// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Virtio balloon implementation, inspired by Dor Laor and Marcelo
 * Tosatti's implementations.
 *
 *  Copyright 2008 Rusty Russell IBM Corporation
 */

#include <linux/virtio.h>
#include <linux/virtio_balloon.h>
#include <linux/swap.h>
#include <linux/workqueue.h>
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/balloon_compaction.h>
#include <linux/oom.h>
#include <linux/wait.h>
#include <linux/mm.h>
#include <linux/mount.h>
#include <linux/magic.h>
#include <linux/pseudo_fs.h>
#include <linux/page_reporting.h>

/*
 * Balloon device works in 4K page units.  So each page is pointed to by
 * multiple balloon pages.  All memory counters in this driver are in balloon
 * page units.
 */
#define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
#define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
/* Maximum number of (4k) pages to deflate on OOM notifications. */
#define VIRTIO_BALLOON_OOM_NR_PAGES 256
#define VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY 80

#define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \
                                             __GFP_NOMEMALLOC)
/* The order of free page blocks to report to host */
#define VIRTIO_BALLOON_HINT_BLOCK_ORDER (MAX_ORDER - 1)
/* The size of a free page block in bytes */
#define VIRTIO_BALLOON_HINT_BLOCK_BYTES \
        (1 << (VIRTIO_BALLOON_HINT_BLOCK_ORDER + PAGE_SHIFT))
#define VIRTIO_BALLOON_HINT_BLOCK_PAGES (1 << VIRTIO_BALLOON_HINT_BLOCK_ORDER)

#ifdef CONFIG_BALLOON_COMPACTION
static struct vfsmount *balloon_mnt;
#endif

enum virtio_balloon_vq {
        VIRTIO_BALLOON_VQ_INFLATE,
        VIRTIO_BALLOON_VQ_DEFLATE,
        VIRTIO_BALLOON_VQ_STATS,
        VIRTIO_BALLOON_VQ_FREE_PAGE,
        VIRTIO_BALLOON_VQ_REPORTING,
        VIRTIO_BALLOON_VQ_MAX
};

enum virtio_balloon_config_read {
        VIRTIO_BALLOON_CONFIG_READ_CMD_ID = 0,
};

struct virtio_balloon {
        struct virtio_device *vdev;
        struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;

        /* Balloon's own wq for cpu-intensive work items */
        struct workqueue_struct *balloon_wq;
        /* The free page reporting work item submitted to the balloon wq */
        struct work_struct report_free_page_work;

        /* The balloon servicing is delegated to a freezable workqueue. */
        struct work_struct update_balloon_stats_work;
        struct work_struct update_balloon_size_work;

        /* Prevent updating balloon when it is being canceled. */
        spinlock_t stop_update_lock;
        bool stop_update;
        /* Bitmap to indicate if reading the related config fields are needed */
        unsigned long config_read_bitmap;

        /* The list of allocated free pages, waiting to be given back to mm */
        struct list_head free_page_list;
        spinlock_t free_page_list_lock;
        /* The number of free page blocks on the above list */
        unsigned long num_free_page_blocks;
        /*
         * The cmd id received from host.
         * Read it via virtio_balloon_cmd_id_received to get the latest value
         * sent from host.
         */
        u32 cmd_id_received_cache;
        /* The cmd id that is actively in use */
        __virtio32 cmd_id_active;
        /* Buffer to store the stop sign */
        __virtio32 cmd_id_stop;

        /* Waiting for host to ack the pages we released. */
        wait_queue_head_t acked;

        /* Number of balloon pages we've told the Host we're not using. */
        unsigned int num_pages;
        /*
         * The pages we've told the Host we're not using are enqueued
         * at vb_dev_info->pages list.
         * Each page on this list adds VIRTIO_BALLOON_PAGES_PER_PAGE
         * to num_pages above.
         */
        struct balloon_dev_info vb_dev_info;

        /* Synchronize access/update to this struct virtio_balloon elements */
        struct mutex balloon_lock;

        /* The array of pfns we tell the Host about. */
        unsigned int num_pfns;
        __virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];

        /* Memory statistics */
        struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];

        /* Shrinker to return free pages - VIRTIO_BALLOON_F_FREE_PAGE_HINT */
        struct shrinker shrinker;

        /* OOM notifier to deflate on OOM - VIRTIO_BALLOON_F_DEFLATE_ON_OOM */
        struct notifier_block oom_nb;

        /* Free page reporting device */
        struct virtqueue *reporting_vq;
        struct page_reporting_dev_info pr_dev_info;
};

static struct virtio_device_id id_table[] = {
        { VIRTIO_ID_BALLOON, VIRTIO_DEV_ANY_ID },
        { 0 },
};

static u32 page_to_balloon_pfn(struct page *page)
{
        unsigned long pfn = page_to_pfn(page);

        BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
        /* Convert pfn from Linux page size to balloon page size. */
        return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
}

static void balloon_ack(struct virtqueue *vq)
{
        struct virtio_balloon *vb = vq->vdev->priv;

        wake_up(&vb->acked);
}

static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
{
        struct scatterlist sg;
        unsigned int len;

        sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);

        /* We should always be able to add one buffer to an empty queue. */
        virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
        virtqueue_kick(vq);

        /* When host has read buffer, this completes via balloon_ack */
        wait_event(vb->acked, virtqueue_get_buf(vq, &len));

}

static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
                                   struct scatterlist *sg, unsigned int nents)
{
        struct virtio_balloon *vb =
                container_of(pr_dev_info, struct virtio_balloon, pr_dev_info);
        struct virtqueue *vq = vb->reporting_vq;
        unsigned int unused, err;

        /* We should always be able to add these buffers to an empty queue. */
        err = virtqueue_add_inbuf(vq, sg, nents, vb, GFP_NOWAIT | __GFP_NOWARN);

        /*
         * In the extremely unlikely case that something has occurred and we
         * are able to trigger an error we will simply display a warning
         * and exit without actually processing the pages.
         */
        if (WARN_ON_ONCE(err))
                return err;

        virtqueue_kick(vq);

        /* When host has read buffer, this completes via balloon_ack */
        wait_event(vb->acked, virtqueue_get_buf(vq, &unused));

        return 0;
}

static void set_page_pfns(struct virtio_balloon *vb,
                          __virtio32 pfns[], struct page *page)
{
        unsigned int i;

        BUILD_BUG_ON(VIRTIO_BALLOON_PAGES_PER_PAGE > VIRTIO_BALLOON_ARRAY_PFNS_MAX);

        /*
         * Set balloon pfns pointing at this page.
         * Note that the first pfn points at start of the page.
         */
        for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
                pfns[i] = cpu_to_virtio32(vb->vdev,
                                          page_to_balloon_pfn(page) + i);
}

static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
{
        unsigned num_allocated_pages;
        unsigned num_pfns;
        struct page *page;
        LIST_HEAD(pages);

        /* We can only do one array worth at a time. */
        num = min(num, ARRAY_SIZE(vb->pfns));

        for (num_pfns = 0; num_pfns < num;
             num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
                struct page *page = balloon_page_alloc();

                if (!page) {
                        dev_info_ratelimited(&vb->vdev->dev,
                                             "Out of puff! Can't get %u pages\n",
                                             VIRTIO_BALLOON_PAGES_PER_PAGE);
                        /* Sleep for at least 1/5 of a second before retry. */
                        msleep(200);
                        break;
                }

                balloon_page_push(&pages, page);
        }

        mutex_lock(&vb->balloon_lock);

        vb->num_pfns = 0;

        while ((page = balloon_page_pop(&pages))) {
                balloon_page_enqueue(&vb->vb_dev_info, page);

                set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
                vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
                if (!virtio_has_feature(vb->vdev,
                                        VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
                        adjust_managed_page_count(page, -1);
                vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
        }

        num_allocated_pages = vb->num_pfns;
        /* Did we get any? */
        if (vb->num_pfns != 0)
                tell_host(vb, vb->inflate_vq);
        mutex_unlock(&vb->balloon_lock);

        return num_allocated_pages;
}

static void release_pages_balloon(struct virtio_balloon *vb,
                                 struct list_head *pages)
{
        struct page *page, *next;

        list_for_each_entry_safe(page, next, pages, lru) {
                if (!virtio_has_feature(vb->vdev,
                                        VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
                        adjust_managed_page_count(page, 1);
                list_del(&page->lru);
                put_page(page); /* balloon reference */
        }
}

static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
{
        unsigned num_freed_pages;
        struct page *page;
        struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
        LIST_HEAD(pages);

        /* We can only do one array worth at a time. */
        num = min(num, ARRAY_SIZE(vb->pfns));

        mutex_lock(&vb->balloon_lock);
        /* We can't release more pages than taken */
        num = min(num, (size_t)vb->num_pages);
        for (vb->num_pfns = 0; vb->num_pfns < num;
             vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
                page = balloon_page_dequeue(vb_dev_info);
                if (!page)
                        break;
                set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
                list_add(&page->lru, &pages);
                vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
        }

        num_freed_pages = vb->num_pfns;
        /*
         * Note that if
         * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
         * is true, we *have* to do it in this order
         */
        if (vb->num_pfns != 0)
                tell_host(vb, vb->deflate_vq);
        release_pages_balloon(vb, &pages);
        mutex_unlock(&vb->balloon_lock);
        return num_freed_pages;
}

static inline void update_stat(struct virtio_balloon *vb, int idx,
                               u16 tag, u64 val)
{
        BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
        vb->stats[idx].tag = cpu_to_virtio16(vb->vdev, tag);
        vb->stats[idx].val = cpu_to_virtio64(vb->vdev, val);
}

#define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)

static unsigned int update_balloon_stats(struct virtio_balloon *vb)
{
        unsigned long events[NR_VM_EVENT_ITEMS];
        struct sysinfo i;
        unsigned int idx = 0;
        long available;
        unsigned long caches;

        all_vm_events(events);
        si_meminfo(&i);

        available = si_mem_available();
        caches = global_node_page_state(NR_FILE_PAGES);

#ifdef CONFIG_VM_EVENT_COUNTERS
        update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
                                pages_to_bytes(events[PSWPIN]));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_OUT,
                                pages_to_bytes(events[PSWPOUT]));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
#ifdef CONFIG_HUGETLB_PAGE
        update_stat(vb, idx++, VIRTIO_BALLOON_S_HTLB_PGALLOC,
                    events[HTLB_BUDDY_PGALLOC]);
        update_stat(vb, idx++, VIRTIO_BALLOON_S_HTLB_PGFAIL,
                    events[HTLB_BUDDY_PGALLOC_FAIL]);
#endif
#endif
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMFREE,
                                pages_to_bytes(i.freeram));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,
                                pages_to_bytes(i.totalram));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_AVAIL,
                                pages_to_bytes(available));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_CACHES,
                                pages_to_bytes(caches));

        return idx;
}

/*
 * While most virtqueues communicate guest-initiated requests to the hypervisor,
 * the stats queue operates in reverse.  The driver initializes the virtqueue
 * with a single buffer.  From that point forward, all conversations consist of
 * a hypervisor request (a call to this function) which directs us to refill
 * the virtqueue with a fresh stats buffer.  Since stats collection can sleep,
 * we delegate the job to a freezable workqueue that will do the actual work via
 * stats_handle_request().
 */
static void stats_request(struct virtqueue *vq)
{
        struct virtio_balloon *vb = vq->vdev->priv;

        spin_lock(&vb->stop_update_lock);
        if (!vb->stop_update)
                queue_work(system_freezable_wq, &vb->update_balloon_stats_work);
        spin_unlock(&vb->stop_update_lock);
}

static void stats_handle_request(struct virtio_balloon *vb)
{
        struct virtqueue *vq;
        struct scatterlist sg;
        unsigned int len, num_stats;

        num_stats = update_balloon_stats(vb);

        vq = vb->stats_vq;
        if (!virtqueue_get_buf(vq, &len))
                return;
        sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
        virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
        virtqueue_kick(vq);
}

static inline s64 towards_target(struct virtio_balloon *vb)
{
        s64 target;
        u32 num_pages;

        virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages,
                     &num_pages);

        /* Legacy balloon config space is LE, unlike all other devices. */
        if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
                num_pages = le32_to_cpu((__force __le32)num_pages);

        target = num_pages;
        return target - vb->num_pages;
}

/* Gives back @num_to_return blocks of free pages to mm. */
static unsigned long return_free_pages_to_mm(struct virtio_balloon *vb,
                                             unsigned long num_to_return)
{
        struct page *page;
        unsigned long num_returned;

        spin_lock_irq(&vb->free_page_list_lock);
        for (num_returned = 0; num_returned < num_to_return; num_returned++) {
                page = balloon_page_pop(&vb->free_page_list);
                if (!page)
                        break;
                free_pages((unsigned long)page_address(page),
                           VIRTIO_BALLOON_HINT_BLOCK_ORDER);
        }
        vb->num_free_page_blocks -= num_returned;
        spin_unlock_irq(&vb->free_page_list_lock);

        return num_returned;
}

static void virtio_balloon_queue_free_page_work(struct virtio_balloon *vb)
{
        if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                return;

        /* No need to queue the work if the bit was already set. */
        if (test_and_set_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
                             &vb->config_read_bitmap))
                return;

        queue_work(vb->balloon_wq, &vb->report_free_page_work);
}

static void virtballoon_changed(struct virtio_device *vdev)
{
        struct virtio_balloon *vb = vdev->priv;
        unsigned long flags;

        spin_lock_irqsave(&vb->stop_update_lock, flags);
        if (!vb->stop_update) {
                queue_work(system_freezable_wq,
                           &vb->update_balloon_size_work);
                virtio_balloon_queue_free_page_work(vb);
        }
        spin_unlock_irqrestore(&vb->stop_update_lock, flags);
}

static void update_balloon_size(struct virtio_balloon *vb)
{
        u32 actual = vb->num_pages;

        /* Legacy balloon config space is LE, unlike all other devices. */
        if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
                actual = (__force u32)cpu_to_le32(actual);

        virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
                      &actual);
}

static void update_balloon_stats_func(struct work_struct *work)
{
        struct virtio_balloon *vb;

        vb = container_of(work, struct virtio_balloon,
                          update_balloon_stats_work);
        stats_handle_request(vb);
}

static void update_balloon_size_func(struct work_struct *work)
{
        struct virtio_balloon *vb;
        s64 diff;

        vb = container_of(work, struct virtio_balloon,
                          update_balloon_size_work);
        diff = towards_target(vb);

        if (!diff)
                return;

        if (diff > 0)
                diff -= fill_balloon(vb, diff);
        else
                diff += leak_balloon(vb, -diff);
        update_balloon_size(vb);

        if (diff)
                queue_work(system_freezable_wq, work);
}

static int init_vqs(struct virtio_balloon *vb)
{
        struct virtqueue *vqs[VIRTIO_BALLOON_VQ_MAX];
        vq_callback_t *callbacks[VIRTIO_BALLOON_VQ_MAX];
        const char *names[VIRTIO_BALLOON_VQ_MAX];
        int err;

        /*
         * Inflateq and deflateq are used unconditionally. The names[]
         * will be NULL if the related feature is not enabled, which will
         * cause no allocation for the corresponding virtqueue in find_vqs.
         */
        callbacks[VIRTIO_BALLOON_VQ_INFLATE] = balloon_ack;
        names[VIRTIO_BALLOON_VQ_INFLATE] = "inflate";
        callbacks[VIRTIO_BALLOON_VQ_DEFLATE] = balloon_ack;
        names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
        callbacks[VIRTIO_BALLOON_VQ_STATS] = NULL;
        names[VIRTIO_BALLOON_VQ_STATS] = NULL;
        callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
        names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
        names[VIRTIO_BALLOON_VQ_REPORTING] = NULL;

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
                names[VIRTIO_BALLOON_VQ_STATS] = "stats";
                callbacks[VIRTIO_BALLOON_VQ_STATS] = stats_request;
        }

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
                names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "free_page_vq";
                callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
        }

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
                names[VIRTIO_BALLOON_VQ_REPORTING] = "reporting_vq";
                callbacks[VIRTIO_BALLOON_VQ_REPORTING] = balloon_ack;
        }

        err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
                                         vqs, callbacks, names, NULL, NULL);
        if (err)
                return err;

        vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
        vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
                struct scatterlist sg;
                unsigned int num_stats;
                vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];

                /*
                 * Prime this virtqueue with one buffer so the hypervisor can
                 * use it to signal us later (it can't be broken yet!).
                 */
                num_stats = update_balloon_stats(vb);

                sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
                err = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
                                           GFP_KERNEL);
                if (err) {
                        dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
                                 __func__);
                        return err;
                }
                virtqueue_kick(vb->stats_vq);
        }

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                vb->free_page_vq = vqs[VIRTIO_BALLOON_VQ_FREE_PAGE];

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
                vb->reporting_vq = vqs[VIRTIO_BALLOON_VQ_REPORTING];

        return 0;
}

static u32 virtio_balloon_cmd_id_received(struct virtio_balloon *vb)
{
        if (test_and_clear_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
                               &vb->config_read_bitmap)) {
                virtio_cread(vb->vdev, struct virtio_balloon_config,
                             free_page_hint_cmd_id,
                             &vb->cmd_id_received_cache);
                /* Legacy balloon config space is LE, unlike all other devices. */
                if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
                        vb->cmd_id_received_cache = le32_to_cpu((__force __le32)vb->cmd_id_received_cache);
        }

        return vb->cmd_id_received_cache;
}

static int send_cmd_id_start(struct virtio_balloon *vb)
{
        struct scatterlist sg;
        struct virtqueue *vq = vb->free_page_vq;
        int err, unused;

        /* Detach all the used buffers from the vq */
        while (virtqueue_get_buf(vq, &unused))
                ;

        vb->cmd_id_active = virtio32_to_cpu(vb->vdev,
                                        virtio_balloon_cmd_id_received(vb));
        sg_init_one(&sg, &vb->cmd_id_active, sizeof(vb->cmd_id_active));
        err = virtqueue_add_outbuf(vq, &sg, 1, &vb->cmd_id_active, GFP_KERNEL);
        if (!err)
                virtqueue_kick(vq);
        return err;
}

static int send_cmd_id_stop(struct virtio_balloon *vb)
{
        struct scatterlist sg;
        struct virtqueue *vq = vb->free_page_vq;
        int err, unused;

        /* Detach all the used buffers from the vq */
        while (virtqueue_get_buf(vq, &unused))
                ;

        sg_init_one(&sg, &vb->cmd_id_stop, sizeof(vb->cmd_id_stop));
        err = virtqueue_add_outbuf(vq, &sg, 1, &vb->cmd_id_stop, GFP_KERNEL);
        if (!err)
                virtqueue_kick(vq);
        return err;
}

static int get_free_page_and_send(struct virtio_balloon *vb)
{
        struct virtqueue *vq = vb->free_page_vq;
        struct page *page;
        struct scatterlist sg;
        int err, unused;
        void *p;

        /* Detach all the used buffers from the vq */
        while (virtqueue_get_buf(vq, &unused))
                ;

        page = alloc_pages(VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG,
                           VIRTIO_BALLOON_HINT_BLOCK_ORDER);
        /*
         * When the allocation returns NULL, it indicates that we have got all
         * the possible free pages, so return -EINTR to stop.
         */
        if (!page)
                return -EINTR;

        p = page_address(page);
        sg_init_one(&sg, p, VIRTIO_BALLOON_HINT_BLOCK_BYTES);
        /* There is always 1 entry reserved for the cmd id to use. */
        if (vq->num_free > 1) {
                err = virtqueue_add_inbuf(vq, &sg, 1, p, GFP_KERNEL);
                if (unlikely(err)) {
                        free_pages((unsigned long)p,
                                   VIRTIO_BALLOON_HINT_BLOCK_ORDER);
                        return err;
                }
                virtqueue_kick(vq);
                spin_lock_irq(&vb->free_page_list_lock);
                balloon_page_push(&vb->free_page_list, page);
                vb->num_free_page_blocks++;
                spin_unlock_irq(&vb->free_page_list_lock);
        } else {
                /*
                 * The vq has no available entry to add this page block, so
                 * just free it.
                 */
                free_pages((unsigned long)p, VIRTIO_BALLOON_HINT_BLOCK_ORDER);
        }

        return 0;
}

static int send_free_pages(struct virtio_balloon *vb)
{
        int err;
        u32 cmd_id_active;

        while (1) {
                /*
                 * If a stop id or a new cmd id was just received from host,
                 * stop the reporting.
                 */
                cmd_id_active = virtio32_to_cpu(vb->vdev, vb->cmd_id_active);
                if (unlikely(cmd_id_active !=
                             virtio_balloon_cmd_id_received(vb)))
                        break;

                /*
                 * The free page blocks are allocated and sent to host one by
                 * one.
                 */
                err = get_free_page_and_send(vb);
                if (err == -EINTR)
                        break;
                else if (unlikely(err))
                        return err;
        }

        return 0;
}

static void virtio_balloon_report_free_page(struct virtio_balloon *vb)
{
        int err;
        struct device *dev = &vb->vdev->dev;

        /* Start by sending the received cmd id to host with an outbuf. */
        err = send_cmd_id_start(vb);
        if (unlikely(err))
                dev_err(dev, "Failed to send a start id, err = %d\n", err);

        err = send_free_pages(vb);
        if (unlikely(err))
                dev_err(dev, "Failed to send a free page, err = %d\n", err);

        /* End by sending a stop id to host with an outbuf. */
        err = send_cmd_id_stop(vb);
        if (unlikely(err))
                dev_err(dev, "Failed to send a stop id, err = %d\n", err);
}

static void report_free_page_func(struct work_struct *work)
{
        struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
                                                 report_free_page_work);
        u32 cmd_id_received;

        cmd_id_received = virtio_balloon_cmd_id_received(vb);
        if (cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
                /* Pass ULONG_MAX to give back all the free pages */
                return_free_pages_to_mm(vb, ULONG_MAX);
        } else if (cmd_id_received != VIRTIO_BALLOON_CMD_ID_STOP &&
                   cmd_id_received !=
                   virtio32_to_cpu(vb->vdev, vb->cmd_id_active)) {
                virtio_balloon_report_free_page(vb);
        }
}

#ifdef CONFIG_BALLOON_COMPACTION
/*
 * virtballoon_migratepage - perform the balloon page migration on behalf of
 *                           a compation thread.     (called under page lock)
 * @vb_dev_info: the balloon device
 * @newpage: page that will replace the isolated page after migration finishes.
 * @page   : the isolated (old) page that is about to be migrated to newpage.
 * @mode   : compaction mode -- not used for balloon page migration.
 *
 * After a ballooned page gets isolated by compaction procedures, this is the
 * function that performs the page migration on behalf of a compaction thread
 * The page migration for virtio balloon is done in a simple swap fashion which
 * follows these two macro steps:
 *  1) insert newpage into vb->pages list and update the host about it;
 *  2) update the host about the old page removed from vb->pages list;
 *
 * This function preforms the balloon page migration task.
 * Called through balloon_mapping->a_ops->migratepage
 */
static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
                struct page *newpage, struct page *page, enum migrate_mode mode)
{
        struct virtio_balloon *vb = container_of(vb_dev_info,
                        struct virtio_balloon, vb_dev_info);
        unsigned long flags;

        /*
         * In order to avoid lock contention while migrating pages concurrently
         * to leak_balloon() or fill_balloon() we just give up the balloon_lock
         * this turn, as it is easier to retry the page migration later.
         * This also prevents fill_balloon() getting stuck into a mutex
         * recursion in the case it ends up triggering memory compaction
         * while it is attempting to inflate the ballon.
         */
        if (!mutex_trylock(&vb->balloon_lock))
                return -EAGAIN;

        get_page(newpage); /* balloon reference */

        /*
          * When we migrate a page to a different zone and adjusted the
          * managed page count when inflating, we have to fixup the count of
          * both involved zones.
          */
        if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM) &&
            page_zone(page) != page_zone(newpage)) {
                adjust_managed_page_count(page, 1);
                adjust_managed_page_count(newpage, -1);
        }

        /* balloon's page migration 1st step  -- inflate "newpage" */
        spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
        balloon_page_insert(vb_dev_info, newpage);
        vb_dev_info->isolated_pages--;
        __count_vm_event(BALLOON_MIGRATE);
        spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
        vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
        set_page_pfns(vb, vb->pfns, newpage);
        tell_host(vb, vb->inflate_vq);

        /* balloon's page migration 2nd step -- deflate "page" */
        spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
        balloon_page_delete(page);
        spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
        vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
        set_page_pfns(vb, vb->pfns, page);
        tell_host(vb, vb->deflate_vq);

        mutex_unlock(&vb->balloon_lock);

        put_page(page); /* balloon reference */

        return MIGRATEPAGE_SUCCESS;
}

static int balloon_init_fs_context(struct fs_context *fc)
{
        return init_pseudo(fc, BALLOON_KVM_MAGIC) ? 0 : -ENOMEM;
}

static struct file_system_type balloon_fs = {
        .name           = "balloon-kvm",
        .init_fs_context = balloon_init_fs_context,
        .kill_sb        = kill_anon_super,
};

#endif /* CONFIG_BALLOON_COMPACTION */

static unsigned long shrink_free_pages(struct virtio_balloon *vb,
                                       unsigned long pages_to_free)
{
        unsigned long blocks_to_free, blocks_freed;

        pages_to_free = round_up(pages_to_free,
                                 VIRTIO_BALLOON_HINT_BLOCK_PAGES);
        blocks_to_free = pages_to_free / VIRTIO_BALLOON_HINT_BLOCK_PAGES;
        blocks_freed = return_free_pages_to_mm(vb, blocks_to_free);

        return blocks_freed * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
}

static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
                                                  struct shrink_control *sc)
{
        struct virtio_balloon *vb = container_of(shrinker,
                                        struct virtio_balloon, shrinker);

        return shrink_free_pages(vb, sc->nr_to_scan);
}

static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
                                                   struct shrink_control *sc)
{
        struct virtio_balloon *vb = container_of(shrinker,
                                        struct virtio_balloon, shrinker);

        return vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
}

static int virtio_balloon_oom_notify(struct notifier_block *nb,
                                     unsigned long dummy, void *parm)
{
        struct virtio_balloon *vb = container_of(nb,
                                                 struct virtio_balloon, oom_nb);
        unsigned long *freed = parm;

        *freed += leak_balloon(vb, VIRTIO_BALLOON_OOM_NR_PAGES) /
                  VIRTIO_BALLOON_PAGES_PER_PAGE;
        update_balloon_size(vb);

        return NOTIFY_OK;
}

static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb)
{
        unregister_shrinker(&vb->shrinker);
}

static int virtio_balloon_register_shrinker(struct virtio_balloon *vb)
{
        vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
        vb->shrinker.count_objects = virtio_balloon_shrinker_count;
        vb->shrinker.seeks = DEFAULT_SEEKS;

        return register_shrinker(&vb->shrinker);
}

static int virtballoon_probe(struct virtio_device *vdev)
{
        struct virtio_balloon *vb;
        int err;

        if (!vdev->config->get) {
                dev_err(&vdev->dev, "%s failure: config access disabled\n",
                        __func__);
                return -EINVAL;
        }

        vdev->priv = vb = kzalloc(sizeof(*vb), GFP_KERNEL);
        if (!vb) {
                err = -ENOMEM;
                goto out;
        }

        INIT_WORK(&vb->update_balloon_stats_work, update_balloon_stats_func);
        INIT_WORK(&vb->update_balloon_size_work, update_balloon_size_func);
        spin_lock_init(&vb->stop_update_lock);
        mutex_init(&vb->balloon_lock);
        init_waitqueue_head(&vb->acked);
        vb->vdev = vdev;

        balloon_devinfo_init(&vb->vb_dev_info);

        err = init_vqs(vb);
        if (err)
                goto out_free_vb;

#ifdef CONFIG_BALLOON_COMPACTION
        balloon_mnt = kern_mount(&balloon_fs);
        if (IS_ERR(balloon_mnt)) {
                err = PTR_ERR(balloon_mnt);
                goto out_del_vqs;
        }

        vb->vb_dev_info.migratepage = virtballoon_migratepage;
        vb->vb_dev_info.inode = alloc_anon_inode(balloon_mnt->mnt_sb);
        if (IS_ERR(vb->vb_dev_info.inode)) {
                err = PTR_ERR(vb->vb_dev_info.inode);
                goto out_kern_unmount;
        }
        vb->vb_dev_info.inode->i_mapping->a_ops = &balloon_aops;
#endif
        if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
                /*
                 * There is always one entry reserved for cmd id, so the ring
                 * size needs to be at least two to report free page hints.
                 */
                if (virtqueue_get_vring_size(vb->free_page_vq) < 2) {
                        err = -ENOSPC;
                        goto out_iput;
                }
                vb->balloon_wq = alloc_workqueue("balloon-wq",
                                        WQ_FREEZABLE | WQ_CPU_INTENSIVE, 0);
                if (!vb->balloon_wq) {
                        err = -ENOMEM;
                        goto out_iput;
                }
                INIT_WORK(&vb->report_free_page_work, report_free_page_func);
                vb->cmd_id_received_cache = VIRTIO_BALLOON_CMD_ID_STOP;
                vb->cmd_id_active = cpu_to_virtio32(vb->vdev,
                                                  VIRTIO_BALLOON_CMD_ID_STOP);
                vb->cmd_id_stop = cpu_to_virtio32(vb->vdev,
                                                  VIRTIO_BALLOON_CMD_ID_STOP);
                spin_lock_init(&vb->free_page_list_lock);
                INIT_LIST_HEAD(&vb->free_page_list);
                /*
                 * We're allowed to reuse any free pages, even if they are
                 * still to be processed by the host.
                 */
                err = virtio_balloon_register_shrinker(vb);
                if (err)
                        goto out_del_balloon_wq;
        }

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
                vb->oom_nb.notifier_call = virtio_balloon_oom_notify;
                vb->oom_nb.priority = VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY;
                err = register_oom_notifier(&vb->oom_nb);
                if (err < 0)
                        goto out_unregister_shrinker;
        }

        if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) {
                /* Start with poison val of 0 representing general init */
                __u32 poison_val = 0;

                /*
                 * Let the hypervisor know that we are expecting a
                 * specific value to be written back in balloon pages.
                 *
                 * If the PAGE_POISON value was larger than a byte we would
                 * need to byte swap poison_val here to guarantee it is
                 * little-endian. However for now it is a single byte so we
                 * can pass it as-is.
                 */
                if (!want_init_on_free())
                        memset(&poison_val, PAGE_POISON, sizeof(poison_val));

                virtio_cwrite(vb->vdev, struct virtio_balloon_config,
                              poison_val, &poison_val);
        }

        vb->pr_dev_info.report = virtballoon_free_page_report;
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
                unsigned int capacity;

                capacity = virtqueue_get_vring_size(vb->reporting_vq);
                if (capacity < PAGE_REPORTING_CAPACITY) {
                        err = -ENOSPC;

                        goto out_unregister_oom;
                }

                err = page_reporting_register(&vb->pr_dev_info);
                if (err)
                        goto out_unregister_oom;
        }

        virtio_device_ready(vdev);

        if (towards_target(vb))
                virtballoon_changed(vdev);
        return 0;

out_unregister_oom:
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
                unregister_oom_notifier(&vb->oom_nb);
out_unregister_shrinker:
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                virtio_balloon_unregister_shrinker(vb);
out_del_balloon_wq:
        if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                destroy_workqueue(vb->balloon_wq);
out_iput:
#ifdef CONFIG_BALLOON_COMPACTION
        iput(vb->vb_dev_info.inode);
out_kern_unmount:
        kern_unmount(balloon_mnt);
out_del_vqs:
#endif
        vdev->config->del_vqs(vdev);
out_free_vb:
        kfree(vb);
out:
        return err;
}

static void remove_common(struct virtio_balloon *vb)
{
        /* There might be pages left in the balloon: free them. */
        while (vb->num_pages)
                leak_balloon(vb, vb->num_pages);
        update_balloon_size(vb);

        /* There might be free pages that are being reported: release them. */
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                return_free_pages_to_mm(vb, ULONG_MAX);

        /* Now we reset the device so we can clean up the queues. */
        vb->vdev->config->reset(vb->vdev);

        vb->vdev->config->del_vqs(vb->vdev);
}

static void virtballoon_remove(struct virtio_device *vdev)
{
        struct virtio_balloon *vb = vdev->priv;

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
                page_reporting_unregister(&vb->pr_dev_info);
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
                unregister_oom_notifier(&vb->oom_nb);
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
                virtio_balloon_unregister_shrinker(vb);
        spin_lock_irq(&vb->stop_update_lock);
        vb->stop_update = true;
        spin_unlock_irq(&vb->stop_update_lock);
        cancel_work_sync(&vb->update_balloon_size_work);
        cancel_work_sync(&vb->update_balloon_stats_work);

        if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
                cancel_work_sync(&vb->report_free_page_work);
                destroy_workqueue(vb->balloon_wq);
        }

        remove_common(vb);
#ifdef CONFIG_BALLOON_COMPACTION
        if (vb->vb_dev_info.inode)
                iput(vb->vb_dev_info.inode);

        kern_unmount(balloon_mnt);
#endif
        kfree(vb);
}

#ifdef CONFIG_PM_SLEEP
static int virtballoon_freeze(struct virtio_device *vdev)
{
        struct virtio_balloon *vb = vdev->priv;

        /*
         * The workqueue is already frozen by the PM core before this
         * function is called.
         */
        remove_common(vb);
        return 0;
}

static int virtballoon_restore(struct virtio_device *vdev)
{
        struct virtio_balloon *vb = vdev->priv;
        int ret;

        ret = init_vqs(vdev->priv);
        if (ret)
                return ret;

        virtio_device_ready(vdev);

        if (towards_target(vb))
                virtballoon_changed(vdev);
        update_balloon_size(vb);
        return 0;
}
#endif

static int virtballoon_validate(struct virtio_device *vdev)
{
        /*
         * Inform the hypervisor that our pages are poisoned or
         * initialized. If we cannot do that then we should disable
         * page reporting as it could potentially change the contents
         * of our free pages.
         */
        if (!want_init_on_free() &&
            (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY) ||
             !page_poisoning_enabled()))
                __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
        else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON))
                __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);

        __virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
        return 0;
}

static unsigned int features[] = {
        VIRTIO_BALLOON_F_MUST_TELL_HOST,
        VIRTIO_BALLOON_F_STATS_VQ,
        VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
        VIRTIO_BALLOON_F_FREE_PAGE_HINT,
        VIRTIO_BALLOON_F_PAGE_POISON,
        VIRTIO_BALLOON_F_REPORTING,
};

static struct virtio_driver virtio_balloon_driver = {
        .feature_table = features,
        .feature_table_size = ARRAY_SIZE(features),
        .driver.name =  KBUILD_MODNAME,
        .driver.owner = THIS_MODULE,
        .id_table =     id_table,
        .validate =     virtballoon_validate,
        .probe =        virtballoon_probe,
        .remove =       virtballoon_remove,
        .config_changed = virtballoon_changed,
#ifdef CONFIG_PM_SLEEP
        .freeze =       virtballoon_freeze,
        .restore =      virtballoon_restore,
#endif
};

module_virtio_driver(virtio_balloon_driver);
MODULE_DEVICE_TABLE(virtio, id_table);
MODULE_DESCRIPTION("Virtio balloon driver");
MODULE_LICENSE("GPL");