings->x86_64, rc));
+			break;
+		default:
+			BUG();
+		}
+		blk_rings->common.req_cons = ++rc; /* before make_response() */
+
+		/* Apply all sanity checks to /private copy/ of request. */
+		barrier();
+
+		switch (req.operation) {
+		case BLKIF_OP_READ:
+		case BLKIF_OP_WRITE:
+		case BLKIF_OP_WRITE_BARRIER:
+		case BLKIF_OP_FLUSH_DISKCACHE:
+		case BLKIF_OP_INDIRECT:
+			if (dispatch_rw_block_io(ring, &req, pending_req))
+				goto done;
+			break;
+		case BLKIF_OP_DISCARD:
+			free_req(ring, pending_req);
+			if (dispatch_discard_io(ring, &req))
+				goto done;
+			break;
+		default:
+			if (dispatch_other_io(ring, &req, pending_req))
+				goto done;
+			break;
+		}
+
+		/* Yield point for this unbounded loop. */
+		cond_resched();
+	}
+done:
+	return more_to_do;
+}
+
+static int
+do_block_io_op(struct xen_blkif_ring *ring, unsigned int *eoi_flags)
+{
+	union blkif_back_rings *blk_rings = &ring->blk_rings;
+	int more_to_do;
+
+	do {
+		more_to_do = __do_block_io_op(ring, eoi_flags);
+		if (more_to_do)
+			break;
+
+		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
+	} while (more_to_do);
+
+	return more_to_do;
+}
+/*
+ * Transmutation of the 'struct blkif_request' to a proper 'struct bio'
+ * and call the 'submit_bio' to pass it to the underlying storage.
+ */
+static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
+				struct blkif_request *req,
+				struct pending_req *pending_req)
+{
+	struct phys_req preq;
+	struct seg_buf *seg = pending_req->seg;
+	unsigned int nseg;
+	struct bio *bio = NULL;
+	struct bio **biolist = pending_req->biolist;
+	int i, nbio = 0;
+	enum req_op operation;
+	blk_opf_t operation_flags = 0;
+	struct blk_plug plug;
+	bool drain = false;
+	struct grant_page **pages = pending_req->segments;
+	unsigned short req_operation;
+
+	req_operation = req->operation == BLKIF_OP_INDIRECT ?
+			req->u.indirect.indirect_op : req->operation;
+
+	if ((req->operation == BLKIF_OP_INDIRECT) &&
+	    (req_operation != BLKIF_OP_READ) &&
+	    (req_operation != BLKIF_OP_WRITE)) {
+		pr_debug("Invalid indirect operation (%u)\n", req_operation);
+		goto fail_response;
+	}
+
+	switch (req_operation) {
+	case BLKIF_OP_READ:
+		ring->st_rd_req++;
+		operation = REQ_OP_READ;
+		break;
+	case BLKIF_OP_WRITE:
+		ring->st_wr_req++;
+		operation = REQ_OP_WRITE;
+		operation_flags = REQ_SYNC | REQ_IDLE;
+		break;
+	case BLKIF_OP_WRITE_BARRIER:
+		drain = true;
+		fallthrough;
+	case BLKIF_OP_FLUSH_DISKCACHE:
+		ring->st_f_req++;
+		operation = REQ_OP_WRITE;
+		operation_flags = REQ_PREFLUSH;
+		break;
+	default:
+		operation = 0; /* make gcc happy */
+		goto fail_response;
+		break;
+	}
+
+	/* Check that the number of segments is sane. */
+	nseg = req->operation == BLKIF_OP_INDIRECT ?
+	       req->u.indirect.nr_segments : req->u.rw.nr_segments;
+
+	if (unlikely(nseg == 0 && operation_flags != REQ_PREFLUSH) ||
+	    unlikely((req->operation != BLKIF_OP_INDIRECT) &&
+		     (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) ||
+	    unlikely((req->operation == BLKIF_OP_INDIRECT) &&
+		     (nseg > MAX_INDIRECT_SEGMENTS))) {
+		pr_debug("Bad number of segments in request (%d)\n", nseg);
+		/* Haven't submitted any bio's yet. */
+		goto fail_response;
+	}
+
+	preq.nr_sects      = 0;
+
+	pending_req->ring      = ring;
+	pending_req->id        = req->u.rw.id;
+	pending_req->operation = req_operation;
+	pending_req->status    = BLKIF_RSP_OKAY;
+	pending_req->nr_segs   = nseg;
+
+	if (req->operation != BLKIF_OP_INDIRECT) {
+		preq.dev               = req->u.rw.handle;
+		preq.sector_number     = req->u.rw.sector_number;
+		for (i = 0; i < nseg; i++) {
+			pages[i]->gref = req->u.rw.seg[i].gref;
+			seg[i].nsec = req->u.rw.seg[i].last_sect -
+				req->u.rw.seg[i].first_sect + 1;
+			seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
+			if ((req->u.rw.seg[i].last_sect >= (XEN_PAGE_SIZE >> 9)) ||
+			    (req->u.rw.seg[i].last_sect <
+			     req->u.rw.seg[i].first_sect))
+				goto fail_response;
+			preq.nr_sects += seg[i].nsec;
+		}
+	} else {
+		preq.dev               = req->u.indirect.handle;
+		preq.sector_number     = req->u.indirect.sector_number;
+		if (xen_blkbk_parse_indirect(req, pending_req, seg, &preq))
+			goto fail_response;
+	}
+
+	if (xen_vbd_translate(&preq, ring->blkif, operation) != 0) {
+		pr_debug("access denied: %s of [%llu,%llu] on dev=%04x\n",
+			 operation == REQ_OP_READ ? "read" : "write",
+			 preq.sector_number,
+			 preq.sector_number + preq.nr_sects,
+			 ring->blkif->vbd.pdevice);
+		goto fail_response;
+	}
+
+	/*
+	 * This check _MUST_ be done after xen_vbd_translate as the preq.bdev
+	 * is set there.
+	 */
+	for (i = 0; i < nseg; i++) {
+		if (((int)preq.sector_number|(int)seg[i].nsec) &
+		    ((bdev_logical_block_size(preq.bdev) >> 9) - 1)) {
+			pr_debug("Misaligned I/O request from domain %d\n",
+				 ring->blkif->domid);
+			goto fail_response;
+		}
+	}
+
+	/* Wait on all outstanding I/O's and once that has been completed
+	 * issue the flush.
+	 */
+	if (drain)
+		xen_blk_drain_io(pending_req->ring);
+
+	/*
+	 * If we have failed at this point, we need to undo the M2P override,
+	 * set gnttab_set_unmap_op on all of the grant references and perform
+	 * the hypercall to unmap the grants - that is all done in
+	 * xen_blkbk_unmap.
+	 */
+	if (xen_blkbk_map_seg(pending_req))
+		goto fail_flush;
+
+	/*
+	 * This corresponding xen_blkif_put is done in __end_block_io_op, or
+	 * below (in "!bio") if we are handling a BLKIF_OP_DISCARD.
+	 */
+	xen_blkif_get(ring->blkif);
+	atomic_inc(&ring->inflight);
+
+	for (i = 0; i < nseg; i++) {
+		while ((bio == NULL) ||
+		       (bio_add_page(bio,
+				     pages[i]->page,
+				     seg[i].nsec << 9,
+				     seg[i].offset) == 0)) {
+			bio = bio_alloc(preq.bdev, bio_max_segs(nseg - i),
+					operation | operation_flags,
+					GFP_KERNEL);
+			biolist[nbio++] = bio;
+			bio->bi_private = pending_req;
+			bio->bi_end_io  = end_block_io_op;
+			bio->bi_iter.bi_sector  = preq.sector_number;
+		}
+
+		preq.sector_number += seg[i].nsec;
+	}
+
+	/* This will be hit if the operation was a flush or discard. */
+	if (!bio) {
+		BUG_ON(operation_flags != REQ_PREFLUSH);
+
+		bio = bio_alloc(preq.bdev, 0, operation | operation_flags,
+				GFP_KERNEL);
+		biolist[nbio++] = bio;
+		bio->bi_private = pending_req;
+		bio->bi_end_io  = end_block_io_op;
+	}
+
+	atomic_set(&pending_req->pendcnt, nbio);
+	blk_start_plug(&plug);
+
+	for (i = 0; i < nbio; i++)
+		submit_bio(biolist[i]);
+
+	/* Let the I/Os go.. */
+	blk_finish_plug(&plug);
+
+	if (operation == REQ_OP_READ)
+		ring->st_rd_sect += preq.nr_sects;
+	else if (operation == REQ_OP_WRITE)
+		ring->st_wr_sect += preq.nr_sects;
+
+	return 0;
+
+ fail_flush:
+	xen_blkbk_unmap(ring, pending_req->segments,
+	                pending_req->nr_segs);
+ fail_response:
+	/* Haven't submitted any bio's yet. */
+	make_response(ring, req->u.rw.id, req_operation, BLKIF_RSP_ERROR);
+	free_req(ring, pending_req);
+	msleep(1); /* back off a bit */
+	return -EIO;
+}
+
+
+
+/*
+ * Put a response on the ring on how the operation fared.
+ */
+static void make_response(struct xen_blkif_ring *ring, u64 id,
+			  unsigned short op, int st)
+{
+	struct blkif_response *resp;
+	unsigned long     flags;
+	union blkif_back_rings *blk_rings;
+	int notify;
+
+	spin_lock_irqsave(&ring->blk_ring_lock, flags);
+	blk_rings = &ring->blk_rings;
+	/* Place on the response ring for the relevant domain. */
+	switch (ring->blkif->blk_protocol) {
+	case BLKIF_PROTOCOL_NATIVE:
+		resp = RING_GET_RESPONSE(&blk_rings->native,
+					 blk_rings->native.rsp_prod_pvt);
+		break;
+	case BLKIF_PROTOCOL_X86_32:
+		resp = RING_GET_RESPONSE(&blk_rings->x86_32,
+					 blk_rings->x86_32.rsp_prod_pvt);
+		break;
+	case BLKIF_PROTOCOL_X86_64:
+		resp = RING_GET_RESPONSE(&blk_rings->x86_64,
+					 blk_rings->x86_64.rsp_prod_pvt);
+		break;
+	default:
+		BUG();
+	}
+
+	resp->id        = id;
+	resp->operation = op;
+	resp->status    = st;
+
+	blk_rings->common.rsp_prod_pvt++;
+	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blk_rings->common, notify);
+	spin_unlock_irqrestore(&ring->blk_ring_lock, flags);
+	if (notify)
+		notify_remote_via_irq(ring->irq);
+}
+
+static int __init xen_blkif_init(void)
+{
+	int rc = 0;
+
+	if (!xen_domain())
+		return -ENODEV;
+
+	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_GRANT_ORDER) {
+		pr_info("Invalid max_ring_order (%d), will use default max: %d.\n",
+			xen_blkif_max_ring_order, XENBUS_MAX_RING_GRANT_ORDER);
+		xen_blkif_max_ring_order = XENBUS_MAX_RING_GRANT_ORDER;
+	}
+
+	if (xenblk_max_queues == 0)
+		xenblk_max_queues = num_online_cpus();
+
+	rc = xen_blkif_interface_init();
+	if (rc)
+		goto failed_init;
+
+	rc = xen_blkif_xenbus_init();
+	if (rc)
+		goto failed_init;
+
+ failed_init:
+	return rc;
+}
+
+module_init(xen_blkif_init);
+
+static void __exit xen_blkif_fini(void)
+{
+	xen_blkif_xenbus_fini();
+	xen_blkif_interface_fini();
+}
+
+module_exit(xen_blkif_fini);
+
+MODULE_LICENSE("Dual BSD/GPL");
+MODULE_ALIAS("xen-backend:vbd");