btrfs: init device stats for seed devices

[mirror_ubuntu-hirsute-kernel.git] / lib / test_hmm.c

// SPDX-License-Identifier: GPL-2.0
/*
 * This is a module to test the HMM (Heterogeneous Memory Management)
 * mirror and zone device private memory migration APIs of the kernel.
 * Userspace programs can register with the driver to mirror their own address
 * space and can use the device to read/write any valid virtual address.
 */
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/mutex.h>
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/highmem.h>
#include <linux/delay.h>
#include <linux/pagemap.h>
#include <linux/hmm.h>
#include <linux/vmalloc.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/sched/mm.h>
#include <linux/platform_device.h>

#include "test_hmm_uapi.h"

#define DMIRROR_NDEVICES                2
#define DMIRROR_RANGE_FAULT_TIMEOUT     1000
#define DEVMEM_CHUNK_SIZE               (256 * 1024 * 1024U)
#define DEVMEM_CHUNKS_RESERVE           16

static const struct dev_pagemap_ops dmirror_devmem_ops;
static const struct mmu_interval_notifier_ops dmirror_min_ops;
static dev_t dmirror_dev;
static struct page *dmirror_zero_page;

struct dmirror_device;

struct dmirror_bounce {
        void                    *ptr;
        unsigned long           size;
        unsigned long           addr;
        unsigned long           cpages;
};

#define DPT_XA_TAG_WRITE 3UL

/*
 * Data structure to track address ranges and register for mmu interval
 * notifier updates.
 */
struct dmirror_interval {
        struct mmu_interval_notifier    notifier;
        struct dmirror                  *dmirror;
};

/*
 * Data attached to the open device file.
 * Note that it might be shared after a fork().
 */
struct dmirror {
        struct dmirror_device           *mdevice;
        struct xarray                   pt;
        struct mmu_interval_notifier    notifier;
        struct mutex                    mutex;
};

/*
 * ZONE_DEVICE pages for migration and simulating device memory.
 */
struct dmirror_chunk {
        struct dev_pagemap      pagemap;
        struct dmirror_device   *mdevice;
};

/*
 * Per device data.
 */
struct dmirror_device {
        struct cdev             cdevice;
        struct hmm_devmem       *devmem;

        unsigned int            devmem_capacity;
        unsigned int            devmem_count;
        struct dmirror_chunk    **devmem_chunks;
        struct mutex            devmem_lock;    /* protects the above */

        unsigned long           calloc;
        unsigned long           cfree;
        struct page             *free_pages;
        spinlock_t              lock;           /* protects the above */
};

static struct dmirror_device dmirror_devices[DMIRROR_NDEVICES];

static int dmirror_bounce_init(struct dmirror_bounce *bounce,
                               unsigned long addr,
                               unsigned long size)
{
        bounce->addr = addr;
        bounce->size = size;
        bounce->cpages = 0;
        bounce->ptr = vmalloc(size);
        if (!bounce->ptr)
                return -ENOMEM;
        return 0;
}

static void dmirror_bounce_fini(struct dmirror_bounce *bounce)
{
        vfree(bounce->ptr);
}

static int dmirror_fops_open(struct inode *inode, struct file *filp)
{
        struct cdev *cdev = inode->i_cdev;
        struct dmirror *dmirror;
        int ret;

        /* Mirror this process address space */
        dmirror = kzalloc(sizeof(*dmirror), GFP_KERNEL);
        if (dmirror == NULL)
                return -ENOMEM;

        dmirror->mdevice = container_of(cdev, struct dmirror_device, cdevice);
        mutex_init(&dmirror->mutex);
        xa_init(&dmirror->pt);

        ret = mmu_interval_notifier_insert(&dmirror->notifier, current->mm,
                                0, ULONG_MAX & PAGE_MASK, &dmirror_min_ops);
        if (ret) {
                kfree(dmirror);
                return ret;
        }

        filp->private_data = dmirror;
        return 0;
}

static int dmirror_fops_release(struct inode *inode, struct file *filp)
{
        struct dmirror *dmirror = filp->private_data;

        mmu_interval_notifier_remove(&dmirror->notifier);
        xa_destroy(&dmirror->pt);
        kfree(dmirror);
        return 0;
}

static struct dmirror_device *dmirror_page_to_device(struct page *page)

{
        return container_of(page->pgmap, struct dmirror_chunk,
                            pagemap)->mdevice;
}

static int dmirror_do_fault(struct dmirror *dmirror, struct hmm_range *range)
{
        unsigned long *pfns = range->hmm_pfns;
        unsigned long pfn;

        for (pfn = (range->start >> PAGE_SHIFT);
             pfn < (range->end >> PAGE_SHIFT);
             pfn++, pfns++) {
                struct page *page;
                void *entry;

                /*
                 * Since we asked for hmm_range_fault() to populate pages,
                 * it shouldn't return an error entry on success.
                 */
                WARN_ON(*pfns & HMM_PFN_ERROR);
                WARN_ON(!(*pfns & HMM_PFN_VALID));

                page = hmm_pfn_to_page(*pfns);
                WARN_ON(!page);

                entry = page;
                if (*pfns & HMM_PFN_WRITE)
                        entry = xa_tag_pointer(entry, DPT_XA_TAG_WRITE);
                else if (WARN_ON(range->default_flags & HMM_PFN_WRITE))
                        return -EFAULT;
                entry = xa_store(&dmirror->pt, pfn, entry, GFP_ATOMIC);
                if (xa_is_err(entry))
                        return xa_err(entry);
        }

        return 0;
}

static void dmirror_do_update(struct dmirror *dmirror, unsigned long start,
                              unsigned long end)
{
        unsigned long pfn;
        void *entry;

        /*
         * The XArray doesn't hold references to pages since it relies on
         * the mmu notifier to clear page pointers when they become stale.
         * Therefore, it is OK to just clear the entry.
         */
        xa_for_each_range(&dmirror->pt, pfn, entry, start >> PAGE_SHIFT,
                          end >> PAGE_SHIFT)
                xa_erase(&dmirror->pt, pfn);
}

static bool dmirror_interval_invalidate(struct mmu_interval_notifier *mni,
                                const struct mmu_notifier_range *range,
                                unsigned long cur_seq)
{
        struct dmirror *dmirror = container_of(mni, struct dmirror, notifier);

        /*
         * Ignore invalidation callbacks for device private pages since
         * the invalidation is handled as part of the migration process.
         */
        if (range->event == MMU_NOTIFY_MIGRATE &&
            range->migrate_pgmap_owner == dmirror->mdevice)
                return true;

        if (mmu_notifier_range_blockable(range))
                mutex_lock(&dmirror->mutex);
        else if (!mutex_trylock(&dmirror->mutex))
                return false;

        mmu_interval_set_seq(mni, cur_seq);
        dmirror_do_update(dmirror, range->start, range->end);

        mutex_unlock(&dmirror->mutex);
        return true;
}

static const struct mmu_interval_notifier_ops dmirror_min_ops = {
        .invalidate = dmirror_interval_invalidate,
};

static int dmirror_range_fault(struct dmirror *dmirror,
                                struct hmm_range *range)
{
        struct mm_struct *mm = dmirror->notifier.mm;
        unsigned long timeout =
                jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
        int ret;

        while (true) {
                if (time_after(jiffies, timeout)) {
                        ret = -EBUSY;
                        goto out;
                }

                range->notifier_seq = mmu_interval_read_begin(range->notifier);
                mmap_read_lock(mm);
                ret = hmm_range_fault(range);
                mmap_read_unlock(mm);
                if (ret) {
                        if (ret == -EBUSY)
                                continue;
                        goto out;
                }

                mutex_lock(&dmirror->mutex);
                if (mmu_interval_read_retry(range->notifier,
                                            range->notifier_seq)) {
                        mutex_unlock(&dmirror->mutex);
                        continue;
                }
                break;
        }

        ret = dmirror_do_fault(dmirror, range);

        mutex_unlock(&dmirror->mutex);
out:
        return ret;
}

static int dmirror_fault(struct dmirror *dmirror, unsigned long start,
                         unsigned long end, bool write)
{
        struct mm_struct *mm = dmirror->notifier.mm;
        unsigned long addr;
        unsigned long pfns[64];
        struct hmm_range range = {
                .notifier = &dmirror->notifier,
                .hmm_pfns = pfns,
                .pfn_flags_mask = 0,
                .default_flags =
                        HMM_PFN_REQ_FAULT | (write ? HMM_PFN_REQ_WRITE : 0),
                .dev_private_owner = dmirror->mdevice,
        };
        int ret = 0;

        /* Since the mm is for the mirrored process, get a reference first. */
        if (!mmget_not_zero(mm))
                return 0;

        for (addr = start; addr < end; addr = range.end) {
                range.start = addr;
                range.end = min(addr + (ARRAY_SIZE(pfns) << PAGE_SHIFT), end);

                ret = dmirror_range_fault(dmirror, &range);
                if (ret)
                        break;
        }

        mmput(mm);
        return ret;
}

static int dmirror_do_read(struct dmirror *dmirror, unsigned long start,
                           unsigned long end, struct dmirror_bounce *bounce)
{
        unsigned long pfn;
        void *ptr;

        ptr = bounce->ptr + ((start - bounce->addr) & PAGE_MASK);

        for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++) {
                void *entry;
                struct page *page;
                void *tmp;

                entry = xa_load(&dmirror->pt, pfn);
                page = xa_untag_pointer(entry);
                if (!page)
                        return -ENOENT;

                tmp = kmap(page);
                memcpy(ptr, tmp, PAGE_SIZE);
                kunmap(page);

                ptr += PAGE_SIZE;
                bounce->cpages++;
        }

        return 0;
}

static int dmirror_read(struct dmirror *dmirror, struct hmm_dmirror_cmd *cmd)
{
        struct dmirror_bounce bounce;
        unsigned long start, end;
        unsigned long size = cmd->npages << PAGE_SHIFT;
        int ret;

        start = cmd->addr;
        end = start + size;
        if (end < start)
                return -EINVAL;

        ret = dmirror_bounce_init(&bounce, start, size);
        if (ret)
                return ret;

        while (1) {
                mutex_lock(&dmirror->mutex);
                ret = dmirror_do_read(dmirror, start, end, &bounce);
                mutex_unlock(&dmirror->mutex);
                if (ret != -ENOENT)
                        break;

                start = cmd->addr + (bounce.cpages << PAGE_SHIFT);
                ret = dmirror_fault(dmirror, start, end, false);
                if (ret)
                        break;
                cmd->faults++;
        }

        if (ret == 0) {
                if (copy_to_user(u64_to_user_ptr(cmd->ptr), bounce.ptr,
                                 bounce.size))
                        ret = -EFAULT;
        }
        cmd->cpages = bounce.cpages;
        dmirror_bounce_fini(&bounce);
        return ret;
}

static int dmirror_do_write(struct dmirror *dmirror, unsigned long start,
                            unsigned long end, struct dmirror_bounce *bounce)
{
        unsigned long pfn;
        void *ptr;

        ptr = bounce->ptr + ((start - bounce->addr) & PAGE_MASK);

        for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++) {
                void *entry;
                struct page *page;
                void *tmp;

                entry = xa_load(&dmirror->pt, pfn);
                page = xa_untag_pointer(entry);
                if (!page || xa_pointer_tag(entry) != DPT_XA_TAG_WRITE)
                        return -ENOENT;

                tmp = kmap(page);
                memcpy(tmp, ptr, PAGE_SIZE);
                kunmap(page);

                ptr += PAGE_SIZE;
                bounce->cpages++;
        }

        return 0;
}

static int dmirror_write(struct dmirror *dmirror, struct hmm_dmirror_cmd *cmd)
{
        struct dmirror_bounce bounce;
        unsigned long start, end;
        unsigned long size = cmd->npages << PAGE_SHIFT;
        int ret;

        start = cmd->addr;
        end = start + size;
        if (end < start)
                return -EINVAL;

        ret = dmirror_bounce_init(&bounce, start, size);
        if (ret)
                return ret;
        if (copy_from_user(bounce.ptr, u64_to_user_ptr(cmd->ptr),
                           bounce.size)) {
                ret = -EFAULT;
                goto fini;
        }

        while (1) {
                mutex_lock(&dmirror->mutex);
                ret = dmirror_do_write(dmirror, start, end, &bounce);
                mutex_unlock(&dmirror->mutex);
                if (ret != -ENOENT)
                        break;

                start = cmd->addr + (bounce.cpages << PAGE_SHIFT);
                ret = dmirror_fault(dmirror, start, end, true);
                if (ret)
                        break;
                cmd->faults++;
        }

fini:
        cmd->cpages = bounce.cpages;
        dmirror_bounce_fini(&bounce);
        return ret;
}

static bool dmirror_allocate_chunk(struct dmirror_device *mdevice,
                                   struct page **ppage)
{
        struct dmirror_chunk *devmem;
        struct resource *res;
        unsigned long pfn;
        unsigned long pfn_first;
        unsigned long pfn_last;
        void *ptr;

        mutex_lock(&mdevice->devmem_lock);

        if (mdevice->devmem_count == mdevice->devmem_capacity) {
                struct dmirror_chunk **new_chunks;
                unsigned int new_capacity;

                new_capacity = mdevice->devmem_capacity +
                                DEVMEM_CHUNKS_RESERVE;
                new_chunks = krealloc(mdevice->devmem_chunks,
                                sizeof(new_chunks[0]) * new_capacity,
                                GFP_KERNEL);
                if (!new_chunks)
                        goto err;
                mdevice->devmem_capacity = new_capacity;
                mdevice->devmem_chunks = new_chunks;
        }

        res = request_free_mem_region(&iomem_resource, DEVMEM_CHUNK_SIZE,
                                        "hmm_dmirror");
        if (IS_ERR(res))
                goto err;

        devmem = kzalloc(sizeof(*devmem), GFP_KERNEL);
        if (!devmem)
                goto err_release;

        devmem->pagemap.type = MEMORY_DEVICE_PRIVATE;
        devmem->pagemap.res = *res;
        devmem->pagemap.ops = &dmirror_devmem_ops;
        devmem->pagemap.owner = mdevice;

        ptr = memremap_pages(&devmem->pagemap, numa_node_id());
        if (IS_ERR(ptr))
                goto err_free;

        devmem->mdevice = mdevice;
        pfn_first = devmem->pagemap.res.start >> PAGE_SHIFT;
        pfn_last = pfn_first +
                (resource_size(&devmem->pagemap.res) >> PAGE_SHIFT);
        mdevice->devmem_chunks[mdevice->devmem_count++] = devmem;

        mutex_unlock(&mdevice->devmem_lock);

        pr_info("added new %u MB chunk (total %u chunks, %u MB) PFNs [0x%lx 0x%lx)\n",
                DEVMEM_CHUNK_SIZE / (1024 * 1024),
                mdevice->devmem_count,
                mdevice->devmem_count * (DEVMEM_CHUNK_SIZE / (1024 * 1024)),
                pfn_first, pfn_last);

        spin_lock(&mdevice->lock);
        for (pfn = pfn_first; pfn < pfn_last; pfn++) {
                struct page *page = pfn_to_page(pfn);

                page->zone_device_data = mdevice->free_pages;
                mdevice->free_pages = page;
        }
        if (ppage) {
                *ppage = mdevice->free_pages;
                mdevice->free_pages = (*ppage)->zone_device_data;
                mdevice->calloc++;
        }
        spin_unlock(&mdevice->lock);

        return true;

err_free:
        kfree(devmem);
err_release:
        release_mem_region(res->start, resource_size(res));
err:
        mutex_unlock(&mdevice->devmem_lock);
        return false;
}

static struct page *dmirror_devmem_alloc_page(struct dmirror_device *mdevice)
{
        struct page *dpage = NULL;
        struct page *rpage;

        /*
         * This is a fake device so we alloc real system memory to store
         * our device memory.
         */
        rpage = alloc_page(GFP_HIGHUSER);
        if (!rpage)
                return NULL;

        spin_lock(&mdevice->lock);

        if (mdevice->free_pages) {
                dpage = mdevice->free_pages;
                mdevice->free_pages = dpage->zone_device_data;
                mdevice->calloc++;
                spin_unlock(&mdevice->lock);
        } else {
                spin_unlock(&mdevice->lock);
                if (!dmirror_allocate_chunk(mdevice, &dpage))
                        goto error;
        }

        dpage->zone_device_data = rpage;
        get_page(dpage);
        lock_page(dpage);
        return dpage;

error:
        __free_page(rpage);
        return NULL;
}

static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
                                           struct dmirror *dmirror)
{
        struct dmirror_device *mdevice = dmirror->mdevice;
        const unsigned long *src = args->src;
        unsigned long *dst = args->dst;
        unsigned long addr;

        for (addr = args->start; addr < args->end; addr += PAGE_SIZE,
                                                   src++, dst++) {
                struct page *spage;
                struct page *dpage;
                struct page *rpage;

                if (!(*src & MIGRATE_PFN_MIGRATE))
                        continue;

                /*
                 * Note that spage might be NULL which is OK since it is an
                 * unallocated pte_none() or read-only zero page.
                 */
                spage = migrate_pfn_to_page(*src);

                dpage = dmirror_devmem_alloc_page(mdevice);
                if (!dpage)
                        continue;

                rpage = dpage->zone_device_data;
                if (spage)
                        copy_highpage(rpage, spage);
                else
                        clear_highpage(rpage);

                /*
                 * Normally, a device would use the page->zone_device_data to
                 * point to the mirror but here we use it to hold the page for
                 * the simulated device memory and that page holds the pointer
                 * to the mirror.
                 */
                rpage->zone_device_data = dmirror;

                *dst = migrate_pfn(page_to_pfn(dpage)) |
                            MIGRATE_PFN_LOCKED;
                if ((*src & MIGRATE_PFN_WRITE) ||
                    (!spage && args->vma->vm_flags & VM_WRITE))
                        *dst |= MIGRATE_PFN_WRITE;
        }
}

static int dmirror_migrate_finalize_and_map(struct migrate_vma *args,
                                            struct dmirror *dmirror)
{
        unsigned long start = args->start;
        unsigned long end = args->end;
        const unsigned long *src = args->src;
        const unsigned long *dst = args->dst;
        unsigned long pfn;

        /* Map the migrated pages into the device's page tables. */
        mutex_lock(&dmirror->mutex);

        for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++,
                                                                src++, dst++) {
                struct page *dpage;
                void *entry;

                if (!(*src & MIGRATE_PFN_MIGRATE))
                        continue;

                dpage = migrate_pfn_to_page(*dst);
                if (!dpage)
                        continue;

                /*
                 * Store the page that holds the data so the page table
                 * doesn't have to deal with ZONE_DEVICE private pages.
                 */
                entry = dpage->zone_device_data;
                if (*dst & MIGRATE_PFN_WRITE)
                        entry = xa_tag_pointer(entry, DPT_XA_TAG_WRITE);
                entry = xa_store(&dmirror->pt, pfn, entry, GFP_ATOMIC);
                if (xa_is_err(entry)) {
                        mutex_unlock(&dmirror->mutex);
                        return xa_err(entry);
                }
        }

        mutex_unlock(&dmirror->mutex);
        return 0;
}

static int dmirror_migrate(struct dmirror *dmirror,
                           struct hmm_dmirror_cmd *cmd)
{
        unsigned long start, end, addr;
        unsigned long size = cmd->npages << PAGE_SHIFT;
        struct mm_struct *mm = dmirror->notifier.mm;
        struct vm_area_struct *vma;
        unsigned long src_pfns[64];
        unsigned long dst_pfns[64];
        struct dmirror_bounce bounce;
        struct migrate_vma args;
        unsigned long next;
        int ret;

        start = cmd->addr;
        end = start + size;
        if (end < start)
                return -EINVAL;

        /* Since the mm is for the mirrored process, get a reference first. */
        if (!mmget_not_zero(mm))
                return -EINVAL;

        mmap_read_lock(mm);
        for (addr = start; addr < end; addr = next) {
                vma = find_vma(mm, addr);
                if (!vma || addr < vma->vm_start ||
                    !(vma->vm_flags & VM_READ)) {
                        ret = -EINVAL;
                        goto out;
                }
                next = min(end, addr + (ARRAY_SIZE(src_pfns) << PAGE_SHIFT));
                if (next > vma->vm_end)
                        next = vma->vm_end;

                args.vma = vma;
                args.src = src_pfns;
                args.dst = dst_pfns;
                args.start = addr;
                args.end = next;
                args.pgmap_owner = dmirror->mdevice;
                args.flags = MIGRATE_VMA_SELECT_SYSTEM;
                ret = migrate_vma_setup(&args);
                if (ret)
                        goto out;

                dmirror_migrate_alloc_and_copy(&args, dmirror);
                migrate_vma_pages(&args);
                dmirror_migrate_finalize_and_map(&args, dmirror);
                migrate_vma_finalize(&args);
        }
        mmap_read_unlock(mm);
        mmput(mm);

        /* Return the migrated data for verification. */
        ret = dmirror_bounce_init(&bounce, start, size);
        if (ret)
                return ret;
        mutex_lock(&dmirror->mutex);
        ret = dmirror_do_read(dmirror, start, end, &bounce);
        mutex_unlock(&dmirror->mutex);
        if (ret == 0) {
                if (copy_to_user(u64_to_user_ptr(cmd->ptr), bounce.ptr,
                                 bounce.size))
                        ret = -EFAULT;
        }
        cmd->cpages = bounce.cpages;
        dmirror_bounce_fini(&bounce);
        return ret;

out:
        mmap_read_unlock(mm);
        mmput(mm);
        return ret;
}

static void dmirror_mkentry(struct dmirror *dmirror, struct hmm_range *range,
                            unsigned char *perm, unsigned long entry)
{
        struct page *page;

        if (entry & HMM_PFN_ERROR) {
                *perm = HMM_DMIRROR_PROT_ERROR;
                return;
        }
        if (!(entry & HMM_PFN_VALID)) {
                *perm = HMM_DMIRROR_PROT_NONE;
                return;
        }

        page = hmm_pfn_to_page(entry);
        if (is_device_private_page(page)) {
                /* Is the page migrated to this device or some other? */
                if (dmirror->mdevice == dmirror_page_to_device(page))
                        *perm = HMM_DMIRROR_PROT_DEV_PRIVATE_LOCAL;
                else
                        *perm = HMM_DMIRROR_PROT_DEV_PRIVATE_REMOTE;
        } else if (is_zero_pfn(page_to_pfn(page)))
                *perm = HMM_DMIRROR_PROT_ZERO;
        else
                *perm = HMM_DMIRROR_PROT_NONE;
        if (entry & HMM_PFN_WRITE)
                *perm |= HMM_DMIRROR_PROT_WRITE;
        else
                *perm |= HMM_DMIRROR_PROT_READ;
        if (hmm_pfn_to_map_order(entry) + PAGE_SHIFT == PMD_SHIFT)
                *perm |= HMM_DMIRROR_PROT_PMD;
        else if (hmm_pfn_to_map_order(entry) + PAGE_SHIFT == PUD_SHIFT)
                *perm |= HMM_DMIRROR_PROT_PUD;
}

static bool dmirror_snapshot_invalidate(struct mmu_interval_notifier *mni,
                                const struct mmu_notifier_range *range,
                                unsigned long cur_seq)
{
        struct dmirror_interval *dmi =
                container_of(mni, struct dmirror_interval, notifier);
        struct dmirror *dmirror = dmi->dmirror;

        if (mmu_notifier_range_blockable(range))
                mutex_lock(&dmirror->mutex);
        else if (!mutex_trylock(&dmirror->mutex))
                return false;

        /*
         * Snapshots only need to set the sequence number since any
         * invalidation in the interval invalidates the whole snapshot.
         */
        mmu_interval_set_seq(mni, cur_seq);

        mutex_unlock(&dmirror->mutex);
        return true;
}

static const struct mmu_interval_notifier_ops dmirror_mrn_ops = {
        .invalidate = dmirror_snapshot_invalidate,
};

static int dmirror_range_snapshot(struct dmirror *dmirror,
                                  struct hmm_range *range,
                                  unsigned char *perm)
{
        struct mm_struct *mm = dmirror->notifier.mm;
        struct dmirror_interval notifier;
        unsigned long timeout =
                jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
        unsigned long i;
        unsigned long n;
        int ret = 0;

        notifier.dmirror = dmirror;
        range->notifier = &notifier.notifier;

        ret = mmu_interval_notifier_insert(range->notifier, mm,
                        range->start, range->end - range->start,
                        &dmirror_mrn_ops);
        if (ret)
                return ret;

        while (true) {
                if (time_after(jiffies, timeout)) {
                        ret = -EBUSY;
                        goto out;
                }

                range->notifier_seq = mmu_interval_read_begin(range->notifier);

                mmap_read_lock(mm);
                ret = hmm_range_fault(range);
                mmap_read_unlock(mm);
                if (ret) {
                        if (ret == -EBUSY)
                                continue;
                        goto out;
                }

                mutex_lock(&dmirror->mutex);
                if (mmu_interval_read_retry(range->notifier,
                                            range->notifier_seq)) {
                        mutex_unlock(&dmirror->mutex);
                        continue;
                }
                break;
        }

        n = (range->end - range->start) >> PAGE_SHIFT;
        for (i = 0; i < n; i++)
                dmirror_mkentry(dmirror, range, perm + i, range->hmm_pfns[i]);

        mutex_unlock(&dmirror->mutex);
out:
        mmu_interval_notifier_remove(range->notifier);
        return ret;
}

static int dmirror_snapshot(struct dmirror *dmirror,
                            struct hmm_dmirror_cmd *cmd)
{
        struct mm_struct *mm = dmirror->notifier.mm;
        unsigned long start, end;
        unsigned long size = cmd->npages << PAGE_SHIFT;
        unsigned long addr;
        unsigned long next;
        unsigned long pfns[64];
        unsigned char perm[64];
        char __user *uptr;
        struct hmm_range range = {
                .hmm_pfns = pfns,
                .dev_private_owner = dmirror->mdevice,
        };
        int ret = 0;

        start = cmd->addr;
        end = start + size;
        if (end < start)
                return -EINVAL;

        /* Since the mm is for the mirrored process, get a reference first. */
        if (!mmget_not_zero(mm))
                return -EINVAL;

        /*
         * Register a temporary notifier to detect invalidations even if it
         * overlaps with other mmu_interval_notifiers.
         */
        uptr = u64_to_user_ptr(cmd->ptr);
        for (addr = start; addr < end; addr = next) {
                unsigned long n;

                next = min(addr + (ARRAY_SIZE(pfns) << PAGE_SHIFT), end);
                range.start = addr;
                range.end = next;

                ret = dmirror_range_snapshot(dmirror, &range, perm);
                if (ret)
                        break;

                n = (range.end - range.start) >> PAGE_SHIFT;
                if (copy_to_user(uptr, perm, n)) {
                        ret = -EFAULT;
                        break;
                }

                cmd->cpages += n;
                uptr += n;
        }
        mmput(mm);

        return ret;
}

static long dmirror_fops_unlocked_ioctl(struct file *filp,
                                        unsigned int command,
                                        unsigned long arg)
{
        void __user *uarg = (void __user *)arg;
        struct hmm_dmirror_cmd cmd;
        struct dmirror *dmirror;
        int ret;

        dmirror = filp->private_data;
        if (!dmirror)
                return -EINVAL;

        if (copy_from_user(&cmd, uarg, sizeof(cmd)))
                return -EFAULT;

        if (cmd.addr & ~PAGE_MASK)
                return -EINVAL;
        if (cmd.addr >= (cmd.addr + (cmd.npages << PAGE_SHIFT)))
                return -EINVAL;

        cmd.cpages = 0;
        cmd.faults = 0;

        switch (command) {
        case HMM_DMIRROR_READ:
                ret = dmirror_read(dmirror, &cmd);
                break;

        case HMM_DMIRROR_WRITE:
                ret = dmirror_write(dmirror, &cmd);
                break;

        case HMM_DMIRROR_MIGRATE:
                ret = dmirror_migrate(dmirror, &cmd);
                break;

        case HMM_DMIRROR_SNAPSHOT:
                ret = dmirror_snapshot(dmirror, &cmd);
                break;

        default:
                return -EINVAL;
        }
        if (ret)
                return ret;

        if (copy_to_user(uarg, &cmd, sizeof(cmd)))
                return -EFAULT;

        return 0;
}

static const struct file_operations dmirror_fops = {
        .open           = dmirror_fops_open,
        .release        = dmirror_fops_release,
        .unlocked_ioctl = dmirror_fops_unlocked_ioctl,
        .llseek         = default_llseek,
        .owner          = THIS_MODULE,
};

static void dmirror_devmem_free(struct page *page)
{
        struct page *rpage = page->zone_device_data;
        struct dmirror_device *mdevice;

        if (rpage)
                __free_page(rpage);

        mdevice = dmirror_page_to_device(page);

        spin_lock(&mdevice->lock);
        mdevice->cfree++;
        page->zone_device_data = mdevice->free_pages;
        mdevice->free_pages = page;
        spin_unlock(&mdevice->lock);
}

static vm_fault_t dmirror_devmem_fault_alloc_and_copy(struct migrate_vma *args,
                                                      struct dmirror *dmirror)
{
        const unsigned long *src = args->src;
        unsigned long *dst = args->dst;
        unsigned long start = args->start;
        unsigned long end = args->end;
        unsigned long addr;

        for (addr = start; addr < end; addr += PAGE_SIZE,
                                       src++, dst++) {
                struct page *dpage, *spage;

                spage = migrate_pfn_to_page(*src);
                if (!spage || !(*src & MIGRATE_PFN_MIGRATE))
                        continue;
                spage = spage->zone_device_data;

                dpage = alloc_page_vma(GFP_HIGHUSER_MOVABLE, args->vma, addr);
                if (!dpage)
                        continue;

                lock_page(dpage);
                xa_erase(&dmirror->pt, addr >> PAGE_SHIFT);
                copy_highpage(dpage, spage);
                *dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
                if (*src & MIGRATE_PFN_WRITE)
                        *dst |= MIGRATE_PFN_WRITE;
        }
        return 0;
}

static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
{
        struct migrate_vma args;
        unsigned long src_pfns;
        unsigned long dst_pfns;
        struct page *rpage;
        struct dmirror *dmirror;
        vm_fault_t ret;

        /*
         * Normally, a device would use the page->zone_device_data to point to
         * the mirror but here we use it to hold the page for the simulated
         * device memory and that page holds the pointer to the mirror.
         */
        rpage = vmf->page->zone_device_data;
        dmirror = rpage->zone_device_data;

        /* FIXME demonstrate how we can adjust migrate range */
        args.vma = vmf->vma;
        args.start = vmf->address;
        args.end = args.start + PAGE_SIZE;
        args.src = &src_pfns;
        args.dst = &dst_pfns;
        args.pgmap_owner = dmirror->mdevice;
        args.flags = MIGRATE_VMA_SELECT_DEVICE_PRIVATE;

        if (migrate_vma_setup(&args))
                return VM_FAULT_SIGBUS;

        ret = dmirror_devmem_fault_alloc_and_copy(&args, dmirror);
        if (ret)
                return ret;
        migrate_vma_pages(&args);
        /*
         * No device finalize step is needed since
         * dmirror_devmem_fault_alloc_and_copy() will have already
         * invalidated the device page table.
         */
        migrate_vma_finalize(&args);
        return 0;
}

static const struct dev_pagemap_ops dmirror_devmem_ops = {
        .page_free      = dmirror_devmem_free,
        .migrate_to_ram = dmirror_devmem_fault,
};

static int dmirror_device_init(struct dmirror_device *mdevice, int id)
{
        dev_t dev;
        int ret;

        dev = MKDEV(MAJOR(dmirror_dev), id);
        mutex_init(&mdevice->devmem_lock);
        spin_lock_init(&mdevice->lock);

        cdev_init(&mdevice->cdevice, &dmirror_fops);
        mdevice->cdevice.owner = THIS_MODULE;
        ret = cdev_add(&mdevice->cdevice, dev, 1);
        if (ret)
                return ret;

        /* Build a list of free ZONE_DEVICE private struct pages */
        dmirror_allocate_chunk(mdevice, NULL);

        return 0;
}

static void dmirror_device_remove(struct dmirror_device *mdevice)
{
        unsigned int i;

        if (mdevice->devmem_chunks) {
                for (i = 0; i < mdevice->devmem_count; i++) {
                        struct dmirror_chunk *devmem =
                                mdevice->devmem_chunks[i];

                        memunmap_pages(&devmem->pagemap);
                        release_mem_region(devmem->pagemap.res.start,
                                           resource_size(&devmem->pagemap.res));
                        kfree(devmem);
                }
                kfree(mdevice->devmem_chunks);
        }

        cdev_del(&mdevice->cdevice);
}

static int __init hmm_dmirror_init(void)
{
        int ret;
        int id;

        ret = alloc_chrdev_region(&dmirror_dev, 0, DMIRROR_NDEVICES,
                                  "HMM_DMIRROR");
        if (ret)
                goto err_unreg;

        for (id = 0; id < DMIRROR_NDEVICES; id++) {
                ret = dmirror_device_init(dmirror_devices + id, id);
                if (ret)
                        goto err_chrdev;
        }

        /*
         * Allocate a zero page to simulate a reserved page of device private
         * memory which is always zero. The zero_pfn page isn't used just to
         * make the code here simpler (i.e., we need a struct page for it).
         */
        dmirror_zero_page = alloc_page(GFP_HIGHUSER | __GFP_ZERO);
        if (!dmirror_zero_page) {
                ret = -ENOMEM;
                goto err_chrdev;
        }

        pr_info("HMM test module loaded. This is only for testing HMM.\n");
        return 0;

err_chrdev:
        while (--id >= 0)
                dmirror_device_remove(dmirror_devices + id);
        unregister_chrdev_region(dmirror_dev, DMIRROR_NDEVICES);
err_unreg:
        return ret;
}

static void __exit hmm_dmirror_exit(void)
{
        int id;

        if (dmirror_zero_page)
                __free_page(dmirror_zero_page);
        for (id = 0; id < DMIRROR_NDEVICES; id++)
                dmirror_device_remove(dmirror_devices + id);
        unregister_chrdev_region(dmirror_dev, DMIRROR_NDEVICES);
}

module_init(hmm_dmirror_init);
module_exit(hmm_dmirror_exit);
MODULE_LICENSE("GPL");