split dev_queue

[cor.git] / mm / hmm.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright 2013 Red Hat Inc.
 *
 * Authors: Jérôme Glisse <jglisse@redhat.com>
 */
/*
 * Refer to include/linux/hmm.h for information about heterogeneous memory
 * management or HMM for short.
 */
#include <linux/pagewalk.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/sched/mm.h>
#include <linux/jump_label.h>
#include <linux/dma-mapping.h>
#include <linux/mmu_notifier.h>
#include <linux/memory_hotplug.h>

struct hmm_vma_walk {
        struct hmm_range        *range;
        struct dev_pagemap      *pgmap;
        unsigned long           last;
        unsigned int            flags;
};

static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
                            bool write_fault, uint64_t *pfn)
{
        unsigned int flags = 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;

        if (!vma)
                goto err;

        if (hmm_vma_walk->flags & HMM_FAULT_ALLOW_RETRY)
                flags |= FAULT_FLAG_ALLOW_RETRY;
        if (write_fault)
                flags |= FAULT_FLAG_WRITE;

        ret = handle_mm_fault(vma, addr, flags);
        if (ret & VM_FAULT_RETRY) {
                /* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */
                return -EAGAIN;
        }
        if (ret & VM_FAULT_ERROR)
                goto err;

        return -EBUSY;

err:
        *pfn = range->values[HMM_PFN_ERROR];
        return -EFAULT;
}

static int hmm_pfns_fill(unsigned long addr, unsigned long end,
                struct hmm_range *range, enum hmm_pfn_value_e value)
{
        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[value];

        return 0;
}

/*
 * hmm_vma_walk_hole_() - handle a range lacking valid pmd or pte(s)
 * @addr: 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
 * Return: 0 on success, -EBUSY 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;

        if (write_fault && walk->vma && !(walk->vma->vm_flags & VM_WRITE))
                return -EPERM;

        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 != -EBUSY)
                                return ret;
                }
        }

        return (fault || write_fault) ? -EBUSY : 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;

        if (hmm_vma_walk->flags & HMM_FAULT_SNAPSHOT)
                return;

        /*
         * So we not only consider the individual per page request we also
         * consider the default flags requested for the range. The API can
         * be used 2 ways. The first one where the HMM user coalesces
         * multiple page faults into one request and sets flags per pfn for
         * those faults. The second one where the HMM user wants to pre-
         * fault a range with specific flags. For the latter one it is a
         * waste to have the user pre-fill the pfn arrays with a default
         * flags value.
         */
        pfns = (pfns & range->pfn_flags_mask) | range->default_flags;

        /* We aren't ask to do anything ... */
        if (!(pfns & range->flags[HMM_PFN_VALID]))
                return;
        /* If this is device memory then 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->flags & HMM_FAULT_SNAPSHOT) {
                *fault = *write_fault = false;
                return;
        }

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

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
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) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
        for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
                if (pmd_devmap(pmd)) {
                        hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
                                              hmm_vma_walk->pgmap);
                        if (unlikely(!hmm_vma_walk->pgmap))
                                return -EBUSY;
                }
                pfns[i] = hmm_device_entry_from_pfn(range, pfn) | cpu_flags;
        }
        if (hmm_vma_walk->pgmap) {
                put_dev_pagemap(hmm_vma_walk->pgmap);
                hmm_vma_walk->pgmap = NULL;
        }
        hmm_vma_walk->last = end;
        return 0;
}
#else /* CONFIG_TRANSPARENT_HUGEPAGE */
/* stub to allow the code below to compile */
int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
                unsigned long end, uint64_t *pfns, pmd_t pmd);
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

static inline uint64_t pte_to_hmm_pfn_flags(struct hmm_range *range, pte_t pte)
{
        if (pte_none(pte) || !pte_present(pte) || pte_protnone(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;
        bool fault, write_fault;
        uint64_t cpu_flags;
        pte_t pte = *ptep;
        uint64_t orig_pfn = *pfn;

        *pfn = range->values[HMM_PFN_NONE];
        fault = write_fault = false;

        if (pte_none(pte)) {
                hmm_pte_need_fault(hmm_vma_walk, orig_pfn, 0,
                                   &fault, &write_fault);
                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)) {
                        cpu_flags = pte_to_hmm_pfn_flags(range, pte);
                        hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
                                           &fault, &write_fault);
                        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_device_entry_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(walk->mm, pmdp, addr);
                                return -EBUSY;
                        }
                        return 0;
                }

                /* Report error for everything else */
                *pfn = range->values[HMM_PFN_ERROR];
                return -EFAULT;
        } else {
                cpu_flags = pte_to_hmm_pfn_flags(range, pte);
                hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
                                   &fault, &write_fault);
        }

        if (fault || write_fault)
                goto fault;

        if (pte_devmap(pte)) {
                hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
                                              hmm_vma_walk->pgmap);
                if (unlikely(!hmm_vma_walk->pgmap))
                        return -EBUSY;
        } else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
                if (!is_zero_pfn(pte_pfn(pte))) {
                        *pfn = range->values[HMM_PFN_SPECIAL];
                        return -EFAULT;
                }
                /*
                 * Since each architecture defines a struct page for the zero
                 * page, just fall through and treat it like a normal page.
                 */
        }

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

fault:
        if (hmm_vma_walk->pgmap) {
                put_dev_pagemap(hmm_vma_walk->pgmap);
                hmm_vma_walk->pgmap = NULL;
        }
        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;
        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 (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(walk->mm, pmdp);
                        return -EBUSY;
                }
                return 0;
        } else if (!pmd_present(pmd))
                return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);

        if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
                /*
                 * No need to take pmd_lock here, even if some other thread
                 * is splitting the huge pmd we will get that event through
                 * mmu_notifier callback.
                 *
                 * So just read pmd value and check again it's 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 cases 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_fill(start, end, range, HMM_PFN_ERROR);

        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;
                }
        }
        if (hmm_vma_walk->pgmap) {
                /*
                 * We do put_dev_pagemap() here and not in hmm_vma_handle_pte()
                 * so that we can leverage get_dev_pagemap() optimization which
                 * will not re-take a reference on a pgmap if we already have
                 * one.
                 */
                put_dev_pagemap(hmm_vma_walk->pgmap);
                hmm_vma_walk->pgmap = NULL;
        }
        pte_unmap(ptep - 1);

        hmm_vma_walk->last = addr;
        return 0;
}

#if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && \
    defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
static inline uint64_t pud_to_hmm_pfn_flags(struct hmm_range *range, pud_t pud)
{
        if (!pud_present(pud))
                return 0;
        return pud_write(pud) ? range->flags[HMM_PFN_VALID] |
                                range->flags[HMM_PFN_WRITE] :
                                range->flags[HMM_PFN_VALID];
}

static int hmm_vma_walk_pud(pud_t *pudp, 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;
        unsigned long addr = start, next;
        pmd_t *pmdp;
        pud_t pud;
        int ret;

again:
        pud = READ_ONCE(*pudp);
        if (pud_none(pud))
                return hmm_vma_walk_hole(start, end, walk);

        if (pud_huge(pud) && pud_devmap(pud)) {
                unsigned long i, npages, pfn;
                uint64_t *pfns, cpu_flags;
                bool fault, write_fault;

                if (!pud_present(pud))
                        return hmm_vma_walk_hole(start, end, walk);

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

                cpu_flags = pud_to_hmm_pfn_flags(range, pud);
                hmm_range_need_fault(hmm_vma_walk, pfns, npages,
                                     cpu_flags, &fault, &write_fault);
                if (fault || write_fault)
                        return hmm_vma_walk_hole_(addr, end, fault,
                                                write_fault, walk);

                pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
                for (i = 0; i < npages; ++i, ++pfn) {
                        hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
                                              hmm_vma_walk->pgmap);
                        if (unlikely(!hmm_vma_walk->pgmap))
                                return -EBUSY;
                        pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
                                  cpu_flags;
                }
                if (hmm_vma_walk->pgmap) {
                        put_dev_pagemap(hmm_vma_walk->pgmap);
                        hmm_vma_walk->pgmap = NULL;
                }
                hmm_vma_walk->last = end;
                return 0;
        }

        split_huge_pud(walk->vma, pudp, addr);
        if (pud_none(*pudp))
                goto again;

        pmdp = pmd_offset(pudp, addr);
        do {
                next = pmd_addr_end(addr, end);
                ret = hmm_vma_walk_pmd(pmdp, addr, next, walk);
                if (ret)
                        return ret;
        } while (pmdp++, addr = next, addr != end);

        return 0;
}
#else
#define hmm_vma_walk_pud        NULL
#endif

#ifdef CONFIG_HUGETLB_PAGE
static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
                                      unsigned long start, unsigned long end,
                                      struct mm_walk *walk)
{
        unsigned long addr = start, i, 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;
        uint64_t orig_pfn, cpu_flags;
        bool fault, write_fault;
        spinlock_t *ptl;
        pte_t entry;
        int ret = 0;

        ptl = huge_pte_lock(hstate_vma(vma), walk->mm, pte);
        entry = huge_ptep_get(pte);

        i = (start - range->start) >> PAGE_SHIFT;
        orig_pfn = range->pfns[i];
        range->pfns[i] = range->values[HMM_PFN_NONE];
        cpu_flags = pte_to_hmm_pfn_flags(range, entry);
        fault = write_fault = false;
        hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
                           &fault, &write_fault);
        if (fault || write_fault) {
                ret = -ENOENT;
                goto unlock;
        }

        pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
        for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
                range->pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
                                 cpu_flags;
        hmm_vma_walk->last = end;

unlock:
        spin_unlock(ptl);

        if (ret == -ENOENT)
                return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);

        return ret;
}
#else
#define hmm_vma_walk_hugetlb_entry NULL
#endif /* CONFIG_HUGETLB_PAGE */

static int hmm_vma_walk_test(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;

        /*
         * Skip vma ranges that don't have struct page backing them or
         * map I/O devices directly.
         */
        if (vma->vm_flags & (VM_IO | VM_PFNMAP | VM_MIXEDMAP))
                return -EFAULT;

        /*
         * If the vma does not allow read access, then assume that it does not
         * allow write access either. HMM does not support architectures
         * that allow write without read.
         */
        if (!(vma->vm_flags & VM_READ)) {
                bool fault, write_fault;

                /*
                 * Check to see if a fault is requested for any page in the
                 * range.
                 */
                hmm_range_need_fault(hmm_vma_walk, range->pfns +
                                        ((start - range->start) >> PAGE_SHIFT),
                                        (end - start) >> PAGE_SHIFT,
                                        0, &fault, &write_fault);
                if (fault || write_fault)
                        return -EFAULT;

                hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
                hmm_vma_walk->last = end;

                /* Skip this vma and continue processing the next vma. */
                return 1;
        }

        return 0;
}

static const struct mm_walk_ops hmm_walk_ops = {
        .pud_entry      = hmm_vma_walk_pud,
        .pmd_entry      = hmm_vma_walk_pmd,
        .pte_hole       = hmm_vma_walk_hole,
        .hugetlb_entry  = hmm_vma_walk_hugetlb_entry,
        .test_walk      = hmm_vma_walk_test,
};

/**
 * hmm_range_fault - try to fault some address in a virtual address range
 * @range:      range being faulted
 * @flags:      HMM_FAULT_* flags
 *
 * Return: the number of valid pages in range->pfns[] (from range start
 * address), which may be zero.  On error one of the following status codes
 * can be returned:
 *
 * -EINVAL:     Invalid arguments or mm or virtual address is in an invalid vma
 *              (e.g., device file vma).
 * -ENOMEM:     Out of memory.
 * -EPERM:      Invalid permission (e.g., asking for write and range is read
 *              only).
 * -EAGAIN:     A page fault needs to be retried and mmap_sem was dropped.
 * -EBUSY:      The range has been invalidated and the caller needs to wait for
 *              the invalidation to finish.
 * -EFAULT:     Invalid (i.e., either no valid vma or it is illegal to access
 *              that range) number of valid pages in range->pfns[] (from
 *              range start address).
 *
 * 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
 * and caller does not ask for migration.
 *
 * On error, for one virtual address in the range, the function will mark the
 * corresponding HMM pfn entry with an error flag.
 */
long hmm_range_fault(struct hmm_range *range, unsigned int flags)
{
        struct hmm_vma_walk hmm_vma_walk = {
                .range = range,
                .last = range->start,
                .flags = flags,
        };
        struct mm_struct *mm = range->notifier->mm;
        int ret;

        lockdep_assert_held(&mm->mmap_sem);

        do {
                /* If range is no longer valid force retry. */
                if (mmu_interval_check_retry(range->notifier,
                                             range->notifier_seq))
                        return -EBUSY;
                ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
                                      &hmm_walk_ops, &hmm_vma_walk);
        } while (ret == -EBUSY);

        if (ret)
                return ret;
        return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
}
EXPORT_SYMBOL(hmm_range_fault);