[linux-2.6-microblaze.git] / drivers / virtio / virtio_mem.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Virtio-mem device driver.
 *
 * Copyright Red Hat, Inc. 2020
 *
 * Author(s): David Hildenbrand <david@redhat.com>
 */

#include <linux/virtio.h>
#include <linux/virtio_mem.h>
#include <linux/workqueue.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/memory_hotplug.h>
#include <linux/memory.h>
#include <linux/hrtimer.h>
#include <linux/crash_dump.h>
#include <linux/mutex.h>
#include <linux/bitmap.h>
#include <linux/lockdep.h>

#include <acpi/acpi_numa.h>

static bool unplug_online = true;
module_param(unplug_online, bool, 0644);
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
 *   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.
 *
 * 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
 * onlined to the same zone - virtio-mem relies on this behavior.
 */

/*
 * State of a Linux memory block in SBM.
 */
enum virtio_mem_sbm_mb_state {
        /* Unplugged, not added to Linux. Can be reused later. */
        VIRTIO_MEM_SBM_MB_UNUSED = 0,
        /* (Partially) plugged, not added to Linux. Error on add_memory(). */
        VIRTIO_MEM_SBM_MB_PLUGGED,
        /* Fully plugged, fully added to Linux, offline. */
        VIRTIO_MEM_SBM_MB_OFFLINE,
        /* Partially plugged, fully added to Linux, offline. */
        VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL,
        /* Fully plugged, fully added to Linux, online. */
        VIRTIO_MEM_SBM_MB_ONLINE,
        /* Partially plugged, fully added to Linux, online. */
        VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL,
        VIRTIO_MEM_SBM_MB_COUNT
};

struct virtio_mem {
        struct virtio_device *vdev;

        /* We might first have to unplug all memory when starting up. */
        bool unplug_all_required;

        /* Workqueue that processes the plug/unplug requests. */
        struct work_struct wq;
        atomic_t wq_active;
        atomic_t config_changed;

        /* Virtqueue for guest->host requests. */
        struct virtqueue *vq;

        /* Wait for a host response to a guest request. */
        wait_queue_head_t host_resp;

        /* Space for one guest request and the host response. */
        struct virtio_mem_req req;
        struct virtio_mem_resp resp;

        /* The current size of the device. */
        uint64_t plugged_size;
        /* The requested size of the device. */
        uint64_t requested_size;

        /* The device block size (for communicating with the device). */
        uint64_t device_block_size;
        /* The determined node id for all memory of the device. */
        int nid;
        /* Physical start address of the memory region. */
        uint64_t addr;
        /* Maximum region size in bytes. */
        uint64_t region_size;

        /* The parent resource for all memory added via this device. */
        struct resource *parent_resource;
        /*
         * Copy of "System RAM (virtio_mem)" to be used for
         * add_memory_driver_managed().
         */
        const char *resource_name;

        /*
         * We don't want to add too much memory if it's not getting onlined,
         * to avoid running OOM. Besides this threshold, we allow to have at
         * least two offline blocks at a time (whatever is bigger).
         */
#define VIRTIO_MEM_DEFAULT_OFFLINE_THRESHOLD            (1024 * 1024 * 1024)
        atomic64_t offline_size;
        uint64_t offline_threshold;

        struct {
                /* Id of the first memory block of this device. */
                unsigned long first_mb_id;
                /* Id of the last usable memory block of this device. */
                unsigned long last_usable_mb_id;
                /* Id of the next memory bock to prepare when needed. */
                unsigned long next_mb_id;

                /* The subblock size. */
                uint64_t sb_size;
                /* The number of subblocks per Linux memory block. */
                uint32_t sbs_per_mb;

                /* Summary of all memory block states. */
                unsigned long mb_count[VIRTIO_MEM_SBM_MB_COUNT];

                /*
                 * 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.
                 */
                uint8_t *mb_states;

                /*
                 * Bitmap: one bit per subblock. Allocated similar to
                 * sbm.mb_states.
                 *
                 * 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.
                 */
                unsigned long *sb_states;
        } sbm;

        /*
         * Mutex that protects the sbm.mb_count, sbm.mb_states, and
         * sbm.sb_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
         * plugged/unplugged.
         */
        struct mutex hotplug_mutex;
        bool hotplug_active;

        /* An error occurred we cannot handle - stop processing requests. */
        bool broken;

        /* The driver is being removed. */
        spinlock_t removal_lock;
        bool removing;

        /* Timer for retrying to plug/unplug memory. */
        struct hrtimer retry_timer;
        unsigned int retry_timer_ms;
#define VIRTIO_MEM_RETRY_TIMER_MIN_MS           50000
#define VIRTIO_MEM_RETRY_TIMER_MAX_MS           300000

        /* Memory notifier (online/offline events). */
        struct notifier_block memory_notifier;

        /* Next device in the list of virtio-mem devices. */
        struct list_head next;
};

/*
 * We have to share a single online_page callback among all virtio-mem
 * devices. We use RCU to iterate the list in the callback.
 */
static DEFINE_MUTEX(virtio_mem_mutex);
static LIST_HEAD(virtio_mem_devices);

static void virtio_mem_online_page_cb(struct page *page, unsigned int order);
static void virtio_mem_fake_offline_going_offline(unsigned long pfn,
                                                  unsigned long nr_pages);
static void virtio_mem_fake_offline_cancel_offline(unsigned long pfn,
                                                   unsigned long nr_pages);
static void virtio_mem_retry(struct virtio_mem *vm);

/*
 * Register a virtio-mem device so it will be considered for the online_page
 * callback.
 */
static int register_virtio_mem_device(struct virtio_mem *vm)
{
        int rc = 0;

        /* First device registers the callback. */
        mutex_lock(&virtio_mem_mutex);
        if (list_empty(&virtio_mem_devices))
                rc = set_online_page_callback(&virtio_mem_online_page_cb);
        if (!rc)
                list_add_rcu(&vm->next, &virtio_mem_devices);
        mutex_unlock(&virtio_mem_mutex);

        return rc;
}

/*
 * Unregister a virtio-mem device so it will no longer be considered for the
 * online_page callback.
 */
static void unregister_virtio_mem_device(struct virtio_mem *vm)
{
        /* Last device unregisters the callback. */
        mutex_lock(&virtio_mem_mutex);
        list_del_rcu(&vm->next);
        if (list_empty(&virtio_mem_devices))
                restore_online_page_callback(&virtio_mem_online_page_cb);
        mutex_unlock(&virtio_mem_mutex);

        synchronize_rcu();
}

/*
 * Calculate the memory block id of a given address.
 */
static unsigned long virtio_mem_phys_to_mb_id(unsigned long addr)
{
        return addr / memory_block_size_bytes();
}

/*
 * Calculate the physical start address of a given memory block id.
 */
static unsigned long virtio_mem_mb_id_to_phys(unsigned long mb_id)
{
        return mb_id * memory_block_size_bytes();
}

/*
 * Calculate the subblock id of a given address.
 */
static unsigned long virtio_mem_phys_to_sb_id(struct virtio_mem *vm,
                                              unsigned long addr)
{
        const unsigned long mb_id = virtio_mem_phys_to_mb_id(addr);
        const unsigned long mb_addr = virtio_mem_mb_id_to_phys(mb_id);

        return (addr - mb_addr) / vm->sbm.sb_size;
}

/*
 * Set the state of a memory block, taking care of the state counter.
 */
static void virtio_mem_sbm_set_mb_state(struct virtio_mem *vm,
                                        unsigned long mb_id, uint8_t state)
{
        const unsigned long idx = mb_id - vm->sbm.first_mb_id;
        uint8_t old_state;

        old_state = vm->sbm.mb_states[idx];
        vm->sbm.mb_states[idx] = state;

        BUG_ON(vm->sbm.mb_count[old_state] == 0);
        vm->sbm.mb_count[old_state]--;
        vm->sbm.mb_count[state]++;
}

/*
 * Get the state of a memory block.
 */
static uint8_t virtio_mem_sbm_get_mb_state(struct virtio_mem *vm,
                                           unsigned long mb_id)
{
        const unsigned long idx = mb_id - vm->sbm.first_mb_id;

        return vm->sbm.mb_states[idx];
}

/*
 * Prepare the state array for the next memory block.
 */
static int virtio_mem_sbm_mb_states_prepare_next_mb(struct virtio_mem *vm)
{
        int old_pages = PFN_UP(vm->sbm.next_mb_id - vm->sbm.first_mb_id);
        int new_pages = PFN_UP(vm->sbm.next_mb_id - vm->sbm.first_mb_id + 1);
        uint8_t *new_array;

        if (vm->sbm.mb_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->sbm.mb_states)
                memcpy(new_array, vm->sbm.mb_states, old_pages * PAGE_SIZE);
        vfree(vm->sbm.mb_states);
        vm->sbm.mb_states = new_array;
        mutex_unlock(&vm->hotplug_mutex);

        return 0;
}

#define virtio_mem_sbm_for_each_mb(_vm, _mb_id, _state) \
        for (_mb_id = _vm->sbm.first_mb_id; \
             _mb_id < _vm->sbm.next_mb_id && _vm->sbm.mb_count[_state]; \
             _mb_id++) \
                if (virtio_mem_sbm_get_mb_state(_vm, _mb_id) == _state)

#define virtio_mem_sbm_for_each_mb_rev(_vm, _mb_id, _state) \
        for (_mb_id = _vm->sbm.next_mb_id - 1; \
             _mb_id >= _vm->sbm.first_mb_id && _vm->sbm.mb_count[_state]; \
             _mb_id--) \
                if (virtio_mem_sbm_get_mb_state(_vm, _mb_id) == _state)

/*
 * Calculate the bit number in the subblock bitmap for the given subblock
 * inside the given memory block.
 */
static int virtio_mem_sbm_sb_state_bit_nr(struct virtio_mem *vm,
                                          unsigned long mb_id, int sb_id)
{
        return (mb_id - vm->sbm.first_mb_id) * vm->sbm.sbs_per_mb + sb_id;
}

/*
 * Mark all selected subblocks plugged.
 *
 * Will not modify the state of the memory block.
 */
static void virtio_mem_sbm_set_sb_plugged(struct virtio_mem *vm,
                                          unsigned long mb_id, int sb_id,
                                          int count)
{
        const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, sb_id);

        __bitmap_set(vm->sbm.sb_states, bit, count);
}

/*
 * Mark all selected subblocks unplugged.
 *
 * Will not modify the state of the memory block.
 */
static void virtio_mem_sbm_set_sb_unplugged(struct virtio_mem *vm,
                                            unsigned long mb_id, int sb_id,
                                            int count)
{
        const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, sb_id);

        __bitmap_clear(vm->sbm.sb_states, bit, count);
}

/*
 * Test if all selected subblocks are plugged.
 */
static bool virtio_mem_sbm_test_sb_plugged(struct virtio_mem *vm,
                                           unsigned long mb_id, int sb_id,
                                           int count)
{
        const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, sb_id);

        if (count == 1)
                return test_bit(bit, vm->sbm.sb_states);

        /* TODO: Helper similar to bitmap_set() */
        return find_next_zero_bit(vm->sbm.sb_states, bit + count, bit) >=
               bit + count;
}

/*
 * Test if all selected subblocks are unplugged.
 */
static bool virtio_mem_sbm_test_sb_unplugged(struct virtio_mem *vm,
                                             unsigned long mb_id, int sb_id,
                                             int count)
{
        const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, sb_id);

        /* TODO: Helper similar to bitmap_set() */
        return find_next_bit(vm->sbm.sb_states, bit + count, bit) >=
               bit + count;
}

/*
 * Find the first unplugged subblock. Returns vm->sbm.sbs_per_mb in case there is
 * none.
 */
static int virtio_mem_sbm_first_unplugged_sb(struct virtio_mem *vm,
                                            unsigned long mb_id)
{
        const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, 0);

        return find_next_zero_bit(vm->sbm.sb_states,
                                  bit + vm->sbm.sbs_per_mb, bit) - bit;
}

/*
 * Prepare the subblock bitmap for the next memory block.
 */
static int virtio_mem_sbm_sb_states_prepare_next_mb(struct virtio_mem *vm)
{
        const unsigned long old_nb_mb = vm->sbm.next_mb_id - vm->sbm.first_mb_id;
        const unsigned long old_nb_bits = old_nb_mb * vm->sbm.sbs_per_mb;
        const unsigned long new_nb_bits = (old_nb_mb + 1) * vm->sbm.sbs_per_mb;
        int old_pages = PFN_UP(BITS_TO_LONGS(old_nb_bits) * sizeof(long));
        int new_pages = PFN_UP(BITS_TO_LONGS(new_nb_bits) * sizeof(long));
        unsigned long *new_bitmap, *old_bitmap;

        if (vm->sbm.sb_states && old_pages == new_pages)
                return 0;

        new_bitmap = vzalloc(new_pages * PAGE_SIZE);
        if (!new_bitmap)
                return -ENOMEM;

        mutex_lock(&vm->hotplug_mutex);
        if (new_bitmap)
                memcpy(new_bitmap, vm->sbm.sb_states, old_pages * PAGE_SIZE);

        old_bitmap = vm->sbm.sb_states;
        vm->sbm.sb_states = new_bitmap;
        mutex_unlock(&vm->hotplug_mutex);

        vfree(old_bitmap);
        return 0;
}

/*
 * Test if we could add memory without creating too much offline memory -
 * to avoid running OOM if memory is getting onlined deferred.
 */
static bool virtio_mem_could_add_memory(struct virtio_mem *vm, uint64_t size)
{
        if (WARN_ON_ONCE(size > vm->offline_threshold))
                return false;

        return atomic64_read(&vm->offline_size) + size <= vm->offline_threshold;
}

/*
 * Try to add a memory block to Linux. This will usually only fail
 * if out of memory.
 *
 * Must not be called with the vm->hotplug_mutex held (possible deadlock with
 * onlining code).
 *
 * Will not modify the state of the memory block.
 */
static int virtio_mem_mb_add(struct virtio_mem *vm, unsigned long mb_id)
{
        const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
        const uint64_t size = memory_block_size_bytes();
        int rc;

        /*
         * When force-unloading the driver and we still have memory added to
         * Linux, the resource name has to stay.
         */
        if (!vm->resource_name) {
                vm->resource_name = kstrdup_const("System RAM (virtio_mem)",
                                                  GFP_KERNEL);
                if (!vm->resource_name)
                        return -ENOMEM;
        }

        dev_dbg(&vm->vdev->dev, "adding memory block: %lu\n", mb_id);
        /* Memory might get onlined immediately. */
        atomic64_add(size, &vm->offline_size);
        rc = add_memory_driver_managed(vm->nid, addr, size, vm->resource_name,
                                       MEMHP_MERGE_RESOURCE);
        if (rc)
                atomic64_sub(size, &vm->offline_size);
        return rc;
}

/*
 * Try to remove a memory block from Linux. Will only fail if the memory block
 * is not offline.
 *
 * Must not be called with the vm->hotplug_mutex held (possible deadlock with
 * onlining code).
 *
 * Will not modify the state of the memory block.
 */
static int virtio_mem_mb_remove(struct virtio_mem *vm, unsigned long mb_id)
{
        const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
        const uint64_t size = memory_block_size_bytes();
        int rc;

        dev_dbg(&vm->vdev->dev, "removing memory block: %lu\n", mb_id);
        rc = remove_memory(vm->nid, addr, size);
        if (!rc) {
                atomic64_sub(size, &vm->offline_size);
                /*
                 * We might have freed up memory we can now unplug, retry
                 * immediately instead of waiting.
                 */
                virtio_mem_retry(vm);
        }
        return rc;
}

/*
 * Try to offline and remove a memory block from Linux.
 *
 * Must not be called with the vm->hotplug_mutex held (possible deadlock with
 * onlining code).
 *
 * Will not modify the state of the memory block.
 */
static int virtio_mem_mb_offline_and_remove(struct virtio_mem *vm,
                                            unsigned long mb_id)
{
        const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
        const uint64_t size = memory_block_size_bytes();
        int rc;

        dev_dbg(&vm->vdev->dev, "offlining and removing memory block: %lu\n",
                mb_id);
        rc = offline_and_remove_memory(vm->nid, addr, size);
        if (!rc) {
                atomic64_sub(size, &vm->offline_size);
                /*
                 * We might have freed up memory we can now unplug, retry
                 * immediately instead of waiting.
                 */
                virtio_mem_retry(vm);
        }
        return rc;
}

/*
 * Trigger the workqueue so the device can perform its magic.
 */
static void virtio_mem_retry(struct virtio_mem *vm)
{
        unsigned long flags;

        spin_lock_irqsave(&vm->removal_lock, flags);
        if (!vm->removing)
                queue_work(system_freezable_wq, &vm->wq);
        spin_unlock_irqrestore(&vm->removal_lock, flags);
}

static int virtio_mem_translate_node_id(struct virtio_mem *vm, uint16_t node_id)
{
        int node = NUMA_NO_NODE;

#if defined(CONFIG_ACPI_NUMA)
        if (virtio_has_feature(vm->vdev, VIRTIO_MEM_F_ACPI_PXM))
                node = pxm_to_node(node_id);
#endif
        return node;
}

/*
 * Test if a virtio-mem device overlaps with the given range. Can be called
 * from (notifier) callbacks lockless.
 */
static bool virtio_mem_overlaps_range(struct virtio_mem *vm, uint64_t start,
                                      uint64_t size)
{
        return start < vm->addr + vm->region_size && vm->addr < start + size;
}

/*
 * Test if a virtio-mem device contains a given range. Can be called from
 * (notifier) callbacks lockless.
 */
static bool virtio_mem_contains_range(struct virtio_mem *vm, uint64_t start,
                                      uint64_t size)
{
        return start >= vm->addr && start + size <= vm->addr + vm->region_size;
}

static int virtio_mem_notify_going_online(struct virtio_mem *vm,
                                          unsigned long mb_id)
{
        switch (virtio_mem_sbm_get_mb_state(vm, mb_id)) {
        case VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL:
        case VIRTIO_MEM_SBM_MB_OFFLINE:
                return NOTIFY_OK;
        default:
                break;
        }
        dev_warn_ratelimited(&vm->vdev->dev,
                             "memory block onlining denied\n");
        return NOTIFY_BAD;
}

static void virtio_mem_notify_offline(struct virtio_mem *vm,
                                      unsigned long mb_id)
{
        switch (virtio_mem_sbm_get_mb_state(vm, mb_id)) {
        case VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL:
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL);
                break;
        case VIRTIO_MEM_SBM_MB_ONLINE:
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_OFFLINE);
                break;
        default:
                BUG();
                break;
        }
}

static void virtio_mem_notify_online(struct virtio_mem *vm, unsigned long mb_id)
{
        switch (virtio_mem_sbm_get_mb_state(vm, mb_id)) {
        case VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL:
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                        VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL);
                break;
        case VIRTIO_MEM_SBM_MB_OFFLINE:
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_ONLINE);
                break;
        default:
                BUG();
                break;
        }
}

static void virtio_mem_notify_going_offline(struct virtio_mem *vm,
                                            unsigned long mb_id)
{
        const unsigned long nr_pages = PFN_DOWN(vm->sbm.sb_size);
        unsigned long pfn;
        int sb_id;

        for (sb_id = 0; sb_id < vm->sbm.sbs_per_mb; sb_id++) {
                if (virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id, 1))
                        continue;
                pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
                               sb_id * vm->sbm.sb_size);
                virtio_mem_fake_offline_going_offline(pfn, nr_pages);
        }
}

static void virtio_mem_notify_cancel_offline(struct virtio_mem *vm,
                                             unsigned long mb_id)
{
        const unsigned long nr_pages = PFN_DOWN(vm->sbm.sb_size);
        unsigned long pfn;
        int sb_id;

        for (sb_id = 0; sb_id < vm->sbm.sbs_per_mb; sb_id++) {
                if (virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id, 1))
                        continue;
                pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
                               sb_id * vm->sbm.sb_size);
                virtio_mem_fake_offline_cancel_offline(pfn, nr_pages);
        }
}

/*
 * This callback will either be called synchronously from add_memory() or
 * asynchronously (e.g., triggered via user space). We have to be careful
 * with locking when calling add_memory().
 */
static int virtio_mem_memory_notifier_cb(struct notifier_block *nb,
                                         unsigned long action, void *arg)
{
        struct virtio_mem *vm = container_of(nb, struct virtio_mem,
                                             memory_notifier);
        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;

        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;

        /*
         * Avoid circular locking lockdep warnings. We lock the mutex
         * e.g., in MEM_GOING_ONLINE and unlock it in MEM_ONLINE. The
         * blocking_notifier_call_chain() has it's own lock, which gets unlocked
         * between both notifier calls and will bail out. False positive.
         */
        lockdep_off();

        switch (action) {
        case MEM_GOING_OFFLINE:
                mutex_lock(&vm->hotplug_mutex);
                if (vm->removing) {
                        rc = notifier_from_errno(-EBUSY);
                        mutex_unlock(&vm->hotplug_mutex);
                        break;
                }
                vm->hotplug_active = true;
                virtio_mem_notify_going_offline(vm, mb_id);
                break;
        case MEM_GOING_ONLINE:
                mutex_lock(&vm->hotplug_mutex);
                if (vm->removing) {
                        rc = notifier_from_errno(-EBUSY);
                        mutex_unlock(&vm->hotplug_mutex);
                        break;
                }
                vm->hotplug_active = true;
                rc = virtio_mem_notify_going_online(vm, mb_id);
                break;
        case MEM_OFFLINE:
                virtio_mem_notify_offline(vm, mb_id);

                atomic64_add(size, &vm->offline_size);
                /*
                 * Trigger the workqueue. Now that we have some offline memory,
                 * maybe we can handle pending unplug requests.
                 */
                if (!unplug_online)
                        virtio_mem_retry(vm);

                vm->hotplug_active = false;
                mutex_unlock(&vm->hotplug_mutex);
                break;
        case MEM_ONLINE:
                virtio_mem_notify_online(vm, mb_id);

                atomic64_sub(size, &vm->offline_size);
                /*
                 * Start adding more memory once we onlined half of our
                 * threshold. Don't trigger if it's possibly due to our actipn
                 * (e.g., us adding memory which gets onlined immediately from
                 * the core).
                 */
                if (!atomic_read(&vm->wq_active) &&
                    virtio_mem_could_add_memory(vm, vm->offline_threshold / 2))
                        virtio_mem_retry(vm);

                vm->hotplug_active = false;
                mutex_unlock(&vm->hotplug_mutex);
                break;
        case MEM_CANCEL_OFFLINE:
                if (!vm->hotplug_active)
                        break;
                virtio_mem_notify_cancel_offline(vm, mb_id);
                vm->hotplug_active = false;
                mutex_unlock(&vm->hotplug_mutex);
                break;
        case MEM_CANCEL_ONLINE:
                if (!vm->hotplug_active)
                        break;
                vm->hotplug_active = false;
                mutex_unlock(&vm->hotplug_mutex);
                break;
        default:
                break;
        }

        lockdep_on();

        return rc;
}

/*
 * Set a range of pages PG_offline. Remember pages that were never onlined
 * (via generic_online_page()) using PageDirty().
 */
static void virtio_mem_set_fake_offline(unsigned long pfn,
                                        unsigned long nr_pages, bool onlined)
{
        for (; nr_pages--; pfn++) {
                struct page *page = pfn_to_page(pfn);

                __SetPageOffline(page);
                if (!onlined) {
                        SetPageDirty(page);
                        /* FIXME: remove after cleanups */
                        ClearPageReserved(page);
                }
        }
}

/*
 * Clear PG_offline from a range of pages. If the pages were never onlined,
 * (via generic_online_page()), clear PageDirty().
 */
static void virtio_mem_clear_fake_offline(unsigned long pfn,
                                          unsigned long nr_pages, bool onlined)
{
        for (; nr_pages--; pfn++) {
                struct page *page = pfn_to_page(pfn);

                __ClearPageOffline(page);
                if (!onlined)
                        ClearPageDirty(page);
        }
}

/*
 * Release a range of fake-offline pages to the buddy, effectively
 * fake-onlining them.
 */
static void virtio_mem_fake_online(unsigned long pfn, unsigned long nr_pages)
{
        const unsigned long max_nr_pages = MAX_ORDER_NR_PAGES;
        unsigned long i;

        /*
         * We are always called at least with MAX_ORDER_NR_PAGES
         * granularity/alignment (e.g., the way subblocks work). All pages
         * inside such a block are alike.
         */
        for (i = 0; i < nr_pages; i += max_nr_pages) {
                struct page *page = pfn_to_page(pfn + i);

                /*
                 * If the page is PageDirty(), it was kept fake-offline when
                 * onlining the memory block. Otherwise, it was allocated
                 * using alloc_contig_range(). All pages in a subblock are
                 * alike.
                 */
                if (PageDirty(page)) {
                        virtio_mem_clear_fake_offline(pfn + i, max_nr_pages,
                                                      false);
                        generic_online_page(page, MAX_ORDER - 1);
                } else {
                        virtio_mem_clear_fake_offline(pfn + i, max_nr_pages,
                                                      true);
                        free_contig_range(pfn + i, max_nr_pages);
                        adjust_managed_page_count(page, max_nr_pages);
                }
        }
}

/*
 * Try to allocate a range, marking pages fake-offline, effectively
 * fake-offlining them.
 */
static int virtio_mem_fake_offline(unsigned long pfn, unsigned long nr_pages)
{
        const bool is_movable = zone_idx(page_zone(pfn_to_page(pfn))) ==
                                ZONE_MOVABLE;
        int rc, retry_count;

        /*
         * TODO: We want an alloc_contig_range() mode that tries to allocate
         * harder (e.g., dealing with temporarily pinned pages, PCP), especially
         * with ZONE_MOVABLE. So for now, retry a couple of times with
         * ZONE_MOVABLE before giving up - because that zone is supposed to give
         * some guarantees.
         */
        for (retry_count = 0; retry_count < 5; retry_count++) {
                rc = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_MOVABLE,
                                        GFP_KERNEL);
                if (rc == -ENOMEM)
                        /* whoops, out of memory */
                        return rc;
                else if (rc && !is_movable)
                        break;
                else if (rc)
                        continue;

                virtio_mem_set_fake_offline(pfn, nr_pages, true);
                adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
                return 0;
        }

        return -EBUSY;
}

/*
 * Handle fake-offline pages when memory is going offline - such that the
 * pages can be skipped by mm-core when offlining.
 */
static void virtio_mem_fake_offline_going_offline(unsigned long pfn,
                                                  unsigned long nr_pages)
{
        struct page *page;
        unsigned long i;

        /*
         * Drop our reference to the pages so the memory can get offlined
         * and add the unplugged pages to the managed page counters (so
         * offlining code can correctly subtract them again).
         */
        adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
        /* Drop our reference to the pages so the memory can get offlined. */
        for (i = 0; i < nr_pages; i++) {
                page = pfn_to_page(pfn + i);
                if (WARN_ON(!page_ref_dec_and_test(page)))
                        dump_page(page, "fake-offline page referenced");
        }
}

/*
 * Handle fake-offline pages when memory offlining is canceled - to undo
 * what we did in virtio_mem_fake_offline_going_offline().
 */
static void virtio_mem_fake_offline_cancel_offline(unsigned long pfn,
                                                   unsigned long nr_pages)
{
        unsigned long i;

        /*
         * Get the reference we dropped when going offline and subtract the
         * unplugged pages from the managed page counters.
         */
        adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
        for (i = 0; i < nr_pages; i++)
                page_ref_inc(pfn_to_page(pfn + i));
}

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);
        struct virtio_mem *vm;
        int sb_id;

        /*
         * 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))
                        generic_online_page(page, order);
                else
                        virtio_mem_set_fake_offline(PFN_DOWN(addr), 1 << order,
                                                    false);
                rcu_read_unlock();
                return;
        }
        rcu_read_unlock();

        /* not virtio-mem memory, but e.g., a DIMM. online it */
        generic_online_page(page, order);
}

static uint64_t virtio_mem_send_request(struct virtio_mem *vm,
                                        const struct virtio_mem_req *req)
{
        struct scatterlist *sgs[2], sg_req, sg_resp;
        unsigned int len;
        int rc;

        /* don't use the request residing on the stack (vaddr) */
        vm->req = *req;

        /* out: buffer for request */
        sg_init_one(&sg_req, &vm->req, sizeof(vm->req));
        sgs[0] = &sg_req;

        /* in: buffer for response */
        sg_init_one(&sg_resp, &vm->resp, sizeof(vm->resp));
        sgs[1] = &sg_resp;

        rc = virtqueue_add_sgs(vm->vq, sgs, 1, 1, vm, GFP_KERNEL);
        if (rc < 0)
                return rc;

        virtqueue_kick(vm->vq);

        /* wait for a response */
        wait_event(vm->host_resp, virtqueue_get_buf(vm->vq, &len));

        return virtio16_to_cpu(vm->vdev, vm->resp.type);
}

static int virtio_mem_send_plug_request(struct virtio_mem *vm, uint64_t addr,
                                        uint64_t size)
{
        const uint64_t nb_vm_blocks = size / vm->device_block_size;
        const struct virtio_mem_req req = {
                .type = cpu_to_virtio16(vm->vdev, VIRTIO_MEM_REQ_PLUG),
                .u.plug.addr = cpu_to_virtio64(vm->vdev, addr),
                .u.plug.nb_blocks = cpu_to_virtio16(vm->vdev, nb_vm_blocks),
        };
        int rc = -ENOMEM;

        if (atomic_read(&vm->config_changed))
                return -EAGAIN;

        dev_dbg(&vm->vdev->dev, "plugging memory: 0x%llx - 0x%llx\n", addr,
                addr + size - 1);

        switch (virtio_mem_send_request(vm, &req)) {
        case VIRTIO_MEM_RESP_ACK:
                vm->plugged_size += size;
                return 0;
        case VIRTIO_MEM_RESP_NACK:
                rc = -EAGAIN;
                break;
        case VIRTIO_MEM_RESP_BUSY:
                rc = -ETXTBSY;
                break;
        case VIRTIO_MEM_RESP_ERROR:
                rc = -EINVAL;
                break;
        default:
                break;
        }

        dev_dbg(&vm->vdev->dev, "plugging memory failed: %d\n", rc);
        return rc;
}

static int virtio_mem_send_unplug_request(struct virtio_mem *vm, uint64_t addr,
                                          uint64_t size)
{
        const uint64_t nb_vm_blocks = size / vm->device_block_size;
        const struct virtio_mem_req req = {
                .type = cpu_to_virtio16(vm->vdev, VIRTIO_MEM_REQ_UNPLUG),
                .u.unplug.addr = cpu_to_virtio64(vm->vdev, addr),
                .u.unplug.nb_blocks = cpu_to_virtio16(vm->vdev, nb_vm_blocks),
        };
        int rc = -ENOMEM;

        if (atomic_read(&vm->config_changed))
                return -EAGAIN;

        dev_dbg(&vm->vdev->dev, "unplugging memory: 0x%llx - 0x%llx\n", addr,
                addr + size - 1);

        switch (virtio_mem_send_request(vm, &req)) {
        case VIRTIO_MEM_RESP_ACK:
                vm->plugged_size -= size;
                return 0;
        case VIRTIO_MEM_RESP_BUSY:
                rc = -ETXTBSY;
                break;
        case VIRTIO_MEM_RESP_ERROR:
                rc = -EINVAL;
                break;
        default:
                break;
        }

        dev_dbg(&vm->vdev->dev, "unplugging memory failed: %d\n", rc);
        return rc;
}

static int virtio_mem_send_unplug_all_request(struct virtio_mem *vm)
{
        const struct virtio_mem_req req = {
                .type = cpu_to_virtio16(vm->vdev, VIRTIO_MEM_REQ_UNPLUG_ALL),
        };
        int rc = -ENOMEM;

        dev_dbg(&vm->vdev->dev, "unplugging all memory");

        switch (virtio_mem_send_request(vm, &req)) {
        case VIRTIO_MEM_RESP_ACK:
                vm->unplug_all_required = false;
                vm->plugged_size = 0;
                /* usable region might have shrunk */
                atomic_set(&vm->config_changed, 1);
                return 0;
        case VIRTIO_MEM_RESP_BUSY:
                rc = -ETXTBSY;
                break;
        default:
                break;
        }

        dev_dbg(&vm->vdev->dev, "unplugging all memory failed: %d\n", rc);
        return rc;
}

/*
 * Plug selected subblocks. Updates the plugged state, but not the state
 * of the memory block.
 */
static int virtio_mem_sbm_plug_sb(struct virtio_mem *vm, unsigned long mb_id,
                                  int sb_id, int count)
{
        const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id) +
                              sb_id * vm->sbm.sb_size;
        const uint64_t size = count * vm->sbm.sb_size;
        int rc;

        rc = virtio_mem_send_plug_request(vm, addr, size);
        if (!rc)
                virtio_mem_sbm_set_sb_plugged(vm, mb_id, sb_id, count);
        return rc;
}

/*
 * Unplug selected subblocks. Updates the plugged state, but not the state
 * of the memory block.
 */
static int virtio_mem_sbm_unplug_sb(struct virtio_mem *vm, unsigned long mb_id,
                                    int sb_id, int count)
{
        const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id) +
                              sb_id * vm->sbm.sb_size;
        const uint64_t size = count * vm->sbm.sb_size;
        int rc;

        rc = virtio_mem_send_unplug_request(vm, addr, size);
        if (!rc)
                virtio_mem_sbm_set_sb_unplugged(vm, mb_id, sb_id, count);
        return rc;
}

/*
 * 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
 * skipping it).
 *
 * Will not modify the state of the memory block.
 *
 * Note: can fail after some subblocks were unplugged.
 */
static int virtio_mem_sbm_unplug_any_sb(struct virtio_mem *vm,
                                        unsigned long mb_id, uint64_t *nb_sb)
{
        int sb_id, count;
        int rc;

        sb_id = vm->sbm.sbs_per_mb - 1;
        while (*nb_sb) {
                /* Find the next candidate subblock */
                while (sb_id >= 0 &&
                       virtio_mem_sbm_test_sb_unplugged(vm, mb_id, sb_id, 1))
                        sb_id--;
                if (sb_id < 0)
                        break;
                /* Try to unplug multiple subblocks at a time */
                count = 1;
                while (count < *nb_sb && sb_id > 0 &&
                       virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id - 1, 1)) {
                        count++;
                        sb_id--;
                }

                rc = virtio_mem_sbm_unplug_sb(vm, mb_id, sb_id, count);
                if (rc)
                        return rc;
                *nb_sb -= count;
                sb_id--;
        }

        return 0;
}

/*
 * Unplug all plugged subblocks of an offline or not-added memory block.
 *
 * Will not modify the state of the memory block.
 *
 * Note: can fail after some subblocks were unplugged.
 */
static int virtio_mem_sbm_unplug_mb(struct virtio_mem *vm, unsigned long mb_id)
{
        uint64_t nb_sb = vm->sbm.sbs_per_mb;

        return virtio_mem_sbm_unplug_any_sb(vm, mb_id, &nb_sb);
}

/*
 * Prepare tracking data for the next memory block.
 */
static int virtio_mem_sbm_prepare_next_mb(struct virtio_mem *vm,
                                          unsigned long *mb_id)
{
        int rc;

        if (vm->sbm.next_mb_id > vm->sbm.last_usable_mb_id)
                return -ENOSPC;

        /* Resize the state array if required. */
        rc = virtio_mem_sbm_mb_states_prepare_next_mb(vm);
        if (rc)
                return rc;

        /* Resize the subblock bitmap if required. */
        rc = virtio_mem_sbm_sb_states_prepare_next_mb(vm);
        if (rc)
                return rc;

        vm->sbm.mb_count[VIRTIO_MEM_SBM_MB_UNUSED]++;
        *mb_id = vm->sbm.next_mb_id++;
        return 0;
}

/*
 * Try to plug the desired number of subblocks and add the memory block
 * to Linux.
 *
 * Will modify the state of the memory block.
 */
static int virtio_mem_sbm_plug_and_add_mb(struct virtio_mem *vm,
                                          unsigned long mb_id, uint64_t *nb_sb)
{
        const int count = min_t(int, *nb_sb, vm->sbm.sbs_per_mb);
        int rc;

        if (WARN_ON_ONCE(!count))
                return -EINVAL;

        /*
         * Plug the requested number of subblocks before adding it to linux,
         * so that onlining will directly online all plugged subblocks.
         */
        rc = virtio_mem_sbm_plug_sb(vm, mb_id, 0, count);
        if (rc)
                return rc;

        /*
         * Mark the block properly offline before adding it to Linux,
         * so the memory notifiers will find the block in the right state.
         */
        if (count == vm->sbm.sbs_per_mb)
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_OFFLINE);
        else
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL);

        /* Add the memory block to linux - if that fails, try to unplug. */
        rc = virtio_mem_mb_add(vm, mb_id);
        if (rc) {
                int new_state = VIRTIO_MEM_SBM_MB_UNUSED;

                dev_err(&vm->vdev->dev,
                        "adding memory block %lu failed with %d\n", mb_id, rc);

                /*
                 * TODO: Linux MM does not properly clean up yet in all cases
                 * where adding of memory failed - especially on -ENOMEM.
                 */
                if (virtio_mem_sbm_unplug_sb(vm, mb_id, 0, count))
                        new_state = VIRTIO_MEM_SBM_MB_PLUGGED;
                virtio_mem_sbm_set_mb_state(vm, mb_id, new_state);
                return rc;
        }

        *nb_sb -= count;
        return 0;
}

/*
 * Try to plug the desired number of subblocks of a memory block that
 * is already added to Linux.
 *
 * Will modify the state of the memory block.
 *
 * Note: Can fail after some subblocks were successfully plugged.
 */
static int virtio_mem_sbm_plug_any_sb(struct virtio_mem *vm,
                                      unsigned long mb_id, uint64_t *nb_sb,
                                      bool online)
{
        unsigned long pfn, nr_pages;
        int sb_id, count;
        int rc;

        if (WARN_ON_ONCE(!*nb_sb))
                return -EINVAL;

        while (*nb_sb) {
                sb_id = virtio_mem_sbm_first_unplugged_sb(vm, mb_id);
                if (sb_id >= vm->sbm.sbs_per_mb)
                        break;
                count = 1;
                while (count < *nb_sb &&
                       sb_id + count < vm->sbm.sbs_per_mb &&
                       !virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id + count, 1))
                        count++;

                rc = virtio_mem_sbm_plug_sb(vm, mb_id, sb_id, count);
                if (rc)
                        return rc;
                *nb_sb -= count;
                if (!online)
                        continue;

                /* fake-online the pages if the memory block is online */
                pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
                               sb_id * vm->sbm.sb_size);
                nr_pages = PFN_DOWN(count * vm->sbm.sb_size);
                virtio_mem_fake_online(pfn, nr_pages);
        }

        if (virtio_mem_sbm_test_sb_plugged(vm, mb_id, 0, vm->sbm.sbs_per_mb)) {
                if (online)
                        virtio_mem_sbm_set_mb_state(vm, mb_id,
                                                    VIRTIO_MEM_SBM_MB_ONLINE);
                else
                        virtio_mem_sbm_set_mb_state(vm, mb_id,
                                                    VIRTIO_MEM_SBM_MB_OFFLINE);
        }

        return 0;
}

/*
 * Try to plug the requested amount of memory.
 */
static int virtio_mem_plug_request(struct virtio_mem *vm, uint64_t diff)
{
        uint64_t nb_sb = diff / vm->sbm.sb_size;
        unsigned long mb_id;
        int rc;

        if (!nb_sb)
                return 0;

        /* Don't race with onlining/offlining */
        mutex_lock(&vm->hotplug_mutex);

        /* Try to plug subblocks of partially plugged online blocks. */
        virtio_mem_sbm_for_each_mb(vm, mb_id,
                                   VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL) {
                rc = virtio_mem_sbm_plug_any_sb(vm, mb_id, &nb_sb, true);
                if (rc || !nb_sb)
                        goto out_unlock;
                cond_resched();
        }

        /* Try to plug subblocks of partially plugged offline blocks. */
        virtio_mem_sbm_for_each_mb(vm, mb_id,
                                   VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL) {
                rc = virtio_mem_sbm_plug_any_sb(vm, mb_id, &nb_sb, false);
                if (rc || !nb_sb)
                        goto out_unlock;
                cond_resched();
        }

        /*
         * We won't be working on online/offline memory blocks from this point,
         * so we can't race with memory onlining/offlining. Drop the mutex.
         */
        mutex_unlock(&vm->hotplug_mutex);

        /* Try to plug and add unused blocks */
        virtio_mem_sbm_for_each_mb(vm, mb_id, VIRTIO_MEM_SBM_MB_UNUSED) {
                if (!virtio_mem_could_add_memory(vm, memory_block_size_bytes()))
                        return -ENOSPC;

                rc = virtio_mem_sbm_plug_and_add_mb(vm, mb_id, &nb_sb);
                if (rc || !nb_sb)
                        return rc;
                cond_resched();
        }

        /* Try to prepare, plug and add new blocks */
        while (nb_sb) {
                if (!virtio_mem_could_add_memory(vm, memory_block_size_bytes()))
                        return -ENOSPC;

                rc = virtio_mem_sbm_prepare_next_mb(vm, &mb_id);
                if (rc)
                        return rc;
                rc = virtio_mem_sbm_plug_and_add_mb(vm, mb_id, &nb_sb);
                if (rc)
                        return rc;
                cond_resched();
        }

        return 0;
out_unlock:
        mutex_unlock(&vm->hotplug_mutex);
        return rc;
}

/*
 * Unplug the desired number of plugged subblocks of an offline memory block.
 * Will fail if any subblock cannot get unplugged (instead of skipping it).
 *
 * Will modify the state of the memory block. Might temporarily drop the
 * hotplug_mutex.
 *
 * Note: Can fail after some subblocks were successfully unplugged.
 */
static int virtio_mem_sbm_unplug_any_sb_offline(struct virtio_mem *vm,
                                                unsigned long mb_id,
                                                uint64_t *nb_sb)
{
        int rc;

        rc = virtio_mem_sbm_unplug_any_sb(vm, mb_id, nb_sb);

        /* some subblocks might have been unplugged even on failure */
        if (!virtio_mem_sbm_test_sb_plugged(vm, mb_id, 0, vm->sbm.sbs_per_mb))
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL);
        if (rc)
                return rc;

        if (virtio_mem_sbm_test_sb_unplugged(vm, mb_id, 0, vm->sbm.sbs_per_mb)) {
                /*
                 * Remove the block from Linux - this should never fail.
                 * Hinder the block from getting onlined by marking it
                 * unplugged. Temporarily drop the mutex, so
                 * any pending GOING_ONLINE requests can be serviced/rejected.
                 */
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_UNUSED);

                mutex_unlock(&vm->hotplug_mutex);
                rc = virtio_mem_mb_remove(vm, mb_id);
                BUG_ON(rc);
                mutex_lock(&vm->hotplug_mutex);
        }
        return 0;
}

/*
 * Unplug the given plugged subblocks of an online memory block.
 *
 * Will modify the state of the memory block.
 */
static int virtio_mem_sbm_unplug_sb_online(struct virtio_mem *vm,
                                           unsigned long mb_id, int sb_id,
                                           int count)
{
        const unsigned long nr_pages = PFN_DOWN(vm->sbm.sb_size) * count;
        unsigned long start_pfn;
        int rc;

        start_pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
                             sb_id * vm->sbm.sb_size);

        rc = virtio_mem_fake_offline(start_pfn, nr_pages);
        if (rc)
                return rc;

        /* Try to unplug the allocated memory */
        rc = virtio_mem_sbm_unplug_sb(vm, mb_id, sb_id, count);
        if (rc) {
                /* Return the memory to the buddy. */
                virtio_mem_fake_online(start_pfn, nr_pages);
                return rc;
        }

        virtio_mem_sbm_set_mb_state(vm, mb_id,
                                    VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL);
        return 0;
}

/*
 * Unplug the desired number of plugged subblocks of an online memory block.
 * Will skip subblock that are busy.
 *
 * Will modify the state of the memory block. Might temporarily drop the
 * hotplug_mutex.
 *
 * Note: Can fail after some subblocks were successfully unplugged. Can
 *       return 0 even if subblocks were busy and could not get unplugged.
 */
static int virtio_mem_sbm_unplug_any_sb_online(struct virtio_mem *vm,
                                               unsigned long mb_id,
                                               uint64_t *nb_sb)
{
        int rc, sb_id;

        /* If possible, try to unplug the complete block in one shot. */
        if (*nb_sb >= vm->sbm.sbs_per_mb &&
            virtio_mem_sbm_test_sb_plugged(vm, mb_id, 0, vm->sbm.sbs_per_mb)) {
                rc = virtio_mem_sbm_unplug_sb_online(vm, mb_id, 0,
                                                     vm->sbm.sbs_per_mb);
                if (!rc) {
                        *nb_sb -= vm->sbm.sbs_per_mb;
                        goto unplugged;
                } else if (rc != -EBUSY)
                        return rc;
        }

        /* Fallback to single subblocks. */
        for (sb_id = vm->sbm.sbs_per_mb - 1; sb_id >= 0 && *nb_sb; sb_id--) {
                /* Find the next candidate subblock */
                while (sb_id >= 0 &&
                       !virtio_mem_sbm_test_sb_plugged(vm, mb_id, sb_id, 1))
                        sb_id--;
                if (sb_id < 0)
                        break;

                rc = virtio_mem_sbm_unplug_sb_online(vm, mb_id, sb_id, 1);
                if (rc == -EBUSY)
                        continue;
                else if (rc)
                        return rc;
                *nb_sb -= 1;
        }

unplugged:
        /*
         * Once all subblocks of a memory block were unplugged, offline and
         * remove it. This will usually not fail, as no memory is in use
         * anymore - however some other notifiers might NACK the request.
         */
        if (virtio_mem_sbm_test_sb_unplugged(vm, mb_id, 0, vm->sbm.sbs_per_mb)) {
                mutex_unlock(&vm->hotplug_mutex);
                rc = virtio_mem_mb_offline_and_remove(vm, mb_id);
                mutex_lock(&vm->hotplug_mutex);
                if (!rc)
                        virtio_mem_sbm_set_mb_state(vm, mb_id,
                                                    VIRTIO_MEM_SBM_MB_UNUSED);
        }

        return 0;
}

/*
 * Try to unplug the requested amount of memory.
 */
static int virtio_mem_unplug_request(struct virtio_mem *vm, uint64_t diff)
{
        uint64_t nb_sb = diff / vm->sbm.sb_size;
        unsigned long mb_id;
        int rc;

        if (!nb_sb)
                return 0;

        /*
         * We'll drop the mutex a couple of times when it is safe to do so.
         * This might result in some blocks switching the state (online/offline)
         * and we could miss them in this run - we will retry again later.
         */
        mutex_lock(&vm->hotplug_mutex);

        /* Try to unplug subblocks of partially plugged offline blocks. */
        virtio_mem_sbm_for_each_mb_rev(vm, mb_id,
                                       VIRTIO_MEM_SBM_MB_OFFLINE_PARTIAL) {
                rc = virtio_mem_sbm_unplug_any_sb_offline(vm, mb_id, &nb_sb);
                if (rc || !nb_sb)
                        goto out_unlock;
                cond_resched();
        }

        /* Try to unplug subblocks of plugged offline blocks. */
        virtio_mem_sbm_for_each_mb_rev(vm, mb_id, VIRTIO_MEM_SBM_MB_OFFLINE) {
                rc = virtio_mem_sbm_unplug_any_sb_offline(vm, mb_id, &nb_sb);
                if (rc || !nb_sb)
                        goto out_unlock;
                cond_resched();
        }

        if (!unplug_online) {
                mutex_unlock(&vm->hotplug_mutex);
                return 0;
        }

        /* Try to unplug subblocks of partially plugged online blocks. */
        virtio_mem_sbm_for_each_mb_rev(vm, mb_id,
                                       VIRTIO_MEM_SBM_MB_ONLINE_PARTIAL) {
                rc = virtio_mem_sbm_unplug_any_sb_online(vm, mb_id, &nb_sb);
                if (rc || !nb_sb)
                        goto out_unlock;
                mutex_unlock(&vm->hotplug_mutex);
                cond_resched();
                mutex_lock(&vm->hotplug_mutex);
        }

        /* Try to unplug subblocks of plugged online blocks. */
        virtio_mem_sbm_for_each_mb_rev(vm, mb_id, VIRTIO_MEM_SBM_MB_ONLINE) {
                rc = virtio_mem_sbm_unplug_any_sb_online(vm, mb_id, &nb_sb);
                if (rc || !nb_sb)
                        goto out_unlock;
                mutex_unlock(&vm->hotplug_mutex);
                cond_resched();
                mutex_lock(&vm->hotplug_mutex);
        }

        mutex_unlock(&vm->hotplug_mutex);
        return nb_sb ? -EBUSY : 0;
out_unlock:
        mutex_unlock(&vm->hotplug_mutex);
        return rc;
}

/*
 * 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;
        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 (rc)
                        return rc;
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_UNUSED);
        }

        return 0;
}

/*
 * Update all parts of the config that could have changed.
 */
static void virtio_mem_refresh_config(struct virtio_mem *vm)
{
        const uint64_t phys_limit = 1UL << MAX_PHYSMEM_BITS;
        uint64_t new_plugged_size, usable_region_size, end_addr;

        /* the plugged_size is just a reflection of what _we_ did previously */
        virtio_cread_le(vm->vdev, struct virtio_mem_config, plugged_size,
                        &new_plugged_size);
        if (WARN_ON_ONCE(new_plugged_size != vm->plugged_size))
                vm->plugged_size = new_plugged_size;

        /* calculate the last usable memory block id */
        virtio_cread_le(vm->vdev, struct virtio_mem_config,
                        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;

        /* see if there is a request to change the size */
        virtio_cread_le(vm->vdev, struct virtio_mem_config, requested_size,
                        &vm->requested_size);

        dev_info(&vm->vdev->dev, "plugged size: 0x%llx", vm->plugged_size);
        dev_info(&vm->vdev->dev, "requested size: 0x%llx", vm->requested_size);
}

/*
 * Workqueue function for handling plug/unplug requests and config updates.
 */
static void virtio_mem_run_wq(struct work_struct *work)
{
        struct virtio_mem *vm = container_of(work, struct virtio_mem, wq);
        uint64_t diff;
        int rc;

        hrtimer_cancel(&vm->retry_timer);

        if (vm->broken)
                return;

        atomic_set(&vm->wq_active, 1);
retry:
        rc = 0;

        /* Make sure we start with a clean state if there are leftovers. */
        if (unlikely(vm->unplug_all_required))
                rc = virtio_mem_send_unplug_all_request(vm);

        if (atomic_read(&vm->config_changed)) {
                atomic_set(&vm->config_changed, 0);
                virtio_mem_refresh_config(vm);
        }

        /* Unplug any leftovers from previous runs */
        if (!rc)
                rc = virtio_mem_unplug_pending_mb(vm);

        if (!rc && vm->requested_size != vm->plugged_size) {
                if (vm->requested_size > vm->plugged_size) {
                        diff = vm->requested_size - vm->plugged_size;
                        rc = virtio_mem_plug_request(vm, diff);
                } else {
                        diff = vm->plugged_size - vm->requested_size;
                        rc = virtio_mem_unplug_request(vm, diff);
                }
        }

        switch (rc) {
        case 0:
                vm->retry_timer_ms = VIRTIO_MEM_RETRY_TIMER_MIN_MS;
                break;
        case -ENOSPC:
                /*
                 * We cannot add any more memory (alignment, physical limit)
                 * or we have too many offline memory blocks.
                 */
                break;
        case -ETXTBSY:
                /*
                 * The hypervisor cannot process our request right now
                 * (e.g., out of memory, migrating);
                 */
        case -EBUSY:
                /*
                 * We cannot free up any memory to unplug it (all plugged memory
                 * is busy).
                 */
        case -ENOMEM:
                /* Out of memory, try again later. */
                hrtimer_start(&vm->retry_timer, ms_to_ktime(vm->retry_timer_ms),
                              HRTIMER_MODE_REL);
                break;
        case -EAGAIN:
                /* Retry immediately (e.g., the config changed). */
                goto retry;
        default:
                /* Unknown error, mark as broken */
                dev_err(&vm->vdev->dev,
                        "unknown error, marking device broken: %d\n", rc);
                vm->broken = true;
        }

        atomic_set(&vm->wq_active, 0);
}

static enum hrtimer_restart virtio_mem_timer_expired(struct hrtimer *timer)
{
        struct virtio_mem *vm = container_of(timer, struct virtio_mem,
                                             retry_timer);

        virtio_mem_retry(vm);
        vm->retry_timer_ms = min_t(unsigned int, vm->retry_timer_ms * 2,
                                   VIRTIO_MEM_RETRY_TIMER_MAX_MS);
        return HRTIMER_NORESTART;
}

static void virtio_mem_handle_response(struct virtqueue *vq)
{
        struct virtio_mem *vm = vq->vdev->priv;

        wake_up(&vm->host_resp);
}

static int virtio_mem_init_vq(struct virtio_mem *vm)
{
        struct virtqueue *vq;

        vq = virtio_find_single_vq(vm->vdev, virtio_mem_handle_response,
                                   "guest-request");
        if (IS_ERR(vq))
                return PTR_ERR(vq);
        vm->vq = vq;

        return 0;
}

static int virtio_mem_init(struct virtio_mem *vm)
{
        const uint64_t phys_limit = 1UL << MAX_PHYSMEM_BITS;
        uint16_t node_id;

        if (!vm->vdev->config->get) {
                dev_err(&vm->vdev->dev, "config access disabled\n");
                return -EINVAL;
        }

        /*
         * We don't want to (un)plug or reuse any memory when in kdump. The
         * memory is still accessible (but not mapped).
         */
        if (is_kdump_kernel()) {
                dev_warn(&vm->vdev->dev, "disabled in kdump kernel\n");
                return -EBUSY;
        }

        /* Fetch all properties that can't change. */
        virtio_cread_le(vm->vdev, struct virtio_mem_config, plugged_size,
                        &vm->plugged_size);
        virtio_cread_le(vm->vdev, struct virtio_mem_config, block_size,
                        &vm->device_block_size);
        virtio_cread_le(vm->vdev, struct virtio_mem_config, node_id,
                        &node_id);
        vm->nid = virtio_mem_translate_node_id(vm, node_id);
        virtio_cread_le(vm->vdev, struct virtio_mem_config, addr, &vm->addr);
        virtio_cread_le(vm->vdev, struct virtio_mem_config, region_size,
                        &vm->region_size);

        /* Determine the nid for the device based on the lowest address. */
        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,
                         "The alignment of the physical start address can make some memory unusable.\n");
        if (!IS_ALIGNED(vm->addr + vm->region_size, memory_block_size_bytes()))
                dev_warn(&vm->vdev->dev,
                         "The alignment of the physical end address can make some memory unusable.\n");
        if (vm->addr + vm->region_size > phys_limit)
                dev_warn(&vm->vdev->dev,
                         "Some memory is not addressable. This can make some memory unusable.\n");

        /*
         * We want subblocks to span at least MAX_ORDER_NR_PAGES and
         * pageblock_nr_pages pages. This:
         * - Simplifies our page onlining code (virtio_mem_online_page_cb)
         *   and fake page onlining code (virtio_mem_fake_online).
         * - 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;

        /* 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;

        /* 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);

        dev_info(&vm->vdev->dev, "start address: 0x%llx", vm->addr);
        dev_info(&vm->vdev->dev, "region size: 0x%llx", vm->region_size);
        dev_info(&vm->vdev->dev, "device block size: 0x%llx",
                 (unsigned long long)vm->device_block_size);
        dev_info(&vm->vdev->dev, "memory block size: 0x%lx",
                 memory_block_size_bytes());
        dev_info(&vm->vdev->dev, "subblock size: 0x%llx",
                 (unsigned long long)vm->sbm.sb_size);
        if (vm->nid != NUMA_NO_NODE && IS_ENABLED(CONFIG_NUMA))
                dev_info(&vm->vdev->dev, "nid: %d", vm->nid);

        return 0;
}

static int virtio_mem_create_resource(struct virtio_mem *vm)
{
        /*
         * When force-unloading the driver and removing the device, we
         * could have a garbage pointer. Duplicate the string.
         */
        const char *name = kstrdup(dev_name(&vm->vdev->dev), GFP_KERNEL);

        if (!name)
                return -ENOMEM;

        vm->parent_resource = __request_mem_region(vm->addr, vm->region_size,
                                                   name, IORESOURCE_SYSTEM_RAM);
        if (!vm->parent_resource) {
                kfree(name);
                dev_warn(&vm->vdev->dev, "could not reserve device region\n");
                dev_info(&vm->vdev->dev,
                         "reloading the driver is not supported\n");
                return -EBUSY;
        }

        /* The memory is not actually busy - make add_memory() work. */
        vm->parent_resource->flags &= ~IORESOURCE_BUSY;
        return 0;
}

static void virtio_mem_delete_resource(struct virtio_mem *vm)
{
        const char *name;

        if (!vm->parent_resource)
                return;

        name = vm->parent_resource->name;
        release_resource(vm->parent_resource);
        kfree(vm->parent_resource);
        kfree(name);
        vm->parent_resource = NULL;
}

static int virtio_mem_range_has_system_ram(struct resource *res, void *arg)
{
        return 1;
}

static bool virtio_mem_has_memory_added(struct virtio_mem *vm)
{
        const unsigned long flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;

        return walk_iomem_res_desc(IORES_DESC_NONE, flags, vm->addr,
                                   vm->addr + vm->region_size, NULL,
                                   virtio_mem_range_has_system_ram) == 1;
}

static int virtio_mem_probe(struct virtio_device *vdev)
{
        struct virtio_mem *vm;
        int rc;

        BUILD_BUG_ON(sizeof(struct virtio_mem_req) != 24);
        BUILD_BUG_ON(sizeof(struct virtio_mem_resp) != 10);

        vdev->priv = vm = kzalloc(sizeof(*vm), GFP_KERNEL);
        if (!vm)
                return -ENOMEM;

        init_waitqueue_head(&vm->host_resp);
        vm->vdev = vdev;
        INIT_WORK(&vm->wq, virtio_mem_run_wq);
        mutex_init(&vm->hotplug_mutex);
        INIT_LIST_HEAD(&vm->next);
        spin_lock_init(&vm->removal_lock);
        hrtimer_init(&vm->retry_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
        vm->retry_timer.function = virtio_mem_timer_expired;
        vm->retry_timer_ms = VIRTIO_MEM_RETRY_TIMER_MIN_MS;

        /* register the virtqueue */
        rc = virtio_mem_init_vq(vm);
        if (rc)
                goto out_free_vm;

        /* initialize the device by querying the config */
        rc = virtio_mem_init(vm);
        if (rc)
                goto out_del_vq;

        /* create the parent resource for all memory */
        rc = virtio_mem_create_resource(vm);
        if (rc)
                goto out_del_vq;

        /*
         * If we still have memory plugged, we have to unplug all memory first.
         * Registering our parent resource makes sure that this memory isn't
         * actually in use (e.g., trying to reload the driver).
         */
        if (vm->plugged_size) {
                vm->unplug_all_required = 1;
                dev_info(&vm->vdev->dev, "unplugging all memory is required\n");
        }

        /* register callbacks */
        vm->memory_notifier.notifier_call = virtio_mem_memory_notifier_cb;
        rc = register_memory_notifier(&vm->memory_notifier);
        if (rc)
                goto out_del_resource;
        rc = register_virtio_mem_device(vm);
        if (rc)
                goto out_unreg_mem;

        virtio_device_ready(vdev);

        /* trigger a config update to start processing the requested_size */
        atomic_set(&vm->config_changed, 1);
        queue_work(system_freezable_wq, &vm->wq);

        return 0;
out_unreg_mem:
        unregister_memory_notifier(&vm->memory_notifier);
out_del_resource:
        virtio_mem_delete_resource(vm);
out_del_vq:
        vdev->config->del_vqs(vdev);
out_free_vm:
        kfree(vm);
        vdev->priv = NULL;

        return rc;
}

static void virtio_mem_remove(struct virtio_device *vdev)
{
        struct virtio_mem *vm = vdev->priv;
        unsigned long mb_id;
        int rc;

        /*
         * Make sure the workqueue won't be triggered anymore and no memory
         * blocks can be onlined/offlined until we're finished here.
         */
        mutex_lock(&vm->hotplug_mutex);
        spin_lock_irq(&vm->removal_lock);
        vm->removing = true;
        spin_unlock_irq(&vm->removal_lock);
        mutex_unlock(&vm->hotplug_mutex);

        /* wait until the workqueue stopped */
        cancel_work_sync(&vm->wq);
        hrtimer_cancel(&vm->retry_timer);

        /*
         * 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) {
                rc = virtio_mem_mb_remove(vm, mb_id);
                BUG_ON(rc);
                virtio_mem_sbm_set_mb_state(vm, mb_id,
                                            VIRTIO_MEM_SBM_MB_UNUSED);
        }
        /*
         * After we unregistered our callbacks, user space can no longer
         * offline partially plugged online memory blocks. No need to worry
         * about them.
         */

        /* unregister callbacks */
        unregister_virtio_mem_device(vm);
        unregister_memory_notifier(&vm->memory_notifier);

        /*
         * There is no way we could reliably remove all memory we have added to
         * the system. And there is no way to stop the driver/device from going
         * away. Warn at least.
         */
        if (virtio_mem_has_memory_added(vm)) {
                dev_warn(&vdev->dev, "device still has system memory added\n");
        } else {
                virtio_mem_delete_resource(vm);
                kfree_const(vm->resource_name);
        }

        /* remove all tracking data - no locking needed */
        vfree(vm->sbm.mb_states);
        vfree(vm->sbm.sb_states);

        /* reset the device and cleanup the queues */
        vdev->config->reset(vdev);
        vdev->config->del_vqs(vdev);

        kfree(vm);
        vdev->priv = NULL;
}

static void virtio_mem_config_changed(struct virtio_device *vdev)
{
        struct virtio_mem *vm = vdev->priv;

        atomic_set(&vm->config_changed, 1);
        virtio_mem_retry(vm);
}

#ifdef CONFIG_PM_SLEEP
static int virtio_mem_freeze(struct virtio_device *vdev)
{
        /*
         * When restarting the VM, all memory is usually unplugged. Don't
         * allow to suspend/hibernate.
         */
        dev_err(&vdev->dev, "save/restore not supported.\n");
        return -EPERM;
}

static int virtio_mem_restore(struct virtio_device *vdev)
{
        return -EPERM;
}
#endif

static unsigned int virtio_mem_features[] = {
#if defined(CONFIG_NUMA) && defined(CONFIG_ACPI_NUMA)
        VIRTIO_MEM_F_ACPI_PXM,
#endif
};

static const struct virtio_device_id virtio_mem_id_table[] = {
        { VIRTIO_ID_MEM, VIRTIO_DEV_ANY_ID },
        { 0 },
};

static struct virtio_driver virtio_mem_driver = {
        .feature_table = virtio_mem_features,
        .feature_table_size = ARRAY_SIZE(virtio_mem_features),
        .driver.name = KBUILD_MODNAME,
        .driver.owner = THIS_MODULE,
        .id_table = virtio_mem_id_table,
        .probe = virtio_mem_probe,
        .remove = virtio_mem_remove,
        .config_changed = virtio_mem_config_changed,
#ifdef CONFIG_PM_SLEEP
        .freeze =       virtio_mem_freeze,
        .restore =      virtio_mem_restore,
#endif
};

module_virtio_driver(virtio_mem_driver);
MODULE_DEVICE_TABLE(virtio, virtio_mem_id_table);
MODULE_AUTHOR("David Hildenbrand <david@redhat.com>");
MODULE_DESCRIPTION("Virtio-mem driver");
MODULE_LICENSE("GPL");