virtio-mem: Big Block Mode (BBM) memory hotplug

Currently, we do not support device block sizes that exceed the Linux
memory block size. For example, having a device block size of 1 GiB (e.g.,
gigantic pages in the hypervisor) won't work with 128 MiB Linux memory
blocks.

Let's implement Big Block Mode (BBM), whereby we add/remove at least
one Linux memory block at a time. With a 1 GiB device block size, a Big
Block (BB) will cover 8 Linux memory blocks.

We'll keep registering the online_page_callback machinery, it will be used
for safe memory hotunplug in BBM next.

Note: BBM is properly prepared for variable-sized Linux memory
blocks that we might see in the future. So we won't care how many Linux
memory blocks a big block actually spans, and how the memory notifier is
called.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-26-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -30,12 +30,18 @@ MODULE_PARM_DESC(unplug_online, "Try to unplug online memory");
 /*
  * virtio-mem currently supports the following modes of operation:
  *
- * * Sub Block Mode (SBM): A Linux memory block spans 1..X subblocks (SB). The
+ * * Sub Block Mode (SBM): A Linux memory block spans 2..X subblocks (SB). The
  *   size of a Sub Block (SB) is determined based on the device block size, the
  *   pageblock size, and the maximum allocation granularity of the buddy.
  *   Subblocks within a Linux memory block might either be plugged or unplugged.
  *   Memory is added/removed to Linux MM in Linux memory block granularity.
  *
+ * * Big Block Mode (BBM): A Big Block (BB) spans 1..X Linux memory blocks.
+ *   Memory is added/removed to Linux MM in Big Block granularity.
+ *
+ * The mode is determined automatically based on the Linux memory block size
+ * and the device block size.
+ *
  * User space / core MM (auto onlining) is responsible for onlining added
  * Linux memory blocks - and for selecting a zone. Linux Memory Blocks are
  * always onlined separately, and all memory within a Linux memory block is
@@ -61,6 +67,19 @@ enum virtio_mem_sbm_mb_state {
 	VIRTIO_MEM_SBM_MB_COUNT
 };
 
+/*
+ * State of a Big Block (BB) in BBM, covering 1..X Linux memory blocks.
+ */
+enum virtio_mem_bbm_bb_state {
+	/* Unplugged, not added to Linux. Can be reused later. */
+	VIRTIO_MEM_BBM_BB_UNUSED = 0,
+	/* Plugged, not added to Linux. Error on add_memory(). */
+	VIRTIO_MEM_BBM_BB_PLUGGED,
+	/* Plugged and added to Linux. */
+	VIRTIO_MEM_BBM_BB_ADDED,
+	VIRTIO_MEM_BBM_BB_COUNT
+};
+
 struct virtio_mem {
 	struct virtio_device *vdev;
 
@@ -113,6 +132,10 @@ struct virtio_mem {
 	atomic64_t offline_size;
 	uint64_t offline_threshold;
 
+	/* If set, the driver is in SBM, otherwise in BBM. */
+	bool in_sbm;
+
+	union {
 		struct {
 			/* Id of the first memory block of this device. */
 			unsigned long first_mb_id;
@@ -130,11 +153,11 @@ struct virtio_mem {
 			unsigned long mb_count[VIRTIO_MEM_SBM_MB_COUNT];
 
 			/*
-		 * One byte state per memory block. Allocated via vmalloc().
-		 * Resized (alloc+copy+free) on demand.
+			 * One byte state per memory block. Allocated via
+			 * vmalloc(). Resized (alloc+copy+free) on demand.
 			 *
-		 * With 128 MiB memory blocks, we have states for 512 GiB of
-		 * memory in one 4 KiB page.
+			 * With 128 MiB memory blocks, we have states for 512
+			 * GiB of memory in one 4 KiB page.
 			 */
 			uint8_t *mb_states;
 
@@ -142,18 +165,37 @@ struct virtio_mem {
 			 * Bitmap: one bit per subblock. Allocated similar to
 			 * sbm.mb_states.
 			 *
-		 * A set bit means the corresponding subblock is plugged,
-		 * otherwise it's unblocked.
+			 * A set bit means the corresponding subblock is
+			 * plugged, otherwise it's unblocked.
 			 *
-		 * With 4 MiB subblocks, we manage 128 GiB of memory in one
-		 * 4 KiB page.
+			 * With 4 MiB subblocks, we manage 128 GiB of memory
+			 * in one 4 KiB page.
 			 */
 			unsigned long *sb_states;
 		} sbm;
 
+		struct {
+			/* Id of the first big block of this device. */
+			unsigned long first_bb_id;
+			/* Id of the last usable big block of this device. */
+			unsigned long last_usable_bb_id;
+			/* Id of the next device bock to prepare when needed. */
+			unsigned long next_bb_id;
+
+			/* Summary of all big block states. */
+			unsigned long bb_count[VIRTIO_MEM_BBM_BB_COUNT];
+
+			/* One byte state per big block. See sbm.mb_states. */
+			uint8_t *bb_states;
+
+			/* The block size used for plugging/adding/removing. */
+			uint64_t bb_size;
+		} bbm;
+	};
+
 	/*
-	 * Mutex that protects the sbm.mb_count, sbm.mb_states, and
-	 * sbm.sb_states.
+	 * Mutex that protects the sbm.mb_count, sbm.mb_states,
+	 * sbm.sb_states, bbm.bb_count, and bbm.bb_states
 	 *
 	 * When this lock is held the pointers can't change, ONLINE and
 	 * OFFLINE blocks can't change the state and no subblocks will get
@@ -247,6 +289,24 @@ static unsigned long virtio_mem_mb_id_to_phys(unsigned long mb_id)
 	return mb_id * memory_block_size_bytes();
 }
 
+/*
+ * Calculate the big block id of a given address.
+ */
+static unsigned long virtio_mem_phys_to_bb_id(struct virtio_mem *vm,
+					      uint64_t addr)
+{
+	return addr / vm->bbm.bb_size;
+}
+
+/*
+ * Calculate the physical start address of a given big block id.
+ */
+static uint64_t virtio_mem_bb_id_to_phys(struct virtio_mem *vm,
+					 unsigned long bb_id)
+{
+	return bb_id * vm->bbm.bb_size;
+}
+
 /*
  * Calculate the subblock id of a given address.
  */
@@ -259,6 +319,67 @@ static unsigned long virtio_mem_phys_to_sb_id(struct virtio_mem *vm,
 	return (addr - mb_addr) / vm->sbm.sb_size;
 }
 
+/*
+ * Set the state of a big block, taking care of the state counter.
+ */
+static void virtio_mem_bbm_set_bb_state(struct virtio_mem *vm,
+					unsigned long bb_id,
+					enum virtio_mem_bbm_bb_state state)
+{
+	const unsigned long idx = bb_id - vm->bbm.first_bb_id;
+	enum virtio_mem_bbm_bb_state old_state;
+
+	old_state = vm->bbm.bb_states[idx];
+	vm->bbm.bb_states[idx] = state;
+
+	BUG_ON(vm->bbm.bb_count[old_state] == 0);
+	vm->bbm.bb_count[old_state]--;
+	vm->bbm.bb_count[state]++;
+}
+
+/*
+ * Get the state of a big block.
+ */
+static enum virtio_mem_bbm_bb_state virtio_mem_bbm_get_bb_state(struct virtio_mem *vm,
+								unsigned long bb_id)
+{
+	return vm->bbm.bb_states[bb_id - vm->bbm.first_bb_id];
+}
+
+/*
+ * Prepare the big block state array for the next big block.
+ */
+static int virtio_mem_bbm_bb_states_prepare_next_bb(struct virtio_mem *vm)
+{
+	unsigned long old_bytes = vm->bbm.next_bb_id - vm->bbm.first_bb_id;
+	unsigned long new_bytes = old_bytes + 1;
+	int old_pages = PFN_UP(old_bytes);
+	int new_pages = PFN_UP(new_bytes);
+	uint8_t *new_array;
+
+	if (vm->bbm.bb_states && old_pages == new_pages)
+		return 0;
+
+	new_array = vzalloc(new_pages * PAGE_SIZE);
+	if (!new_array)
+		return -ENOMEM;
+
+	mutex_lock(&vm->hotplug_mutex);
+	if (vm->bbm.bb_states)
+		memcpy(new_array, vm->bbm.bb_states, old_pages * PAGE_SIZE);
+	vfree(vm->bbm.bb_states);
+	vm->bbm.bb_states = new_array;
+	mutex_unlock(&vm->hotplug_mutex);
+
+	return 0;
+}
+
+#define virtio_mem_bbm_for_each_bb(_vm, _bb_id, _state) \
+	for (_bb_id = vm->bbm.first_bb_id; \
+	     _bb_id < vm->bbm.next_bb_id && _vm->bbm.bb_count[_state]; \
+	     _bb_id++) \
+		if (virtio_mem_bbm_get_bb_state(_vm, _bb_id) == _state)
+
 /*
  * Set the state of a memory block, taking care of the state counter.
  */
@@ -502,6 +623,17 @@ static int virtio_mem_sbm_add_mb(struct virtio_mem *vm, unsigned long mb_id)
 	return virtio_mem_add_memory(vm, addr, size);
 }
 
+/*
+ * See virtio_mem_add_memory(): Try adding a big block.
+ */
+static int virtio_mem_bbm_add_bb(struct virtio_mem *vm, unsigned long bb_id)
+{
+	const uint64_t addr = virtio_mem_bb_id_to_phys(vm, bb_id);
+	const uint64_t size = vm->bbm.bb_size;
+
+	return virtio_mem_add_memory(vm, addr, size);
+}
+
 /*
  * Try removing memory from Linux. Will only fail if memory blocks aren't
  * offline.
@@ -729,20 +861,33 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 	struct memory_notify *mhp = arg;
 	const unsigned long start = PFN_PHYS(mhp->start_pfn);
 	const unsigned long size = PFN_PHYS(mhp->nr_pages);
-	const unsigned long mb_id = virtio_mem_phys_to_mb_id(start);
 	int rc = NOTIFY_OK;
+	unsigned long id;
 
 	if (!virtio_mem_overlaps_range(vm, start, size))
 		return NOTIFY_DONE;
 
+	if (vm->in_sbm) {
+		id = virtio_mem_phys_to_mb_id(start);
 		/*
-	 * Memory is onlined/offlined in memory block granularity. We cannot
-	 * cross virtio-mem device boundaries and memory block boundaries. Bail
-	 * out if this ever changes.
+		 * In SBM, we add memory in separate memory blocks - we expect
+		 * it to be onlined/offlined in the same granularity. Bail out
+		 * if this ever changes.
 		 */
 		if (WARN_ON_ONCE(size != memory_block_size_bytes() ||
 				 !IS_ALIGNED(start, memory_block_size_bytes())))
 			return NOTIFY_BAD;
+	} else {
+		id = virtio_mem_phys_to_bb_id(vm, start);
+		/*
+		 * In BBM, we only care about onlining/offlining happening
+		 * within a single big block, we don't care about the
+		 * actual granularity as we don't track individual Linux
+		 * memory blocks.
+		 */
+		if (WARN_ON_ONCE(id != virtio_mem_phys_to_bb_id(vm, start + size - 1)))
+			return NOTIFY_BAD;
+	}
 
 	/*
 	 * Avoid circular locking lockdep warnings. We lock the mutex
@@ -761,7 +906,8 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 			break;
 		}
 		vm->hotplug_active = true;
-		virtio_mem_sbm_notify_going_offline(vm, mb_id);
+		if (vm->in_sbm)
+			virtio_mem_sbm_notify_going_offline(vm, id);
 		break;
 	case MEM_GOING_ONLINE:
 		mutex_lock(&vm->hotplug_mutex);
@@ -771,10 +917,12 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 			break;
 		}
 		vm->hotplug_active = true;
-		rc = virtio_mem_sbm_notify_going_online(vm, mb_id);
+		if (vm->in_sbm)
+			rc = virtio_mem_sbm_notify_going_online(vm, id);
 		break;
 	case MEM_OFFLINE:
-		virtio_mem_sbm_notify_offline(vm, mb_id);
+		if (vm->in_sbm)
+			virtio_mem_sbm_notify_offline(vm, id);
 
 		atomic64_add(size, &vm->offline_size);
 		/*
@@ -788,7 +936,8 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 		mutex_unlock(&vm->hotplug_mutex);
 		break;
 	case MEM_ONLINE:
-		virtio_mem_sbm_notify_online(vm, mb_id);
+		if (vm->in_sbm)
+			virtio_mem_sbm_notify_online(vm, id);
 
 		atomic64_sub(size, &vm->offline_size);
 		/*
@@ -807,7 +956,8 @@ static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
 	case MEM_CANCEL_OFFLINE:
 		if (!vm->hotplug_active)
 			break;
-		virtio_mem_sbm_notify_cancel_offline(vm, mb_id);
+		if (vm->in_sbm)
+			virtio_mem_sbm_notify_cancel_offline(vm, id);
 		vm->hotplug_active = false;
 		mutex_unlock(&vm->hotplug_mutex);
 		break;
@@ -978,27 +1128,29 @@ static void virtio_mem_fake_offline_cancel_offline(unsigned long pfn,
 static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
 {
 	const unsigned long addr = page_to_phys(page);
-	const unsigned long mb_id = virtio_mem_phys_to_mb_id(addr);
+	unsigned long id, sb_id;
 	struct virtio_mem *vm;
-	int sb_id;
+	bool do_online;
 
-	/*
-	 * We exploit here that subblocks have at least MAX_ORDER_NR_PAGES.
-	 * size/alignment and that this callback is is called with such a
-	 * size/alignment. So we cannot cross subblocks and therefore
-	 * also not memory blocks.
-	 */
 	rcu_read_lock();
 	list_for_each_entry_rcu(vm, &virtio_mem_devices, next) {
 		if (!virtio_mem_contains_range(vm, addr, PFN_PHYS(1 << order)))
 			continue;
 
-		sb_id = virtio_mem_phys_to_sb_id(vm, addr);
+		if (vm->in_sbm) {
 			/*
-		 * If plugged, online the pages, otherwise, set them fake
-		 * offline (PageOffline).
+			 * We exploit here that subblocks have at least
+			 * MAX_ORDER_NR_PAGES size/alignment - so we cannot
+			 * cross subblocks within one call.
 			 */
-		if (virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id, 1))
+			id = virtio_mem_phys_to_mb_id(addr);
+			sb_id = virtio_mem_phys_to_sb_id(vm, addr);
+			do_online = virtio_mem_sbm_test_sb_plugged(vm, id,
+								   sb_id, 1);
+		} else {
+			do_online = true;
+		}
+		if (do_online)
 			generic_online_page(page, order);
 		else
 			virtio_mem_set_fake_offline(PFN_DOWN(addr), 1 << order,
@@ -1178,6 +1330,32 @@ static int virtio_mem_sbm_unplug_sb(struct virtio_mem *vm, unsigned long mb_id,
 	return rc;
 }
 
+/*
+ * Request to unplug a big block.
+ *
+ * Will not modify the state of the big block.
+ */
+static int virtio_mem_bbm_unplug_bb(struct virtio_mem *vm, unsigned long bb_id)
+{
+	const uint64_t addr = virtio_mem_bb_id_to_phys(vm, bb_id);
+	const uint64_t size = vm->bbm.bb_size;
+
+	return virtio_mem_send_unplug_request(vm, addr, size);
+}
+
+/*
+ * Request to plug a big block.
+ *
+ * Will not modify the state of the big block.
+ */
+static int virtio_mem_bbm_plug_bb(struct virtio_mem *vm, unsigned long bb_id)
+{
+	const uint64_t addr = virtio_mem_bb_id_to_phys(vm, bb_id);
+	const uint64_t size = vm->bbm.bb_size;
+
+	return virtio_mem_send_plug_request(vm, addr, size);
+}
+
 /*
  * Unplug the desired number of plugged subblocks of a offline or not-added
  * memory block. Will fail if any subblock cannot get unplugged (instead of
@@ -1363,10 +1541,7 @@ static int virtio_mem_sbm_plug_any_sb(struct virtio_mem *vm,
 	return 0;
 }
 
-/*
- * Try to plug the requested amount of memory.
- */
-static int virtio_mem_plug_request(struct virtio_mem *vm, uint64_t diff)
+static int virtio_mem_sbm_plug_request(struct virtio_mem *vm, uint64_t diff)
 {
 	uint64_t nb_sb = diff / vm->sbm.sb_size;
 	unsigned long mb_id;
@@ -1433,6 +1608,112 @@ static int virtio_mem_plug_request(struct virtio_mem *vm, uint64_t diff)
 	return rc;
 }
 
+/*
+ * Plug a big block and add it to Linux.
+ *
+ * Will modify the state of the big block.
+ */
+static int virtio_mem_bbm_plug_and_add_bb(struct virtio_mem *vm,
+					  unsigned long bb_id)
+{
+	int rc;
+
+	if (WARN_ON_ONCE(virtio_mem_bbm_get_bb_state(vm, bb_id) !=
+			 VIRTIO_MEM_BBM_BB_UNUSED))
+		return -EINVAL;
+
+	rc = virtio_mem_bbm_plug_bb(vm, bb_id);
+	if (rc)
+		return rc;
+	virtio_mem_bbm_set_bb_state(vm, bb_id, VIRTIO_MEM_BBM_BB_ADDED);
+
+	rc = virtio_mem_bbm_add_bb(vm, bb_id);
+	if (rc) {
+		if (!virtio_mem_bbm_unplug_bb(vm, bb_id))
+			virtio_mem_bbm_set_bb_state(vm, bb_id,
+						    VIRTIO_MEM_BBM_BB_UNUSED);
+		else
+			/* Retry from the main loop. */
+			virtio_mem_bbm_set_bb_state(vm, bb_id,
+						    VIRTIO_MEM_BBM_BB_PLUGGED);
+		return rc;
+	}
+	return 0;
+}
+
+/*
+ * Prepare tracking data for the next big block.
+ */
+static int virtio_mem_bbm_prepare_next_bb(struct virtio_mem *vm,
+					  unsigned long *bb_id)
+{
+	int rc;
+
+	if (vm->bbm.next_bb_id > vm->bbm.last_usable_bb_id)
+		return -ENOSPC;
+
+	/* Resize the big block state array if required. */
+	rc = virtio_mem_bbm_bb_states_prepare_next_bb(vm);
+	if (rc)
+		return rc;
+
+	vm->bbm.bb_count[VIRTIO_MEM_BBM_BB_UNUSED]++;
+	*bb_id = vm->bbm.next_bb_id;
+	vm->bbm.next_bb_id++;
+	return 0;
+}
+
+static int virtio_mem_bbm_plug_request(struct virtio_mem *vm, uint64_t diff)
+{
+	uint64_t nb_bb = diff / vm->bbm.bb_size;
+	unsigned long bb_id;
+	int rc;
+
+	if (!nb_bb)
+		return 0;
+
+	/* Try to plug and add unused big blocks */
+	virtio_mem_bbm_for_each_bb(vm, bb_id, VIRTIO_MEM_BBM_BB_UNUSED) {
+		if (!virtio_mem_could_add_memory(vm, vm->bbm.bb_size))
+			return -ENOSPC;
+
+		rc = virtio_mem_bbm_plug_and_add_bb(vm, bb_id);
+		if (!rc)
+			nb_bb--;
+		if (rc || !nb_bb)
+			return rc;
+		cond_resched();
+	}
+
+	/* Try to prepare, plug and add new big blocks */
+	while (nb_bb) {
+		if (!virtio_mem_could_add_memory(vm, vm->bbm.bb_size))
+			return -ENOSPC;
+
+		rc = virtio_mem_bbm_prepare_next_bb(vm, &bb_id);
+		if (rc)
+			return rc;
+		rc = virtio_mem_bbm_plug_and_add_bb(vm, bb_id);
+		if (!rc)
+			nb_bb--;
+		if (rc)
+			return rc;
+		cond_resched();
+	}
+
+	return 0;
+}
+
+/*
+ * Try to plug the requested amount of memory.
+ */
+static int virtio_mem_plug_request(struct virtio_mem *vm, uint64_t diff)
+{
+	if (vm->in_sbm)
+		return virtio_mem_sbm_plug_request(vm, diff);
+	return virtio_mem_bbm_plug_request(vm, diff);
+}
+
 /*
  * Unplug the desired number of plugged subblocks of an offline memory block.
  * Will fail if any subblock cannot get unplugged (instead of skipping it).
@@ -1571,10 +1852,7 @@ static int virtio_mem_sbm_unplug_any_sb_online(struct virtio_mem *vm,
 	return 0;
 }
 
-/*
- * Try to unplug the requested amount of memory.
- */
-static int virtio_mem_unplug_request(struct virtio_mem *vm, uint64_t diff)
+static int virtio_mem_sbm_unplug_request(struct virtio_mem *vm, uint64_t diff)
 {
 	uint64_t nb_sb = diff / vm->sbm.sb_size;
 	unsigned long mb_id;
@@ -1640,20 +1918,42 @@ static int virtio_mem_unplug_request(struct virtio_mem *vm, uint64_t diff)
 	return rc;
 }
 
+/*
+ * Try to unplug the requested amount of memory.
+ */
+static int virtio_mem_unplug_request(struct virtio_mem *vm, uint64_t diff)
+{
+	if (vm->in_sbm)
+		return virtio_mem_sbm_unplug_request(vm, diff);
+	return -EBUSY;
+}
+
 /*
  * Try to unplug all blocks that couldn't be unplugged before, for example,
  * because the hypervisor was busy.
  */
 static int virtio_mem_unplug_pending_mb(struct virtio_mem *vm)
 {
-	unsigned long mb_id;
+	unsigned long id;
 	int rc;
 
-	virtio_mem_sbm_for_each_mb(vm, mb_id, VIRTIO_MEM_SBM_MB_PLUGGED) {
-		rc = virtio_mem_sbm_unplug_mb(vm, mb_id);
+	if (!vm->in_sbm) {
+		virtio_mem_bbm_for_each_bb(vm, id,
+					   VIRTIO_MEM_BBM_BB_PLUGGED) {
+			rc = virtio_mem_bbm_unplug_bb(vm, id);
 			if (rc)
 				return rc;
-		virtio_mem_sbm_set_mb_state(vm, mb_id,
+			virtio_mem_bbm_set_bb_state(vm, id,
+						    VIRTIO_MEM_BBM_BB_UNUSED);
+		}
+		return 0;
+	}
+
+	virtio_mem_sbm_for_each_mb(vm, id, VIRTIO_MEM_SBM_MB_PLUGGED) {
+		rc = virtio_mem_sbm_unplug_mb(vm, id);
+		if (rc)
+			return rc;
+		virtio_mem_sbm_set_mb_state(vm, id,
 					    VIRTIO_MEM_SBM_MB_UNUSED);
 	}
 
@@ -1679,7 +1979,13 @@ static void virtio_mem_refresh_config(struct virtio_mem *vm)
 			usable_region_size, &usable_region_size);
 	end_addr = vm->addr + usable_region_size;
 	end_addr = min(end_addr, phys_limit);
-	vm->sbm.last_usable_mb_id = virtio_mem_phys_to_mb_id(end_addr) - 1;
+
+	if (vm->in_sbm)
+		vm->sbm.last_usable_mb_id =
+					 virtio_mem_phys_to_mb_id(end_addr) - 1;
+	else
+		vm->bbm.last_usable_bb_id =
+				     virtio_mem_phys_to_bb_id(vm, end_addr) - 1;
 
 	/* see if there is a request to change the size */
 	virtio_cread_le(vm->vdev, struct virtio_mem_config, requested_size,
@@ -1802,6 +2108,7 @@ static int virtio_mem_init_vq(struct virtio_mem *vm)
 static int virtio_mem_init(struct virtio_mem *vm)
 {
 	const uint64_t phys_limit = 1UL << MAX_PHYSMEM_BITS;
+	uint64_t sb_size, addr;
 	uint16_t node_id;
 
 	if (!vm->vdev->config->get) {
@@ -1834,16 +2141,6 @@ static int virtio_mem_init(struct virtio_mem *vm)
 	if (vm->nid == NUMA_NO_NODE)
 		vm->nid = memory_add_physaddr_to_nid(vm->addr);
 
-	/*
-	 * We always hotplug memory in memory block granularity. This way,
-	 * we have to wait for exactly one memory block to online.
-	 */
-	if (vm->device_block_size > memory_block_size_bytes()) {
-		dev_err(&vm->vdev->dev,
-			"The block size is not supported (too big).\n");
-		return -EINVAL;
-	}
-
 	/* bad device setup - warn only */
 	if (!IS_ALIGNED(vm->addr, memory_block_size_bytes()))
 		dev_warn(&vm->vdev->dev,
@@ -1863,20 +2160,35 @@ static int virtio_mem_init(struct virtio_mem *vm)
 	 * - Is required for now for alloc_contig_range() to work reliably -
 	 *   it doesn't properly handle smaller granularity on ZONE_NORMAL.
 	 */
-	vm->sbm.sb_size = max_t(uint64_t, MAX_ORDER_NR_PAGES,
+	sb_size = max_t(uint64_t, MAX_ORDER_NR_PAGES,
 			pageblock_nr_pages) * PAGE_SIZE;
-	vm->sbm.sb_size = max_t(uint64_t, vm->device_block_size,
-				vm->sbm.sb_size);
-	vm->sbm.sbs_per_mb = memory_block_size_bytes() / vm->sbm.sb_size;
+	sb_size = max_t(uint64_t, vm->device_block_size, sb_size);
+
+	if (sb_size < memory_block_size_bytes()) {
+		/* SBM: At least two subblocks per Linux memory block. */
+		vm->in_sbm = true;
+		vm->sbm.sb_size = sb_size;
+		vm->sbm.sbs_per_mb = memory_block_size_bytes() /
+				     vm->sbm.sb_size;
 
 		/* Round up to the next full memory block */
-	vm->sbm.first_mb_id = virtio_mem_phys_to_mb_id(vm->addr - 1 +
-						       memory_block_size_bytes());
+		addr = vm->addr + memory_block_size_bytes() - 1;
+		vm->sbm.first_mb_id = virtio_mem_phys_to_mb_id(addr);
 		vm->sbm.next_mb_id = vm->sbm.first_mb_id;
+	} else {
+		/* BBM: At least one Linux memory block. */
+		vm->bbm.bb_size = vm->device_block_size;
+
+		vm->bbm.first_bb_id = virtio_mem_phys_to_bb_id(vm, vm->addr);
+		vm->bbm.next_bb_id = vm->bbm.first_bb_id;
+	}
 
 	/* Prepare the offline threshold - make sure we can add two blocks. */
 	vm->offline_threshold = max_t(uint64_t, 2 * memory_block_size_bytes(),
 				      VIRTIO_MEM_DEFAULT_OFFLINE_THRESHOLD);
+	/* In BBM, we also want at least two big blocks. */
+	vm->offline_threshold = max_t(uint64_t, 2 * vm->bbm.bb_size,
+				      vm->offline_threshold);
 
 	dev_info(&vm->vdev->dev, "start address: 0x%llx", vm->addr);
 	dev_info(&vm->vdev->dev, "region size: 0x%llx", vm->region_size);
@@ -1884,8 +2196,12 @@ static int virtio_mem_init(struct virtio_mem *vm)
 		 (unsigned long long)vm->device_block_size);
 	dev_info(&vm->vdev->dev, "memory block size: 0x%lx",
 		 memory_block_size_bytes());
+	if (vm->in_sbm)
 		dev_info(&vm->vdev->dev, "subblock size: 0x%llx",
 			 (unsigned long long)vm->sbm.sb_size);
+	else
+		dev_info(&vm->vdev->dev, "big block size: 0x%llx",
+			 (unsigned long long)vm->bbm.bb_size);
 	if (vm->nid != NUMA_NO_NODE && IS_ENABLED(CONFIG_NUMA))
 		dev_info(&vm->vdev->dev, "nid: %d", vm->nid);
 
@@ -2042,9 +2358,10 @@ static void virtio_mem_remove(struct virtio_device *vdev)
 	cancel_work_sync(&vm->wq);
 	hrtimer_cancel(&vm->retry_timer);
 
+	if (vm->in_sbm) {
 		/*
-	 * After we unregistered our callbacks, user space can online partially
-	 * plugged offline blocks. Make sure to remove them.
+		 * After we unregistered our callbacks, user space can online
+		 * partially plugged offline blocks. Make sure to remove them.
 		 */
 		virtio_mem_sbm_for_each_mb(vm, mb_id,
 					   VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL) {
@@ -2055,9 +2372,10 @@ static void virtio_mem_remove(struct virtio_device *vdev)
 		}
 		/*
 		 * After we unregistered our callbacks, user space can no longer
-	 * offline partially plugged online memory blocks. No need to worry
-	 * about them.
+		 * offline partially plugged online memory blocks. No need to
+		 * worry about them.
 		 */
+	}
 
 	/* unregister callbacks */
 	unregister_virtio_mem_device(vm);
@@ -2076,8 +2394,12 @@ static void virtio_mem_remove(struct virtio_device *vdev)
 	}
 
 	/* remove all tracking data - no locking needed */
+	if (vm->in_sbm) {
 		vfree(vm->sbm.mb_states);
 		vfree(vm->sbm.sb_states);
+	} else {
+		vfree(vm->bbm.bb_states);
+	}
 
 	/* reset the device and cleanup the queues */
 	vdev->config->reset(vdev);