Linux 4.20.17

[thirdparty/kernel/stable.git] / mm / hmm.c

/*
 * Copyright 2013 Red Hat Inc.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Authors: Jérôme Glisse <jglisse@redhat.com>
 */
/*
 * Refer to include/linux/hmm.h for information about heterogeneous memory
 * management or HMM for short.
 */
#include <linux/mm.h>
#include <linux/hmm.h>
#include <linux/init.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/mmzone.h>
#include <linux/pagemap.h>
#include <linux/swapops.h>
#include <linux/hugetlb.h>
#include <linux/memremap.h>
#include <linux/jump_label.h>
#include <linux/mmu_notifier.h>
#include <linux/memory_hotplug.h>

#define PA_SECTION_SIZE (1UL << PA_SECTION_SHIFT)

#if IS_ENABLED(CONFIG_HMM_MIRROR)
static const struct mmu_notifier_ops hmm_mmu_notifier_ops;

/*
 * struct hmm - HMM per mm struct
 *
 * @mm: mm struct this HMM struct is bound to
 * @lock: lock protecting ranges list
 * @ranges: list of range being snapshotted
 * @mirrors: list of mirrors for this mm
 * @mmu_notifier: mmu notifier to track updates to CPU page table
 * @mirrors_sem: read/write semaphore protecting the mirrors list
 */
struct hmm {
        struct mm_struct        *mm;
        spinlock_t              lock;
        struct list_head        ranges;
        struct list_head        mirrors;
        struct mmu_notifier     mmu_notifier;
        struct rw_semaphore     mirrors_sem;
};

/*
 * hmm_register - register HMM against an mm (HMM internal)
 *
 * @mm: mm struct to attach to
 *
 * This is not intended to be used directly by device drivers. It allocates an
 * HMM struct if mm does not have one, and initializes it.
 */
static struct hmm *hmm_register(struct mm_struct *mm)
{
        struct hmm *hmm = READ_ONCE(mm->hmm);
        bool cleanup = false;

        /*
         * The hmm struct can only be freed once the mm_struct goes away,
         * hence we should always have pre-allocated an new hmm struct
         * above.
         */
        if (hmm)
                return hmm;

        hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
        if (!hmm)
                return NULL;
        INIT_LIST_HEAD(&hmm->mirrors);
        init_rwsem(&hmm->mirrors_sem);
        hmm->mmu_notifier.ops = NULL;
        INIT_LIST_HEAD(&hmm->ranges);
        spin_lock_init(&hmm->lock);
        hmm->mm = mm;

        spin_lock(&mm->page_table_lock);
        if (!mm->hmm)
                mm->hmm = hmm;
        else
                cleanup = true;
        spin_unlock(&mm->page_table_lock);

        if (cleanup)
                goto error;

        /*
         * We should only get here if hold the mmap_sem in write mode ie on
         * registration of first mirror through hmm_mirror_register()
         */
        hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
        if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
                goto error_mm;

        return mm->hmm;

error_mm:
        spin_lock(&mm->page_table_lock);
        if (mm->hmm == hmm)
                mm->hmm = NULL;
        spin_unlock(&mm->page_table_lock);
error:
        kfree(hmm);
        return NULL;
}

void hmm_mm_destroy(struct mm_struct *mm)
{
        kfree(mm->hmm);
}

static int hmm_invalidate_range(struct hmm *hmm, bool device,
                                const struct hmm_update *update)
{
        struct hmm_mirror *mirror;
        struct hmm_range *range;

        spin_lock(&hmm->lock);
        list_for_each_entry(range, &hmm->ranges, list) {
                unsigned long addr, idx, npages;

                if (update->end < range->start || update->start >= range->end)
                        continue;

                range->valid = false;
                addr = max(update->start, range->start);
                idx = (addr - range->start) >> PAGE_SHIFT;
                npages = (min(range->end, update->end) - addr) >> PAGE_SHIFT;
                memset(&range->pfns[idx], 0, sizeof(*range->pfns) * npages);
        }
        spin_unlock(&hmm->lock);

        if (!device)
                return 0;

        down_read(&hmm->mirrors_sem);
        list_for_each_entry(mirror, &hmm->mirrors, list) {
                int ret;

                ret = mirror->ops->sync_cpu_device_pagetables(mirror, update);
                if (!update->blockable && ret == -EAGAIN) {
                        up_read(&hmm->mirrors_sem);
                        return -EAGAIN;
                }
        }
        up_read(&hmm->mirrors_sem);

        return 0;
}

static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
{
        struct hmm_mirror *mirror;
        struct hmm *hmm = mm->hmm;

        down_write(&hmm->mirrors_sem);
        mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
                                          list);
        while (mirror) {
                list_del_init(&mirror->list);
                if (mirror->ops->release) {
                        /*
                         * Drop mirrors_sem so callback can wait on any pending
                         * work that might itself trigger mmu_notifier callback
                         * and thus would deadlock with us.
                         */
                        up_write(&hmm->mirrors_sem);
                        mirror->ops->release(mirror);
                        down_write(&hmm->mirrors_sem);
                }
                mirror = list_first_entry_or_null(&hmm->mirrors,
                                                  struct hmm_mirror, list);
        }
        up_write(&hmm->mirrors_sem);
}

static int hmm_invalidate_range_start(struct mmu_notifier *mn,
                                      struct mm_struct *mm,
                                      unsigned long start,
                                      unsigned long end,
                                      bool blockable)
{
        struct hmm_update update;
        struct hmm *hmm = mm->hmm;

        VM_BUG_ON(!hmm);

        update.start = start;
        update.end = end;
        update.event = HMM_UPDATE_INVALIDATE;
        update.blockable = blockable;
        return hmm_invalidate_range(hmm, true, &update);
}

static void hmm_invalidate_range_end(struct mmu_notifier *mn,
                                     struct mm_struct *mm,
                                     unsigned long start,
                                     unsigned long end)
{
        struct hmm_update update;
        struct hmm *hmm = mm->hmm;

        VM_BUG_ON(!hmm);

        update.start = start;
        update.end = end;
        update.event = HMM_UPDATE_INVALIDATE;
        update.blockable = true;
        hmm_invalidate_range(hmm, false, &update);
}

static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
        .release                = hmm_release,
        .invalidate_range_start = hmm_invalidate_range_start,
        .invalidate_range_end   = hmm_invalidate_range_end,
};

/*
 * hmm_mirror_register() - register a mirror against an mm
 *
 * @mirror: new mirror struct to register
 * @mm: mm to register against
 *
 * To start mirroring a process address space, the device driver must register
 * an HMM mirror struct.
 *
 * THE mm->mmap_sem MUST BE HELD IN WRITE MODE !
 */
int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
{
        /* Sanity check */
        if (!mm || !mirror || !mirror->ops)
                return -EINVAL;

again:
        mirror->hmm = hmm_register(mm);
        if (!mirror->hmm)
                return -ENOMEM;

        down_write(&mirror->hmm->mirrors_sem);
        if (mirror->hmm->mm == NULL) {
                /*
                 * A racing hmm_mirror_unregister() is about to destroy the hmm
                 * struct. Try again to allocate a new one.
                 */
                up_write(&mirror->hmm->mirrors_sem);
                mirror->hmm = NULL;
                goto again;
        } else {
                list_add(&mirror->list, &mirror->hmm->mirrors);
                up_write(&mirror->hmm->mirrors_sem);
        }

        return 0;
}
EXPORT_SYMBOL(hmm_mirror_register);

/*
 * hmm_mirror_unregister() - unregister a mirror
 *
 * @mirror: new mirror struct to register
 *
 * Stop mirroring a process address space, and cleanup.
 */
void hmm_mirror_unregister(struct hmm_mirror *mirror)
{
        bool should_unregister = false;
        struct mm_struct *mm;
        struct hmm *hmm;

        if (mirror->hmm == NULL)
                return;

        hmm = mirror->hmm;
        down_write(&hmm->mirrors_sem);
        list_del_init(&mirror->list);
        should_unregister = list_empty(&hmm->mirrors);
        mirror->hmm = NULL;
        mm = hmm->mm;
        hmm->mm = NULL;
        up_write(&hmm->mirrors_sem);

        if (!should_unregister || mm == NULL)
                return;

        mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);

        spin_lock(&mm->page_table_lock);
        if (mm->hmm == hmm)
                mm->hmm = NULL;
        spin_unlock(&mm->page_table_lock);

        kfree(hmm);
}
EXPORT_SYMBOL(hmm_mirror_unregister);

struct hmm_vma_walk {
        struct hmm_range        *range;
        unsigned long           last;
        bool                    fault;
        bool                    block;
};

static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
                            bool write_fault, uint64_t *pfn)
{
        unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_REMOTE;
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        struct vm_area_struct *vma = walk->vma;
        vm_fault_t ret;

        flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
        flags |= write_fault ? FAULT_FLAG_WRITE : 0;
        ret = handle_mm_fault(vma, addr, flags);
        if (ret & VM_FAULT_RETRY)
                return -EBUSY;
        if (ret & VM_FAULT_ERROR) {
                *pfn = range->values[HMM_PFN_ERROR];
                return -EFAULT;
        }

        return -EAGAIN;
}

static int hmm_pfns_bad(unsigned long addr,
                        unsigned long end,
                        struct mm_walk *walk)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        uint64_t *pfns = range->pfns;
        unsigned long i;

        i = (addr - range->start) >> PAGE_SHIFT;
        for (; addr < end; addr += PAGE_SIZE, i++)
                pfns[i] = range->values[HMM_PFN_ERROR];

        return 0;
}

/*
 * hmm_vma_walk_hole() - handle a range lacking valid pmd or pte(s)
 * @start: range virtual start address (inclusive)
 * @end: range virtual end address (exclusive)
 * @fault: should we fault or not ?
 * @write_fault: write fault ?
 * @walk: mm_walk structure
 * Returns: 0 on success, -EAGAIN after page fault, or page fault error
 *
 * This function will be called whenever pmd_none() or pte_none() returns true,
 * or whenever there is no page directory covering the virtual address range.
 */
static int hmm_vma_walk_hole_(unsigned long addr, unsigned long end,
                              bool fault, bool write_fault,
                              struct mm_walk *walk)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        uint64_t *pfns = range->pfns;
        unsigned long i;

        hmm_vma_walk->last = addr;
        i = (addr - range->start) >> PAGE_SHIFT;
        for (; addr < end; addr += PAGE_SIZE, i++) {
                pfns[i] = range->values[HMM_PFN_NONE];
                if (fault || write_fault) {
                        int ret;

                        ret = hmm_vma_do_fault(walk, addr, write_fault,
                                               &pfns[i]);
                        if (ret != -EAGAIN)
                                return ret;
                }
        }

        return (fault || write_fault) ? -EAGAIN : 0;
}

static inline void hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
                                      uint64_t pfns, uint64_t cpu_flags,
                                      bool *fault, bool *write_fault)
{
        struct hmm_range *range = hmm_vma_walk->range;

        *fault = *write_fault = false;
        if (!hmm_vma_walk->fault)
                return;

        /* We aren't ask to do anything ... */
        if (!(pfns & range->flags[HMM_PFN_VALID]))
                return;
        /* If this is device memory than only fault if explicitly requested */
        if ((cpu_flags & range->flags[HMM_PFN_DEVICE_PRIVATE])) {
                /* Do we fault on device memory ? */
                if (pfns & range->flags[HMM_PFN_DEVICE_PRIVATE]) {
                        *write_fault = pfns & range->flags[HMM_PFN_WRITE];
                        *fault = true;
                }
                return;
        }

        /* If CPU page table is not valid then we need to fault */
        *fault = !(cpu_flags & range->flags[HMM_PFN_VALID]);
        /* Need to write fault ? */
        if ((pfns & range->flags[HMM_PFN_WRITE]) &&
            !(cpu_flags & range->flags[HMM_PFN_WRITE])) {
                *write_fault = true;
                *fault = true;
        }
}

static void hmm_range_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
                                 const uint64_t *pfns, unsigned long npages,
                                 uint64_t cpu_flags, bool *fault,
                                 bool *write_fault)
{
        unsigned long i;

        if (!hmm_vma_walk->fault) {
                *fault = *write_fault = false;
                return;
        }

        for (i = 0; i < npages; ++i) {
                hmm_pte_need_fault(hmm_vma_walk, pfns[i], cpu_flags,
                                   fault, write_fault);
                if ((*fault) || (*write_fault))
                        return;
        }
}

static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
                             struct mm_walk *walk)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        bool fault, write_fault;
        unsigned long i, npages;
        uint64_t *pfns;

        i = (addr - range->start) >> PAGE_SHIFT;
        npages = (end - addr) >> PAGE_SHIFT;
        pfns = &range->pfns[i];
        hmm_range_need_fault(hmm_vma_walk, pfns, npages,
                             0, &fault, &write_fault);
        return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
}

static inline uint64_t pmd_to_hmm_pfn_flags(struct hmm_range *range, pmd_t pmd)
{
        if (pmd_protnone(pmd))
                return 0;
        return pmd_write(pmd) ? range->flags[HMM_PFN_VALID] |
                                range->flags[HMM_PFN_WRITE] :
                                range->flags[HMM_PFN_VALID];
}

static int hmm_vma_handle_pmd(struct mm_walk *walk,
                              unsigned long addr,
                              unsigned long end,
                              uint64_t *pfns,
                              pmd_t pmd)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        unsigned long pfn, npages, i;
        bool fault, write_fault;
        uint64_t cpu_flags;

        npages = (end - addr) >> PAGE_SHIFT;
        cpu_flags = pmd_to_hmm_pfn_flags(range, pmd);
        hmm_range_need_fault(hmm_vma_walk, pfns, npages, cpu_flags,
                             &fault, &write_fault);

        if (pmd_protnone(pmd) || fault || write_fault)
                return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);

        pfn = pmd_pfn(pmd) + pte_index(addr);
        for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
                pfns[i] = hmm_pfn_from_pfn(range, pfn) | cpu_flags;
        hmm_vma_walk->last = end;
        return 0;
}

static inline uint64_t pte_to_hmm_pfn_flags(struct hmm_range *range, pte_t pte)
{
        if (pte_none(pte) || !pte_present(pte))
                return 0;
        return pte_write(pte) ? range->flags[HMM_PFN_VALID] |
                                range->flags[HMM_PFN_WRITE] :
                                range->flags[HMM_PFN_VALID];
}

static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
                              unsigned long end, pmd_t *pmdp, pte_t *ptep,
                              uint64_t *pfn)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        struct vm_area_struct *vma = walk->vma;
        bool fault, write_fault;
        uint64_t cpu_flags;
        pte_t pte = *ptep;
        uint64_t orig_pfn = *pfn;

        *pfn = range->values[HMM_PFN_NONE];
        cpu_flags = pte_to_hmm_pfn_flags(range, pte);
        hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
                           &fault, &write_fault);

        if (pte_none(pte)) {
                if (fault || write_fault)
                        goto fault;
                return 0;
        }

        if (!pte_present(pte)) {
                swp_entry_t entry = pte_to_swp_entry(pte);

                if (!non_swap_entry(entry)) {
                        if (fault || write_fault)
                                goto fault;
                        return 0;
                }

                /*
                 * This is a special swap entry, ignore migration, use
                 * device and report anything else as error.
                 */
                if (is_device_private_entry(entry)) {
                        cpu_flags = range->flags[HMM_PFN_VALID] |
                                range->flags[HMM_PFN_DEVICE_PRIVATE];
                        cpu_flags |= is_write_device_private_entry(entry) ?
                                range->flags[HMM_PFN_WRITE] : 0;
                        hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
                                           &fault, &write_fault);
                        if (fault || write_fault)
                                goto fault;
                        *pfn = hmm_pfn_from_pfn(range, swp_offset(entry));
                        *pfn |= cpu_flags;
                        return 0;
                }

                if (is_migration_entry(entry)) {
                        if (fault || write_fault) {
                                pte_unmap(ptep);
                                hmm_vma_walk->last = addr;
                                migration_entry_wait(vma->vm_mm,
                                                     pmdp, addr);
                                return -EAGAIN;
                        }
                        return 0;
                }

                /* Report error for everything else */
                *pfn = range->values[HMM_PFN_ERROR];
                return -EFAULT;
        }

        if (fault || write_fault)
                goto fault;

        *pfn = hmm_pfn_from_pfn(range, pte_pfn(pte)) | cpu_flags;
        return 0;

fault:
        pte_unmap(ptep);
        /* Fault any virtual address we were asked to fault */
        return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
}

static int hmm_vma_walk_pmd(pmd_t *pmdp,
                            unsigned long start,
                            unsigned long end,
                            struct mm_walk *walk)
{
        struct hmm_vma_walk *hmm_vma_walk = walk->private;
        struct hmm_range *range = hmm_vma_walk->range;
        struct vm_area_struct *vma = walk->vma;
        uint64_t *pfns = range->pfns;
        unsigned long addr = start, i;
        pte_t *ptep;
        pmd_t pmd;


again:
        pmd = READ_ONCE(*pmdp);
        if (pmd_none(pmd))
                return hmm_vma_walk_hole(start, end, walk);

        if (pmd_huge(pmd) && (range->vma->vm_flags & VM_HUGETLB))
                return hmm_pfns_bad(start, end, walk);

        if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
                bool fault, write_fault;
                unsigned long npages;
                uint64_t *pfns;

                i = (addr - range->start) >> PAGE_SHIFT;
                npages = (end - addr) >> PAGE_SHIFT;
                pfns = &range->pfns[i];

                hmm_range_need_fault(hmm_vma_walk, pfns, npages,
                                     0, &fault, &write_fault);
                if (fault || write_fault) {
                        hmm_vma_walk->last = addr;
                        pmd_migration_entry_wait(vma->vm_mm, pmdp);
                        return -EAGAIN;
                }
                return 0;
        } else if (!pmd_present(pmd))
                return hmm_pfns_bad(start, end, walk);

        if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
                /*
                 * No need to take pmd_lock here, even if some other threads
                 * is splitting the huge pmd we will get that event through
                 * mmu_notifier callback.
                 *
                 * So just read pmd value and check again its a transparent
                 * huge or device mapping one and compute corresponding pfn
                 * values.
                 */
                pmd = pmd_read_atomic(pmdp);
                barrier();
                if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
                        goto again;

                i = (addr - range->start) >> PAGE_SHIFT;
                return hmm_vma_handle_pmd(walk, addr, end, &pfns[i], pmd);
        }

        /*
         * We have handled all the valid case above ie either none, migration,
         * huge or transparent huge. At this point either it is a valid pmd
         * entry pointing to pte directory or it is a bad pmd that will not
         * recover.
         */
        if (pmd_bad(pmd))
                return hmm_pfns_bad(start, end, walk);

        ptep = pte_offset_map(pmdp, addr);
        i = (addr - range->start) >> PAGE_SHIFT;
        for (; addr < end; addr += PAGE_SIZE, ptep++, i++) {
                int r;

                r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
                if (r) {
                        /* hmm_vma_handle_pte() did unmap pte directory */
                        hmm_vma_walk->last = addr;
                        return r;
                }
        }
        pte_unmap(ptep - 1);

        hmm_vma_walk->last = addr;
        return 0;
}

static void hmm_pfns_clear(struct hmm_range *range,
                           uint64_t *pfns,
                           unsigned long addr,
                           unsigned long end)
{
        for (; addr < end; addr += PAGE_SIZE, pfns++)
                *pfns = range->values[HMM_PFN_NONE];
}

static void hmm_pfns_special(struct hmm_range *range)
{
        unsigned long addr = range->start, i = 0;

        for (; addr < range->end; addr += PAGE_SIZE, i++)
                range->pfns[i] = range->values[HMM_PFN_SPECIAL];
}

/*
 * hmm_vma_get_pfns() - snapshot CPU page table for a range of virtual addresses
 * @range: range being snapshotted
 * Returns: -EINVAL if invalid argument, -ENOMEM out of memory, -EPERM invalid
 *          vma permission, 0 success
 *
 * This snapshots the CPU page table for a range of virtual addresses. Snapshot
 * validity is tracked by range struct. See hmm_vma_range_done() for further
 * information.
 *
 * The range struct is initialized here. It tracks the CPU page table, but only
 * if the function returns success (0), in which case the caller must then call
 * hmm_vma_range_done() to stop CPU page table update tracking on this range.
 *
 * NOT CALLING hmm_vma_range_done() IF FUNCTION RETURNS 0 WILL LEAD TO SERIOUS
 * MEMORY CORRUPTION ! YOU HAVE BEEN WARNED !
 */
int hmm_vma_get_pfns(struct hmm_range *range)
{
        struct vm_area_struct *vma = range->vma;
        struct hmm_vma_walk hmm_vma_walk;
        struct mm_walk mm_walk;
        struct hmm *hmm;

        /* Sanity check, this really should not happen ! */
        if (range->start < vma->vm_start || range->start >= vma->vm_end)
                return -EINVAL;
        if (range->end < vma->vm_start || range->end > vma->vm_end)
                return -EINVAL;

        hmm = hmm_register(vma->vm_mm);
        if (!hmm)
                return -ENOMEM;
        /* Caller must have registered a mirror, via hmm_mirror_register() ! */
        if (!hmm->mmu_notifier.ops)
                return -EINVAL;

        /* FIXME support hugetlb fs */
        if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL) ||
                        vma_is_dax(vma)) {
                hmm_pfns_special(range);
                return -EINVAL;
        }

        if (!(vma->vm_flags & VM_READ)) {
                /*
                 * If vma do not allow read access, then assume that it does
                 * not allow write access, either. Architecture that allow
                 * write without read access are not supported by HMM, because
                 * operations such has atomic access would not work.
                 */
                hmm_pfns_clear(range, range->pfns, range->start, range->end);
                return -EPERM;
        }

        /* Initialize range to track CPU page table update */
        spin_lock(&hmm->lock);
        range->valid = true;
        list_add_rcu(&range->list, &hmm->ranges);
        spin_unlock(&hmm->lock);

        hmm_vma_walk.fault = false;
        hmm_vma_walk.range = range;
        mm_walk.private = &hmm_vma_walk;

        mm_walk.vma = vma;
        mm_walk.mm = vma->vm_mm;
        mm_walk.pte_entry = NULL;
        mm_walk.test_walk = NULL;
        mm_walk.hugetlb_entry = NULL;
        mm_walk.pmd_entry = hmm_vma_walk_pmd;
        mm_walk.pte_hole = hmm_vma_walk_hole;

        walk_page_range(range->start, range->end, &mm_walk);
        return 0;
}
EXPORT_SYMBOL(hmm_vma_get_pfns);

/*
 * hmm_vma_range_done() - stop tracking change to CPU page table over a range
 * @range: range being tracked
 * Returns: false if range data has been invalidated, true otherwise
 *
 * Range struct is used to track updates to the CPU page table after a call to
 * either hmm_vma_get_pfns() or hmm_vma_fault(). Once the device driver is done
 * using the data,  or wants to lock updates to the data it got from those
 * functions, it must call the hmm_vma_range_done() function, which will then
 * stop tracking CPU page table updates.
 *
 * Note that device driver must still implement general CPU page table update
 * tracking either by using hmm_mirror (see hmm_mirror_register()) or by using
 * the mmu_notifier API directly.
 *
 * CPU page table update tracking done through hmm_range is only temporary and
 * to be used while trying to duplicate CPU page table contents for a range of
 * virtual addresses.
 *
 * There are two ways to use this :
 * again:
 *   hmm_vma_get_pfns(range); or hmm_vma_fault(...);
 *   trans = device_build_page_table_update_transaction(pfns);
 *   device_page_table_lock();
 *   if (!hmm_vma_range_done(range)) {
 *     device_page_table_unlock();
 *     goto again;
 *   }
 *   device_commit_transaction(trans);
 *   device_page_table_unlock();
 *
 * Or:
 *   hmm_vma_get_pfns(range); or hmm_vma_fault(...);
 *   device_page_table_lock();
 *   hmm_vma_range_done(range);
 *   device_update_page_table(range->pfns);
 *   device_page_table_unlock();
 */
bool hmm_vma_range_done(struct hmm_range *range)
{
        unsigned long npages = (range->end - range->start) >> PAGE_SHIFT;
        struct hmm *hmm;

        if (range->end <= range->start) {
                BUG();
                return false;
        }

        hmm = hmm_register(range->vma->vm_mm);
        if (!hmm) {
                memset(range->pfns, 0, sizeof(*range->pfns) * npages);
                return false;
        }

        spin_lock(&hmm->lock);
        list_del_rcu(&range->list);
        spin_unlock(&hmm->lock);

        return range->valid;
}
EXPORT_SYMBOL(hmm_vma_range_done);

/*
 * hmm_vma_fault() - try to fault some address in a virtual address range
 * @range: range being faulted
 * @block: allow blocking on fault (if true it sleeps and do not drop mmap_sem)
 * Returns: 0 success, error otherwise (-EAGAIN means mmap_sem have been drop)
 *
 * This is similar to a regular CPU page fault except that it will not trigger
 * any memory migration if the memory being faulted is not accessible by CPUs.
 *
 * On error, for one virtual address in the range, the function will mark the
 * corresponding HMM pfn entry with an error flag.
 *
 * Expected use pattern:
 * retry:
 *   down_read(&mm->mmap_sem);
 *   // Find vma and address device wants to fault, initialize hmm_pfn_t
 *   // array accordingly
 *   ret = hmm_vma_fault(range, write, block);
 *   switch (ret) {
 *   case -EAGAIN:
 *     hmm_vma_range_done(range);
 *     // You might want to rate limit or yield to play nicely, you may
 *     // also commit any valid pfn in the array assuming that you are
 *     // getting true from hmm_vma_range_monitor_end()
 *     goto retry;
 *   case 0:
 *     break;
 *   case -ENOMEM:
 *   case -EINVAL:
 *   case -EPERM:
 *   default:
 *     // Handle error !
 *     up_read(&mm->mmap_sem)
 *     return;
 *   }
 *   // Take device driver lock that serialize device page table update
 *   driver_lock_device_page_table_update();
 *   hmm_vma_range_done(range);
 *   // Commit pfns we got from hmm_vma_fault()
 *   driver_unlock_device_page_table_update();
 *   up_read(&mm->mmap_sem)
 *
 * YOU MUST CALL hmm_vma_range_done() AFTER THIS FUNCTION RETURN SUCCESS (0)
 * BEFORE FREEING THE range struct OR YOU WILL HAVE SERIOUS MEMORY CORRUPTION !
 *
 * YOU HAVE BEEN WARNED !
 */
int hmm_vma_fault(struct hmm_range *range, bool block)
{
        struct vm_area_struct *vma = range->vma;
        unsigned long start = range->start;
        struct hmm_vma_walk hmm_vma_walk;
        struct mm_walk mm_walk;
        struct hmm *hmm;
        int ret;

        /* Sanity check, this really should not happen ! */
        if (range->start < vma->vm_start || range->start >= vma->vm_end)
                return -EINVAL;
        if (range->end < vma->vm_start || range->end > vma->vm_end)
                return -EINVAL;

        hmm = hmm_register(vma->vm_mm);
        if (!hmm) {
                hmm_pfns_clear(range, range->pfns, range->start, range->end);
                return -ENOMEM;
        }
        /* Caller must have registered a mirror using hmm_mirror_register() */
        if (!hmm->mmu_notifier.ops)
                return -EINVAL;

        /* FIXME support hugetlb fs */
        if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL) ||
                        vma_is_dax(vma)) {
                hmm_pfns_special(range);
                return -EINVAL;
        }

        if (!(vma->vm_flags & VM_READ)) {
                /*
                 * If vma do not allow read access, then assume that it does
                 * not allow write access, either. Architecture that allow
                 * write without read access are not supported by HMM, because
                 * operations such has atomic access would not work.
                 */
                hmm_pfns_clear(range, range->pfns, range->start, range->end);
                return -EPERM;
        }

        /* Initialize range to track CPU page table update */
        spin_lock(&hmm->lock);
        range->valid = true;
        list_add_rcu(&range->list, &hmm->ranges);
        spin_unlock(&hmm->lock);

        hmm_vma_walk.fault = true;
        hmm_vma_walk.block = block;
        hmm_vma_walk.range = range;
        mm_walk.private = &hmm_vma_walk;
        hmm_vma_walk.last = range->start;

        mm_walk.vma = vma;
        mm_walk.mm = vma->vm_mm;
        mm_walk.pte_entry = NULL;
        mm_walk.test_walk = NULL;
        mm_walk.hugetlb_entry = NULL;
        mm_walk.pmd_entry = hmm_vma_walk_pmd;
        mm_walk.pte_hole = hmm_vma_walk_hole;

        do {
                ret = walk_page_range(start, range->end, &mm_walk);
                start = hmm_vma_walk.last;
        } while (ret == -EAGAIN);

        if (ret) {
                unsigned long i;

                i = (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
                hmm_pfns_clear(range, &range->pfns[i], hmm_vma_walk.last,
                               range->end);
                hmm_vma_range_done(range);
        }
        return ret;
}
EXPORT_SYMBOL(hmm_vma_fault);
#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */


#if IS_ENABLED(CONFIG_DEVICE_PRIVATE) ||  IS_ENABLED(CONFIG_DEVICE_PUBLIC)
struct page *hmm_vma_alloc_locked_page(struct vm_area_struct *vma,
                                       unsigned long addr)
{
        struct page *page;

        page = alloc_page_vma(GFP_HIGHUSER, vma, addr);
        if (!page)
                return NULL;
        lock_page(page);
        return page;
}
EXPORT_SYMBOL(hmm_vma_alloc_locked_page);


static void hmm_devmem_ref_release(struct percpu_ref *ref)
{
        struct hmm_devmem *devmem;

        devmem = container_of(ref, struct hmm_devmem, ref);
        complete(&devmem->completion);
}

static void hmm_devmem_ref_exit(void *data)
{
        struct percpu_ref *ref = data;
        struct hmm_devmem *devmem;

        devmem = container_of(ref, struct hmm_devmem, ref);
        wait_for_completion(&devmem->completion);
        percpu_ref_exit(ref);
}

static void hmm_devmem_ref_kill(struct percpu_ref *ref)
{
        struct hmm_devmem *devmem;

        devmem = container_of(ref, struct hmm_devmem, ref);
        percpu_ref_kill(ref);
}

static int hmm_devmem_fault(struct vm_area_struct *vma,
                            unsigned long addr,
                            const struct page *page,
                            unsigned int flags,
                            pmd_t *pmdp)
{
        struct hmm_devmem *devmem = page->pgmap->data;

        return devmem->ops->fault(devmem, vma, addr, page, flags, pmdp);
}

static void hmm_devmem_free(struct page *page, void *data)
{
        struct hmm_devmem *devmem = data;

        page->mapping = NULL;

        devmem->ops->free(devmem, page);
}

/*
 * hmm_devmem_add() - hotplug ZONE_DEVICE memory for device memory
 *
 * @ops: memory event device driver callback (see struct hmm_devmem_ops)
 * @device: device struct to bind the resource too
 * @size: size in bytes of the device memory to add
 * Returns: pointer to new hmm_devmem struct ERR_PTR otherwise
 *
 * This function first finds an empty range of physical address big enough to
 * contain the new resource, and then hotplugs it as ZONE_DEVICE memory, which
 * in turn allocates struct pages. It does not do anything beyond that; all
 * events affecting the memory will go through the various callbacks provided
 * by hmm_devmem_ops struct.
 *
 * Device driver should call this function during device initialization and
 * is then responsible of memory management. HMM only provides helpers.
 */
struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
                                  struct device *device,
                                  unsigned long size)
{
        struct hmm_devmem *devmem;
        resource_size_t addr;
        void *result;
        int ret;

        dev_pagemap_get_ops();

        devmem = devm_kzalloc(device, sizeof(*devmem), GFP_KERNEL);
        if (!devmem)
                return ERR_PTR(-ENOMEM);

        init_completion(&devmem->completion);
        devmem->pfn_first = -1UL;
        devmem->pfn_last = -1UL;
        devmem->resource = NULL;
        devmem->device = device;
        devmem->ops = ops;

        ret = percpu_ref_init(&devmem->ref, &hmm_devmem_ref_release,
                              0, GFP_KERNEL);
        if (ret)
                return ERR_PTR(ret);

        ret = devm_add_action_or_reset(device, hmm_devmem_ref_exit, &devmem->ref);
        if (ret)
                return ERR_PTR(ret);

        size = ALIGN(size, PA_SECTION_SIZE);
        addr = min((unsigned long)iomem_resource.end,
                   (1UL << MAX_PHYSMEM_BITS) - 1);
        addr = addr - size + 1UL;

        /*
         * FIXME add a new helper to quickly walk resource tree and find free
         * range
         *
         * FIXME what about ioport_resource resource ?
         */
        for (; addr > size && addr >= iomem_resource.start; addr -= size) {
                ret = region_intersects(addr, size, 0, IORES_DESC_NONE);
                if (ret != REGION_DISJOINT)
                        continue;

                devmem->resource = devm_request_mem_region(device, addr, size,
                                                           dev_name(device));
                if (!devmem->resource)
                        return ERR_PTR(-ENOMEM);
                break;
        }
        if (!devmem->resource)
                return ERR_PTR(-ERANGE);

        devmem->resource->desc = IORES_DESC_DEVICE_PRIVATE_MEMORY;
        devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
        devmem->pfn_last = devmem->pfn_first +
                           (resource_size(devmem->resource) >> PAGE_SHIFT);

        devmem->pagemap.type = MEMORY_DEVICE_PRIVATE;
        devmem->pagemap.res = *devmem->resource;
        devmem->pagemap.page_fault = hmm_devmem_fault;
        devmem->pagemap.page_free = hmm_devmem_free;
        devmem->pagemap.altmap_valid = false;
        devmem->pagemap.ref = &devmem->ref;
        devmem->pagemap.data = devmem;
        devmem->pagemap.kill = hmm_devmem_ref_kill;

        result = devm_memremap_pages(devmem->device, &devmem->pagemap);
        if (IS_ERR(result))
                return result;
        return devmem;
}
EXPORT_SYMBOL_GPL(hmm_devmem_add);

struct hmm_devmem *hmm_devmem_add_resource(const struct hmm_devmem_ops *ops,
                                           struct device *device,
                                           struct resource *res)
{
        struct hmm_devmem *devmem;
        void *result;
        int ret;

        if (res->desc != IORES_DESC_DEVICE_PUBLIC_MEMORY)
                return ERR_PTR(-EINVAL);

        dev_pagemap_get_ops();

        devmem = devm_kzalloc(device, sizeof(*devmem), GFP_KERNEL);
        if (!devmem)
                return ERR_PTR(-ENOMEM);

        init_completion(&devmem->completion);
        devmem->pfn_first = -1UL;
        devmem->pfn_last = -1UL;
        devmem->resource = res;
        devmem->device = device;
        devmem->ops = ops;

        ret = percpu_ref_init(&devmem->ref, &hmm_devmem_ref_release,
                              0, GFP_KERNEL);
        if (ret)
                return ERR_PTR(ret);

        ret = devm_add_action_or_reset(device, hmm_devmem_ref_exit,
                        &devmem->ref);
        if (ret)
                return ERR_PTR(ret);

        devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
        devmem->pfn_last = devmem->pfn_first +
                           (resource_size(devmem->resource) >> PAGE_SHIFT);

        devmem->pagemap.type = MEMORY_DEVICE_PUBLIC;
        devmem->pagemap.res = *devmem->resource;
        devmem->pagemap.page_fault = hmm_devmem_fault;
        devmem->pagemap.page_free = hmm_devmem_free;
        devmem->pagemap.altmap_valid = false;
        devmem->pagemap.ref = &devmem->ref;
        devmem->pagemap.data = devmem;
        devmem->pagemap.kill = hmm_devmem_ref_kill;

        result = devm_memremap_pages(devmem->device, &devmem->pagemap);
        if (IS_ERR(result))
                return result;
        return devmem;
}
EXPORT_SYMBOL_GPL(hmm_devmem_add_resource);

/*
 * A device driver that wants to handle multiple devices memory through a
 * single fake device can use hmm_device to do so. This is purely a helper
 * and it is not needed to make use of any HMM functionality.
 */
#define HMM_DEVICE_MAX 256

static DECLARE_BITMAP(hmm_device_mask, HMM_DEVICE_MAX);
static DEFINE_SPINLOCK(hmm_device_lock);
static struct class *hmm_device_class;
static dev_t hmm_device_devt;

static void hmm_device_release(struct device *device)
{
        struct hmm_device *hmm_device;

        hmm_device = container_of(device, struct hmm_device, device);
        spin_lock(&hmm_device_lock);
        clear_bit(hmm_device->minor, hmm_device_mask);
        spin_unlock(&hmm_device_lock);

        kfree(hmm_device);
}

struct hmm_device *hmm_device_new(void *drvdata)
{
        struct hmm_device *hmm_device;

        hmm_device = kzalloc(sizeof(*hmm_device), GFP_KERNEL);
        if (!hmm_device)
                return ERR_PTR(-ENOMEM);

        spin_lock(&hmm_device_lock);
        hmm_device->minor = find_first_zero_bit(hmm_device_mask, HMM_DEVICE_MAX);
        if (hmm_device->minor >= HMM_DEVICE_MAX) {
                spin_unlock(&hmm_device_lock);
                kfree(hmm_device);
                return ERR_PTR(-EBUSY);
        }
        set_bit(hmm_device->minor, hmm_device_mask);
        spin_unlock(&hmm_device_lock);

        dev_set_name(&hmm_device->device, "hmm_device%d", hmm_device->minor);
        hmm_device->device.devt = MKDEV(MAJOR(hmm_device_devt),
                                        hmm_device->minor);
        hmm_device->device.release = hmm_device_release;
        dev_set_drvdata(&hmm_device->device, drvdata);
        hmm_device->device.class = hmm_device_class;
        device_initialize(&hmm_device->device);

        return hmm_device;
}
EXPORT_SYMBOL(hmm_device_new);

void hmm_device_put(struct hmm_device *hmm_device)
{
        put_device(&hmm_device->device);
}
EXPORT_SYMBOL(hmm_device_put);

static int __init hmm_init(void)
{
        int ret;

        ret = alloc_chrdev_region(&hmm_device_devt, 0,
                                  HMM_DEVICE_MAX,
                                  "hmm_device");
        if (ret)
                return ret;

        hmm_device_class = class_create(THIS_MODULE, "hmm_device");
        if (IS_ERR(hmm_device_class)) {
                unregister_chrdev_region(hmm_device_devt, HMM_DEVICE_MAX);
                return PTR_ERR(hmm_device_class);
        }
        return 0;
}

device_initcall(hmm_init);
#endif /* CONFIG_DEVICE_PRIVATE || CONFIG_DEVICE_PUBLIC */