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Subject: Re: [PATCH v1 25/29] virtio-mem: Big Block Mode (BBM) memory hotplug
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        virtualization@lists.linux-foundation.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Oscar Salvador <osalvador@suse.de>
References: <20201012125323.17509-1-david@redhat.com>
 <20201012125323.17509-26-david@redhat.com>
 <20201019022630.GB54484@L-31X9LVDL-1304.local>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <c77ecfa5-4664-b6a3-632d-8133c91c296f@redhat.com>
Date:   Mon, 19 Oct 2020 11:15:13 +0200
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On 19.10.20 04:26, Wei Yang wrote:
> On Mon, Oct 12, 2020 at 02:53:19PM +0200, David Hildenbrand wrote:
>> 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>
>> ---
>> drivers/virtio/virtio_mem.c | 484 ++++++++++++++++++++++++++++++------
>> 1 file changed, 402 insertions(+), 82 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
>> index e68d0d99590c..4d396ef98a92 100644
>> --- a/drivers/virtio/virtio_mem.c
>> +++ b/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 +132,9 @@ struct virtio_mem {
>> 	atomic64_t offline_size;
>> 	uint64_t offline_threshold;
>>
>> +	/* If set, the driver is in SBM, otherwise in BBM. */
>> +	bool in_sbm;
>> +
>> 	struct {
>> 		/* Id of the first memory block of this device. */
>> 		unsigned long first_mb_id;
>> @@ -151,9 +173,27 @@ struct virtio_mem {
>> 		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 (un)plugged, 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 +287,24 @@ 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 +317,67 @@ 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.
>>  */
>> @@ -504,6 +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.
>> @@ -731,20 +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;
>>
>> -	/*
>> -	 * 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.
>> -	 */
>> -	if (WARN_ON_ONCE(size != memory_block_size_bytes() ||
>> -			 !IS_ALIGNED(start, memory_block_size_bytes())))
>> -		return NOTIFY_BAD;
>> +	if (vm->in_sbm) {
>> +		id = virtio_mem_phys_to_mb_id(start);
>> +		/*
>> +		 * 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
>> @@ -763,7 +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);
>> @@ -773,10 +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);
>> 		/*
>> @@ -790,7 +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);
>> 		/*
>> @@ -809,7 +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;
>> @@ -980,27 +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 plugged, online the pages, otherwise, set them fake
>> -		 * offline (PageOffline).
>> -		 */
>> -		if (virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id, 1))
>> +		if (vm->in_sbm) {
>> +			/*
>> +			 * We exploit here that subblocks have at least
>> +			 * MAX_ORDER_NR_PAGES size/alignment - so we cannot
>> +			 * cross subblocks within one call.
>> +			 */
>> +			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,
>> @@ -1180,6 +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
>> @@ -1365,10 +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;
>> @@ -1435,6 +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).
>> @@ -1573,10 +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;
>> @@ -1642,20 +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_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, mb_id,
>> +		virtio_mem_sbm_set_mb_state(vm, id,
>> 					    VIRTIO_MEM_SBM_MB_UNUSED);
>> 	}
>>
>> @@ -1681,7 +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,
>> @@ -1804,6 +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) {
>> @@ -1836,16 +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,
>> @@ -1865,20 +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,
>> -				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, MAX_ORDER_NR_PAGES,
>> +			pageblock_nr_pages) * PAGE_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 */
>> +		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;
>>
>> -	/* 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());
>> -	vm->sbm.next_mb_id = vm->sbm.first_mb_id;
>> +		vm->bbm.first_bb_id = virtio_mem_phys_to_bb_id(vm, vm->addr);
> 
> Per my understanding, vm->addr is not guaranteed to be bb_size aligned, right?
> 

The virtio spec enforces alignment to device block size. (QEMU is buggy
with bigger block sizes, though. Fix is on the QEMU list.)

> Why not round up to next big block?

Will implicitly be done in patch #26, where it might no longer be
guaranteed.

-- 
Thanks,

David / dhildenb