Merge branch 'for-4.12/asus' into for-linus

[mirror_ubuntu-artful-kernel.git] / mm / gup.c

#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/err.h>
#include <linux/spinlock.h>

#include <linux/mm.h>
#include <linux/memremap.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/swapops.h>

#include <linux/sched/signal.h>
#include <linux/rwsem.h>
#include <linux/hugetlb.h>

#include <asm/mmu_context.h>
#include <asm/pgtable.h>
#include <asm/tlbflush.h>

#include "internal.h"

static struct page *no_page_table(struct vm_area_struct *vma,
                unsigned int flags)
{
        /*
         * When core dumping an enormous anonymous area that nobody
         * has touched so far, we don't want to allocate unnecessary pages or
         * page tables.  Return error instead of NULL to skip handle_mm_fault,
         * then get_dump_page() will return NULL to leave a hole in the dump.
         * But we can only make this optimization where a hole would surely
         * be zero-filled if handle_mm_fault() actually did handle it.
         */
        if ((flags & FOLL_DUMP) && (!vma->vm_ops || !vma->vm_ops->fault))
                return ERR_PTR(-EFAULT);
        return NULL;
}

static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
                pte_t *pte, unsigned int flags)
{
        /* No page to get reference */
        if (flags & FOLL_GET)
                return -EFAULT;

        if (flags & FOLL_TOUCH) {
                pte_t entry = *pte;

                if (flags & FOLL_WRITE)
                        entry = pte_mkdirty(entry);
                entry = pte_mkyoung(entry);

                if (!pte_same(*pte, entry)) {
                        set_pte_at(vma->vm_mm, address, pte, entry);
                        update_mmu_cache(vma, address, pte);
                }
        }

        /* Proper page table entry exists, but no corresponding struct page */
        return -EEXIST;
}

/*
 * FOLL_FORCE can write to even unwritable pte's, but only
 * after we've gone through a COW cycle and they are dirty.
 */
static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
        return pte_write(pte) ||
                ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}

static struct page *follow_page_pte(struct vm_area_struct *vma,
                unsigned long address, pmd_t *pmd, unsigned int flags)
{
        struct mm_struct *mm = vma->vm_mm;
        struct dev_pagemap *pgmap = NULL;
        struct page *page;
        spinlock_t *ptl;
        pte_t *ptep, pte;

retry:
        if (unlikely(pmd_bad(*pmd)))
                return no_page_table(vma, flags);

        ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
        pte = *ptep;
        if (!pte_present(pte)) {
                swp_entry_t entry;
                /*
                 * KSM's break_ksm() relies upon recognizing a ksm page
                 * even while it is being migrated, so for that case we
                 * need migration_entry_wait().
                 */
                if (likely(!(flags & FOLL_MIGRATION)))
                        goto no_page;
                if (pte_none(pte))
                        goto no_page;
                entry = pte_to_swp_entry(pte);
                if (!is_migration_entry(entry))
                        goto no_page;
                pte_unmap_unlock(ptep, ptl);
                migration_entry_wait(mm, pmd, address);
                goto retry;
        }
        if ((flags & FOLL_NUMA) && pte_protnone(pte))
                goto no_page;
        if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
                pte_unmap_unlock(ptep, ptl);
                return NULL;
        }

        page = vm_normal_page(vma, address, pte);
        if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
                /*
                 * Only return device mapping pages in the FOLL_GET case since
                 * they are only valid while holding the pgmap reference.
                 */
                pgmap = get_dev_pagemap(pte_pfn(pte), NULL);
                if (pgmap)
                        page = pte_page(pte);
                else
                        goto no_page;
        } else if (unlikely(!page)) {
                if (flags & FOLL_DUMP) {
                        /* Avoid special (like zero) pages in core dumps */
                        page = ERR_PTR(-EFAULT);
                        goto out;
                }

                if (is_zero_pfn(pte_pfn(pte))) {
                        page = pte_page(pte);
                } else {
                        int ret;

                        ret = follow_pfn_pte(vma, address, ptep, flags);
                        page = ERR_PTR(ret);
                        goto out;
                }
        }

        if (flags & FOLL_SPLIT && PageTransCompound(page)) {
                int ret;
                get_page(page);
                pte_unmap_unlock(ptep, ptl);
                lock_page(page);
                ret = split_huge_page(page);
                unlock_page(page);
                put_page(page);
                if (ret)
                        return ERR_PTR(ret);
                goto retry;
        }

        if (flags & FOLL_GET) {
                get_page(page);

                /* drop the pgmap reference now that we hold the page */
                if (pgmap) {
                        put_dev_pagemap(pgmap);
                        pgmap = NULL;
                }
        }
        if (flags & FOLL_TOUCH) {
                if ((flags & FOLL_WRITE) &&
                    !pte_dirty(pte) && !PageDirty(page))
                        set_page_dirty(page);
                /*
                 * pte_mkyoung() would be more correct here, but atomic care
                 * is needed to avoid losing the dirty bit: it is easier to use
                 * mark_page_accessed().
                 */
                mark_page_accessed(page);
        }
        if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
                /* Do not mlock pte-mapped THP */
                if (PageTransCompound(page))
                        goto out;

                /*
                 * The preliminary mapping check is mainly to avoid the
                 * pointless overhead of lock_page on the ZERO_PAGE
                 * which might bounce very badly if there is contention.
                 *
                 * If the page is already locked, we don't need to
                 * handle it now - vmscan will handle it later if and
                 * when it attempts to reclaim the page.
                 */
                if (page->mapping && trylock_page(page)) {
                        lru_add_drain();  /* push cached pages to LRU */
                        /*
                         * Because we lock page here, and migration is
                         * blocked by the pte's page reference, and we
                         * know the page is still mapped, we don't even
                         * need to check for file-cache page truncation.
                         */
                        mlock_vma_page(page);
                        unlock_page(page);
                }
        }
out:
        pte_unmap_unlock(ptep, ptl);
        return page;
no_page:
        pte_unmap_unlock(ptep, ptl);
        if (!pte_none(pte))
                return NULL;
        return no_page_table(vma, flags);
}

/**
 * follow_page_mask - look up a page descriptor from a user-virtual address
 * @vma: vm_area_struct mapping @address
 * @address: virtual address to look up
 * @flags: flags modifying lookup behaviour
 * @page_mask: on output, *page_mask is set according to the size of the page
 *
 * @flags can have FOLL_ flags set, defined in <linux/mm.h>
 *
 * Returns the mapped (struct page *), %NULL if no mapping exists, or
 * an error pointer if there is a mapping to something not represented
 * by a page descriptor (see also vm_normal_page()).
 */
struct page *follow_page_mask(struct vm_area_struct *vma,
                              unsigned long address, unsigned int flags,
                              unsigned int *page_mask)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;
        spinlock_t *ptl;
        struct page *page;
        struct mm_struct *mm = vma->vm_mm;

        *page_mask = 0;

        page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
        if (!IS_ERR(page)) {
                BUG_ON(flags & FOLL_GET);
                return page;
        }

        pgd = pgd_offset(mm, address);
        if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
                return no_page_table(vma, flags);
        p4d = p4d_offset(pgd, address);
        if (p4d_none(*p4d))
                return no_page_table(vma, flags);
        BUILD_BUG_ON(p4d_huge(*p4d));
        if (unlikely(p4d_bad(*p4d)))
                return no_page_table(vma, flags);
        pud = pud_offset(p4d, address);
        if (pud_none(*pud))
                return no_page_table(vma, flags);
        if (pud_huge(*pud) && vma->vm_flags & VM_HUGETLB) {
                page = follow_huge_pud(mm, address, pud, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (pud_devmap(*pud)) {
                ptl = pud_lock(mm, pud);
                page = follow_devmap_pud(vma, address, pud, flags);
                spin_unlock(ptl);
                if (page)
                        return page;
        }
        if (unlikely(pud_bad(*pud)))
                return no_page_table(vma, flags);

        pmd = pmd_offset(pud, address);
        if (pmd_none(*pmd))
                return no_page_table(vma, flags);
        if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) {
                page = follow_huge_pmd(mm, address, pmd, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (pmd_devmap(*pmd)) {
                ptl = pmd_lock(mm, pmd);
                page = follow_devmap_pmd(vma, address, pmd, flags);
                spin_unlock(ptl);
                if (page)
                        return page;
        }
        if (likely(!pmd_trans_huge(*pmd)))
                return follow_page_pte(vma, address, pmd, flags);

        if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
                return no_page_table(vma, flags);

        ptl = pmd_lock(mm, pmd);
        if (unlikely(!pmd_trans_huge(*pmd))) {
                spin_unlock(ptl);
                return follow_page_pte(vma, address, pmd, flags);
        }
        if (flags & FOLL_SPLIT) {
                int ret;
                page = pmd_page(*pmd);
                if (is_huge_zero_page(page)) {
                        spin_unlock(ptl);
                        ret = 0;
                        split_huge_pmd(vma, pmd, address);
                        if (pmd_trans_unstable(pmd))
                                ret = -EBUSY;
                } else {
                        get_page(page);
                        spin_unlock(ptl);
                        lock_page(page);
                        ret = split_huge_page(page);
                        unlock_page(page);
                        put_page(page);
                        if (pmd_none(*pmd))
                                return no_page_table(vma, flags);
                }

                return ret ? ERR_PTR(ret) :
                        follow_page_pte(vma, address, pmd, flags);
        }

        page = follow_trans_huge_pmd(vma, address, pmd, flags);
        spin_unlock(ptl);
        *page_mask = HPAGE_PMD_NR - 1;
        return page;
}

static int get_gate_page(struct mm_struct *mm, unsigned long address,
                unsigned int gup_flags, struct vm_area_struct **vma,
                struct page **page)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        int ret = -EFAULT;

        /* user gate pages are read-only */
        if (gup_flags & FOLL_WRITE)
                return -EFAULT;
        if (address > TASK_SIZE)
                pgd = pgd_offset_k(address);
        else
                pgd = pgd_offset_gate(mm, address);
        BUG_ON(pgd_none(*pgd));
        p4d = p4d_offset(pgd, address);
        BUG_ON(p4d_none(*p4d));
        pud = pud_offset(p4d, address);
        BUG_ON(pud_none(*pud));
        pmd = pmd_offset(pud, address);
        if (pmd_none(*pmd))
                return -EFAULT;
        VM_BUG_ON(pmd_trans_huge(*pmd));
        pte = pte_offset_map(pmd, address);
        if (pte_none(*pte))
                goto unmap;
        *vma = get_gate_vma(mm);
        if (!page)
                goto out;
        *page = vm_normal_page(*vma, address, *pte);
        if (!*page) {
                if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
                        goto unmap;
                *page = pte_page(*pte);
        }
        get_page(*page);
out:
        ret = 0;
unmap:
        pte_unmap(pte);
        return ret;
}

/*
 * mmap_sem must be held on entry.  If @nonblocking != NULL and
 * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.
 * If it is, *@nonblocking will be set to 0 and -EBUSY returned.
 */
static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
                unsigned long address, unsigned int *flags, int *nonblocking)
{
        unsigned int fault_flags = 0;
        int ret;

        /* mlock all present pages, but do not fault in new pages */
        if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
                return -ENOENT;
        /* For mm_populate(), just skip the stack guard page. */
        if ((*flags & FOLL_POPULATE) &&
                        (stack_guard_page_start(vma, address) ||
                         stack_guard_page_end(vma, address + PAGE_SIZE)))
                return -ENOENT;
        if (*flags & FOLL_WRITE)
                fault_flags |= FAULT_FLAG_WRITE;
        if (*flags & FOLL_REMOTE)
                fault_flags |= FAULT_FLAG_REMOTE;
        if (nonblocking)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY;
        if (*flags & FOLL_NOWAIT)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
        if (*flags & FOLL_TRIED) {
                VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
                fault_flags |= FAULT_FLAG_TRIED;
        }

        ret = handle_mm_fault(vma, address, fault_flags);
        if (ret & VM_FAULT_ERROR) {
                if (ret & VM_FAULT_OOM)
                        return -ENOMEM;
                if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
                        return *flags & FOLL_HWPOISON ? -EHWPOISON : -EFAULT;
                if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
                        return -EFAULT;
                BUG();
        }

        if (tsk) {
                if (ret & VM_FAULT_MAJOR)
                        tsk->maj_flt++;
                else
                        tsk->min_flt++;
        }

        if (ret & VM_FAULT_RETRY) {
                if (nonblocking)
                        *nonblocking = 0;
                return -EBUSY;
        }

        /*
         * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
         * necessary, even if maybe_mkwrite decided not to set pte_write. We
         * can thus safely do subsequent page lookups as if they were reads.
         * But only do so when looping for pte_write is futile: in some cases
         * userspace may also be wanting to write to the gotten user page,
         * which a read fault here might prevent (a readonly page might get
         * reCOWed by userspace write).
         */
        if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
                *flags |= FOLL_COW;
        return 0;
}

static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
{
        vm_flags_t vm_flags = vma->vm_flags;
        int write = (gup_flags & FOLL_WRITE);
        int foreign = (gup_flags & FOLL_REMOTE);

        if (vm_flags & (VM_IO | VM_PFNMAP))
                return -EFAULT;

        if (write) {
                if (!(vm_flags & VM_WRITE)) {
                        if (!(gup_flags & FOLL_FORCE))
                                return -EFAULT;
                        /*
                         * We used to let the write,force case do COW in a
                         * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
                         * set a breakpoint in a read-only mapping of an
                         * executable, without corrupting the file (yet only
                         * when that file had been opened for writing!).
                         * Anon pages in shared mappings are surprising: now
                         * just reject it.
                         */
                        if (!is_cow_mapping(vm_flags))
                                return -EFAULT;
                }
        } else if (!(vm_flags & VM_READ)) {
                if (!(gup_flags & FOLL_FORCE))
                        return -EFAULT;
                /*
                 * Is there actually any vma we can reach here which does not
                 * have VM_MAYREAD set?
                 */
                if (!(vm_flags & VM_MAYREAD))
                        return -EFAULT;
        }
        /*
         * gups are always data accesses, not instruction
         * fetches, so execute=false here
         */
        if (!arch_vma_access_permitted(vma, write, false, foreign))
                return -EFAULT;
        return 0;
}

/**
 * __get_user_pages() - pin user pages in memory
 * @tsk:        task_struct of target task
 * @mm:         mm_struct of target mm
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying pin behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 * @nonblocking: whether waiting for disk IO or mmap_sem contention
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno. Each page returned must be released
 * with a put_page() call when it is finished with. vmas will only
 * remain valid while mmap_sem is held.
 *
 * Must be called with mmap_sem held.  It may be released.  See below.
 *
 * __get_user_pages walks a process's page tables and takes a reference to
 * each struct page that each user address corresponds to at a given
 * instant. That is, it takes the page that would be accessed if a user
 * thread accesses the given user virtual address at that instant.
 *
 * This does not guarantee that the page exists in the user mappings when
 * __get_user_pages returns, and there may even be a completely different
 * page there in some cases (eg. if mmapped pagecache has been invalidated
 * and subsequently re faulted). However it does guarantee that the page
 * won't be freed completely. And mostly callers simply care that the page
 * contains data that was valid *at some point in time*. Typically, an IO
 * or similar operation cannot guarantee anything stronger anyway because
 * locks can't be held over the syscall boundary.
 *
 * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
 * the page is written to, set_page_dirty (or set_page_dirty_lock, as
 * appropriate) must be called after the page is finished with, and
 * before put_page is called.
 *
 * If @nonblocking != NULL, __get_user_pages will not wait for disk IO
 * or mmap_sem contention, and if waiting is needed to pin all pages,
 * *@nonblocking will be set to 0.  Further, if @gup_flags does not
 * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in
 * this case.
 *
 * A caller using such a combination of @nonblocking and @gup_flags
 * must therefore hold the mmap_sem for reading only, and recognize
 * when it's been released.  Otherwise, it must be held for either
 * reading or writing and will not be released.
 *
 * In most cases, get_user_pages or get_user_pages_fast should be used
 * instead of __get_user_pages. __get_user_pages should be used only if
 * you need some special @gup_flags.
 */
static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas, int *nonblocking)
{
        long i = 0;
        unsigned int page_mask;
        struct vm_area_struct *vma = NULL;

        if (!nr_pages)
                return 0;

        VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));

        /*
         * If FOLL_FORCE is set then do not force a full fault as the hinting
         * fault information is unrelated to the reference behaviour of a task
         * using the address space
         */
        if (!(gup_flags & FOLL_FORCE))
                gup_flags |= FOLL_NUMA;

        do {
                struct page *page;
                unsigned int foll_flags = gup_flags;
                unsigned int page_increm;

                /* first iteration or cross vma bound */
                if (!vma || start >= vma->vm_end) {
                        vma = find_extend_vma(mm, start);
                        if (!vma && in_gate_area(mm, start)) {
                                int ret;
                                ret = get_gate_page(mm, start & PAGE_MASK,
                                                gup_flags, &vma,
                                                pages ? &pages[i] : NULL);
                                if (ret)
                                        return i ? : ret;
                                page_mask = 0;
                                goto next_page;
                        }

                        if (!vma || check_vma_flags(vma, gup_flags))
                                return i ? : -EFAULT;
                        if (is_vm_hugetlb_page(vma)) {
                                i = follow_hugetlb_page(mm, vma, pages, vmas,
                                                &start, &nr_pages, i,
                                                gup_flags, nonblocking);
                                continue;
                        }
                }
retry:
                /*
                 * If we have a pending SIGKILL, don't keep faulting pages and
                 * potentially allocating memory.
                 */
                if (unlikely(fatal_signal_pending(current)))
                        return i ? i : -ERESTARTSYS;
                cond_resched();
                page = follow_page_mask(vma, start, foll_flags, &page_mask);
                if (!page) {
                        int ret;
                        ret = faultin_page(tsk, vma, start, &foll_flags,
                                        nonblocking);
                        switch (ret) {
                        case 0:
                                goto retry;
                        case -EFAULT:
                        case -ENOMEM:
                        case -EHWPOISON:
                                return i ? i : ret;
                        case -EBUSY:
                                return i;
                        case -ENOENT:
                                goto next_page;
                        }
                        BUG();
                } else if (PTR_ERR(page) == -EEXIST) {
                        /*
                         * Proper page table entry exists, but no corresponding
                         * struct page.
                         */
                        goto next_page;
                } else if (IS_ERR(page)) {
                        return i ? i : PTR_ERR(page);
                }
                if (pages) {
                        pages[i] = page;
                        flush_anon_page(vma, page, start);
                        flush_dcache_page(page);
                        page_mask = 0;
                }
next_page:
                if (vmas) {
                        vmas[i] = vma;
                        page_mask = 0;
                }
                page_increm = 1 + (~(start >> PAGE_SHIFT) & page_mask);
                if (page_increm > nr_pages)
                        page_increm = nr_pages;
                i += page_increm;
                start += page_increm * PAGE_SIZE;
                nr_pages -= page_increm;
        } while (nr_pages);
        return i;
}

static bool vma_permits_fault(struct vm_area_struct *vma,
                              unsigned int fault_flags)
{
        bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
        bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);
        vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;

        if (!(vm_flags & vma->vm_flags))
                return false;

        /*
         * The architecture might have a hardware protection
         * mechanism other than read/write that can deny access.
         *
         * gup always represents data access, not instruction
         * fetches, so execute=false here:
         */
        if (!arch_vma_access_permitted(vma, write, false, foreign))
                return false;

        return true;
}

/*
 * fixup_user_fault() - manually resolve a user page fault
 * @tsk:        the task_struct to use for page fault accounting, or
 *              NULL if faults are not to be recorded.
 * @mm:         mm_struct of target mm
 * @address:    user address
 * @fault_flags:flags to pass down to handle_mm_fault()
 * @unlocked:   did we unlock the mmap_sem while retrying, maybe NULL if caller
 *              does not allow retry
 *
 * This is meant to be called in the specific scenario where for locking reasons
 * we try to access user memory in atomic context (within a pagefault_disable()
 * section), this returns -EFAULT, and we want to resolve the user fault before
 * trying again.
 *
 * Typically this is meant to be used by the futex code.
 *
 * The main difference with get_user_pages() is that this function will
 * unconditionally call handle_mm_fault() which will in turn perform all the
 * necessary SW fixup of the dirty and young bits in the PTE, while
 * get_user_pages() only guarantees to update these in the struct page.
 *
 * This is important for some architectures where those bits also gate the
 * access permission to the page because they are maintained in software.  On
 * such architectures, gup() will not be enough to make a subsequent access
 * succeed.
 *
 * This function will not return with an unlocked mmap_sem. So it has not the
 * same semantics wrt the @mm->mmap_sem as does filemap_fault().
 */
int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
                     unsigned long address, unsigned int fault_flags,
                     bool *unlocked)
{
        struct vm_area_struct *vma;
        int ret, major = 0;

        if (unlocked)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY;

retry:
        vma = find_extend_vma(mm, address);
        if (!vma || address < vma->vm_start)
                return -EFAULT;

        if (!vma_permits_fault(vma, fault_flags))
                return -EFAULT;

        ret = handle_mm_fault(vma, address, fault_flags);
        major |= ret & VM_FAULT_MAJOR;
        if (ret & VM_FAULT_ERROR) {
                if (ret & VM_FAULT_OOM)
                        return -ENOMEM;
                if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
                        return -EHWPOISON;
                if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
                        return -EFAULT;
                BUG();
        }

        if (ret & VM_FAULT_RETRY) {
                down_read(&mm->mmap_sem);
                if (!(fault_flags & FAULT_FLAG_TRIED)) {
                        *unlocked = true;
                        fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
                        fault_flags |= FAULT_FLAG_TRIED;
                        goto retry;
                }
        }

        if (tsk) {
                if (major)
                        tsk->maj_flt++;
                else
                        tsk->min_flt++;
        }
        return 0;
}
EXPORT_SYMBOL_GPL(fixup_user_fault);

static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
                                                struct mm_struct *mm,
                                                unsigned long start,
                                                unsigned long nr_pages,
                                                struct page **pages,
                                                struct vm_area_struct **vmas,
                                                int *locked, bool notify_drop,
                                                unsigned int flags)
{
        long ret, pages_done;
        bool lock_dropped;

        if (locked) {
                /* if VM_FAULT_RETRY can be returned, vmas become invalid */
                BUG_ON(vmas);
                /* check caller initialized locked */
                BUG_ON(*locked != 1);
        }

        if (pages)
                flags |= FOLL_GET;

        pages_done = 0;
        lock_dropped = false;
        for (;;) {
                ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
                                       vmas, locked);
                if (!locked)
                        /* VM_FAULT_RETRY couldn't trigger, bypass */
                        return ret;

                /* VM_FAULT_RETRY cannot return errors */
                if (!*locked) {
                        BUG_ON(ret < 0);
                        BUG_ON(ret >= nr_pages);
                }

                if (!pages)
                        /* If it's a prefault don't insist harder */
                        return ret;

                if (ret > 0) {
                        nr_pages -= ret;
                        pages_done += ret;
                        if (!nr_pages)
                                break;
                }
                if (*locked) {
                        /* VM_FAULT_RETRY didn't trigger */
                        if (!pages_done)
                                pages_done = ret;
                        break;
                }
                /* VM_FAULT_RETRY triggered, so seek to the faulting offset */
                pages += ret;
                start += ret << PAGE_SHIFT;

                /*
                 * Repeat on the address that fired VM_FAULT_RETRY
                 * without FAULT_FLAG_ALLOW_RETRY but with
                 * FAULT_FLAG_TRIED.
                 */
                *locked = 1;
                lock_dropped = true;
                down_read(&mm->mmap_sem);
                ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
                                       pages, NULL, NULL);
                if (ret != 1) {
                        BUG_ON(ret > 1);
                        if (!pages_done)
                                pages_done = ret;
                        break;
                }
                nr_pages--;
                pages_done++;
                if (!nr_pages)
                        break;
                pages++;
                start += PAGE_SIZE;
        }
        if (notify_drop && lock_dropped && *locked) {
                /*
                 * We must let the caller know we temporarily dropped the lock
                 * and so the critical section protected by it was lost.
                 */
                up_read(&mm->mmap_sem);
                *locked = 0;
        }
        return pages_done;
}

/*
 * We can leverage the VM_FAULT_RETRY functionality in the page fault
 * paths better by using either get_user_pages_locked() or
 * get_user_pages_unlocked().
 *
 * get_user_pages_locked() is suitable to replace the form:
 *
 *      down_read(&mm->mmap_sem);
 *      do_something()
 *      get_user_pages(tsk, mm, ..., pages, NULL);
 *      up_read(&mm->mmap_sem);
 *
 *  to:
 *
 *      int locked = 1;
 *      down_read(&mm->mmap_sem);
 *      do_something()
 *      get_user_pages_locked(tsk, mm, ..., pages, &locked);
 *      if (locked)
 *          up_read(&mm->mmap_sem);
 */
long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
                           unsigned int gup_flags, struct page **pages,
                           int *locked)
{
        return __get_user_pages_locked(current, current->mm, start, nr_pages,
                                       pages, NULL, locked, true,
                                       gup_flags | FOLL_TOUCH);
}
EXPORT_SYMBOL(get_user_pages_locked);

/*
 * Same as get_user_pages_unlocked(...., FOLL_TOUCH) but it allows for
 * tsk, mm to be specified.
 *
 * NOTE: here FOLL_TOUCH is not set implicitly and must be set by the
 * caller if required (just like with __get_user_pages). "FOLL_GET"
 * is set implicitly if "pages" is non-NULL.
 */
static __always_inline long __get_user_pages_unlocked(struct task_struct *tsk,
                struct mm_struct *mm, unsigned long start,
                unsigned long nr_pages, struct page **pages,
                unsigned int gup_flags)
{
        long ret;
        int locked = 1;

        down_read(&mm->mmap_sem);
        ret = __get_user_pages_locked(tsk, mm, start, nr_pages, pages, NULL,
                                      &locked, false, gup_flags);
        if (locked)
                up_read(&mm->mmap_sem);
        return ret;
}

/*
 * get_user_pages_unlocked() is suitable to replace the form:
 *
 *      down_read(&mm->mmap_sem);
 *      get_user_pages(tsk, mm, ..., pages, NULL);
 *      up_read(&mm->mmap_sem);
 *
 *  with:
 *
 *      get_user_pages_unlocked(tsk, mm, ..., pages);
 *
 * It is functionally equivalent to get_user_pages_fast so
 * get_user_pages_fast should be used instead if specific gup_flags
 * (e.g. FOLL_FORCE) are not required.
 */
long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
                             struct page **pages, unsigned int gup_flags)
{
        return __get_user_pages_unlocked(current, current->mm, start, nr_pages,
                                         pages, gup_flags | FOLL_TOUCH);
}
EXPORT_SYMBOL(get_user_pages_unlocked);

/*
 * get_user_pages_remote() - pin user pages in memory
 * @tsk:        the task_struct to use for page fault accounting, or
 *              NULL if faults are not to be recorded.
 * @mm:         mm_struct of target mm
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying lookup behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 * @locked:     pointer to lock flag indicating whether lock is held and
 *              subsequently whether VM_FAULT_RETRY functionality can be
 *              utilised. Lock must initially be held.
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno. Each page returned must be released
 * with a put_page() call when it is finished with. vmas will only
 * remain valid while mmap_sem is held.
 *
 * Must be called with mmap_sem held for read or write.
 *
 * get_user_pages walks a process's page tables and takes a reference to
 * each struct page that each user address corresponds to at a given
 * instant. That is, it takes the page that would be accessed if a user
 * thread accesses the given user virtual address at that instant.
 *
 * This does not guarantee that the page exists in the user mappings when
 * get_user_pages returns, and there may even be a completely different
 * page there in some cases (eg. if mmapped pagecache has been invalidated
 * and subsequently re faulted). However it does guarantee that the page
 * won't be freed completely. And mostly callers simply care that the page
 * contains data that was valid *at some point in time*. Typically, an IO
 * or similar operation cannot guarantee anything stronger anyway because
 * locks can't be held over the syscall boundary.
 *
 * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
 * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
 * be called after the page is finished with, and before put_page is called.
 *
 * get_user_pages is typically used for fewer-copy IO operations, to get a
 * handle on the memory by some means other than accesses via the user virtual
 * addresses. The pages may be submitted for DMA to devices or accessed via
 * their kernel linear mapping (via the kmap APIs). Care should be taken to
 * use the correct cache flushing APIs.
 *
 * See also get_user_pages_fast, for performance critical applications.
 *
 * get_user_pages should be phased out in favor of
 * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
 * should use get_user_pages because it cannot pass
 * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
 */
long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
                unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas, int *locked)
{
        return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
                                       locked, true,
                                       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
}
EXPORT_SYMBOL(get_user_pages_remote);

/*
 * This is the same as get_user_pages_remote(), just with a
 * less-flexible calling convention where we assume that the task
 * and mm being operated on are the current task's and don't allow
 * passing of a locked parameter.  We also obviously don't pass
 * FOLL_REMOTE in here.
 */
long get_user_pages(unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas)
{
        return __get_user_pages_locked(current, current->mm, start, nr_pages,
                                       pages, vmas, NULL, false,
                                       gup_flags | FOLL_TOUCH);
}
EXPORT_SYMBOL(get_user_pages);

/**
 * populate_vma_page_range() -  populate a range of pages in the vma.
 * @vma:   target vma
 * @start: start address
 * @end:   end address
 * @nonblocking:
 *
 * This takes care of mlocking the pages too if VM_LOCKED is set.
 *
 * return 0 on success, negative error code on error.
 *
 * vma->vm_mm->mmap_sem must be held.
 *
 * If @nonblocking is NULL, it may be held for read or write and will
 * be unperturbed.
 *
 * If @nonblocking is non-NULL, it must held for read only and may be
 * released.  If it's released, *@nonblocking will be set to 0.
 */
long populate_vma_page_range(struct vm_area_struct *vma,
                unsigned long start, unsigned long end, int *nonblocking)
{
        struct mm_struct *mm = vma->vm_mm;
        unsigned long nr_pages = (end - start) / PAGE_SIZE;
        int gup_flags;

        VM_BUG_ON(start & ~PAGE_MASK);
        VM_BUG_ON(end   & ~PAGE_MASK);
        VM_BUG_ON_VMA(start < vma->vm_start, vma);
        VM_BUG_ON_VMA(end   > vma->vm_end, vma);
        VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);

        gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
        if (vma->vm_flags & VM_LOCKONFAULT)
                gup_flags &= ~FOLL_POPULATE;
        /*
         * We want to touch writable mappings with a write fault in order
         * to break COW, except for shared mappings because these don't COW
         * and we would not want to dirty them for nothing.
         */
        if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
                gup_flags |= FOLL_WRITE;

        /*
         * We want mlock to succeed for regions that have any permissions
         * other than PROT_NONE.
         */
        if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
                gup_flags |= FOLL_FORCE;

        /*
         * We made sure addr is within a VMA, so the following will
         * not result in a stack expansion that recurses back here.
         */
        return __get_user_pages(current, mm, start, nr_pages, gup_flags,
                                NULL, NULL, nonblocking);
}

/*
 * __mm_populate - populate and/or mlock pages within a range of address space.
 *
 * This is used to implement mlock() and the MAP_POPULATE / MAP_LOCKED mmap
 * flags. VMAs must be already marked with the desired vm_flags, and
 * mmap_sem must not be held.
 */
int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
{
        struct mm_struct *mm = current->mm;
        unsigned long end, nstart, nend;
        struct vm_area_struct *vma = NULL;
        int locked = 0;
        long ret = 0;

        VM_BUG_ON(start & ~PAGE_MASK);
        VM_BUG_ON(len != PAGE_ALIGN(len));
        end = start + len;

        for (nstart = start; nstart < end; nstart = nend) {
                /*
                 * We want to fault in pages for [nstart; end) address range.
                 * Find first corresponding VMA.
                 */
                if (!locked) {
                        locked = 1;
                        down_read(&mm->mmap_sem);
                        vma = find_vma(mm, nstart);
                } else if (nstart >= vma->vm_end)
                        vma = vma->vm_next;
                if (!vma || vma->vm_start >= end)
                        break;
                /*
                 * Set [nstart; nend) to intersection of desired address
                 * range with the first VMA. Also, skip undesirable VMA types.
                 */
                nend = min(end, vma->vm_end);
                if (vma->vm_flags & (VM_IO | VM_PFNMAP))
                        continue;
                if (nstart < vma->vm_start)
                        nstart = vma->vm_start;
                /*
                 * Now fault in a range of pages. populate_vma_page_range()
                 * double checks the vma flags, so that it won't mlock pages
                 * if the vma was already munlocked.
                 */
                ret = populate_vma_page_range(vma, nstart, nend, &locked);
                if (ret < 0) {
                        if (ignore_errors) {
                                ret = 0;
                                continue;       /* continue at next VMA */
                        }
                        break;
                }
                nend = nstart + ret * PAGE_SIZE;
                ret = 0;
        }
        if (locked)
                up_read(&mm->mmap_sem);
        return ret;     /* 0 or negative error code */
}

/**
 * get_dump_page() - pin user page in memory while writing it to core dump
 * @addr: user address
 *
 * Returns struct page pointer of user page pinned for dump,
 * to be freed afterwards by put_page().
 *
 * Returns NULL on any kind of failure - a hole must then be inserted into
 * the corefile, to preserve alignment with its headers; and also returns
 * NULL wherever the ZERO_PAGE, or an anonymous pte_none, has been found -
 * allowing a hole to be left in the corefile to save diskspace.
 *
 * Called without mmap_sem, but after all other threads have been killed.
 */
#ifdef CONFIG_ELF_CORE
struct page *get_dump_page(unsigned long addr)
{
        struct vm_area_struct *vma;
        struct page *page;

        if (__get_user_pages(current, current->mm, addr, 1,
                             FOLL_FORCE | FOLL_DUMP | FOLL_GET, &page, &vma,
                             NULL) < 1)
                return NULL;
        flush_cache_page(vma, addr, page_to_pfn(page));
        return page;
}
#endif /* CONFIG_ELF_CORE */

/*
 * Generic RCU Fast GUP
 *
 * get_user_pages_fast attempts to pin user pages by walking the page
 * tables directly and avoids taking locks. Thus the walker needs to be
 * protected from page table pages being freed from under it, and should
 * block any THP splits.
 *
 * One way to achieve this is to have the walker disable interrupts, and
 * rely on IPIs from the TLB flushing code blocking before the page table
 * pages are freed. This is unsuitable for architectures that do not need
 * to broadcast an IPI when invalidating TLBs.
 *
 * Another way to achieve this is to batch up page table containing pages
 * belonging to more than one mm_user, then rcu_sched a callback to free those
 * pages. Disabling interrupts will allow the fast_gup walker to both block
 * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
 * (which is a relatively rare event). The code below adopts this strategy.
 *
 * Before activating this code, please be aware that the following assumptions
 * are currently made:
 *
 *  *) HAVE_RCU_TABLE_FREE is enabled, and tlb_remove_table is used to free
 *      pages containing page tables.
 *
 *  *) ptes can be read atomically by the architecture.
 *
 *  *) access_ok is sufficient to validate userspace address ranges.
 *
 * The last two assumptions can be relaxed by the addition of helper functions.
 *
 * This code is based heavily on the PowerPC implementation by Nick Piggin.
 */
#ifdef CONFIG_HAVE_GENERIC_RCU_GUP

#ifdef __HAVE_ARCH_PTE_SPECIAL
static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
                         int write, struct page **pages, int *nr)
{
        pte_t *ptep, *ptem;
        int ret = 0;

        ptem = ptep = pte_offset_map(&pmd, addr);
        do {
                /*
                 * In the line below we are assuming that the pte can be read
                 * atomically. If this is not the case for your architecture,
                 * please wrap this in a helper function!
                 *
                 * for an example see gup_get_pte in arch/x86/mm/gup.c
                 */
                pte_t pte = READ_ONCE(*ptep);
                struct page *head, *page;

                /*
                 * Similar to the PMD case below, NUMA hinting must take slow
                 * path using the pte_protnone check.
                 */
                if (!pte_present(pte) || pte_special(pte) ||
                        pte_protnone(pte) || (write && !pte_write(pte)))
                        goto pte_unmap;

                if (!arch_pte_access_permitted(pte, write))
                        goto pte_unmap;

                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                page = pte_page(pte);
                head = compound_head(page);

                if (!page_cache_get_speculative(head))
                        goto pte_unmap;

                if (unlikely(pte_val(pte) != pte_val(*ptep))) {
                        put_page(head);
                        goto pte_unmap;
                }

                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;

        } while (ptep++, addr += PAGE_SIZE, addr != end);

        ret = 1;

pte_unmap:
        pte_unmap(ptem);
        return ret;
}
#else

/*
 * If we can't determine whether or not a pte is special, then fail immediately
 * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
 * to be special.
 *
 * For a futex to be placed on a THP tail page, get_futex_key requires a
 * __get_user_pages_fast implementation that can pin pages. Thus it's still
 * useful to have gup_huge_pmd even if we can't operate on ptes.
 */
static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
                         int write, struct page **pages, int *nr)
{
        return 0;
}
#endif /* __HAVE_ARCH_PTE_SPECIAL */

static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
{
        struct page *head, *page;
        int refs;

        if (write && !pmd_write(orig))
                return 0;

        refs = 0;
        head = pmd_page(orig);
        page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
        do {
                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;
                refs++;
        } while (addr += PAGE_SIZE, addr != end);

        if (!page_cache_add_speculative(head, refs)) {
                *nr -= refs;
                return 0;
        }

        if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
                *nr -= refs;
                while (refs--)
                        put_page(head);
                return 0;
        }

        return 1;
}

static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
                unsigned long end, int write, struct page **pages, int *nr)
{
        struct page *head, *page;
        int refs;

        if (write && !pud_write(orig))
                return 0;

        refs = 0;
        head = pud_page(orig);
        page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
        do {
                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;
                refs++;
        } while (addr += PAGE_SIZE, addr != end);

        if (!page_cache_add_speculative(head, refs)) {
                *nr -= refs;
                return 0;
        }

        if (unlikely(pud_val(orig) != pud_val(*pudp))) {
                *nr -= refs;
                while (refs--)
                        put_page(head);
                return 0;
        }

        return 1;
}

static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
                        unsigned long end, int write,
                        struct page **pages, int *nr)
{
        int refs;
        struct page *head, *page;

        if (write && !pgd_write(orig))
                return 0;

        refs = 0;
        head = pgd_page(orig);
        page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
        do {
                VM_BUG_ON_PAGE(compound_head(page) != head, page);
                pages[*nr] = page;
                (*nr)++;
                page++;
                refs++;
        } while (addr += PAGE_SIZE, addr != end);

        if (!page_cache_add_speculative(head, refs)) {
                *nr -= refs;
                return 0;
        }

        if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
                *nr -= refs;
                while (refs--)
                        put_page(head);
                return 0;
        }

        return 1;
}

static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
                int write, struct page **pages, int *nr)
{
        unsigned long next;
        pmd_t *pmdp;

        pmdp = pmd_offset(&pud, addr);
        do {
                pmd_t pmd = READ_ONCE(*pmdp);

                next = pmd_addr_end(addr, end);
                if (pmd_none(pmd))
                        return 0;

                if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd))) {
                        /*
                         * NUMA hinting faults need to be handled in the GUP
                         * slowpath for accounting purposes and so that they
                         * can be serialised against THP migration.
                         */
                        if (pmd_protnone(pmd))
                                return 0;

                        if (!gup_huge_pmd(pmd, pmdp, addr, next, write,
                                pages, nr))
                                return 0;

                } else if (unlikely(is_hugepd(__hugepd(pmd_val(pmd))))) {
                        /*
                         * architecture have different format for hugetlbfs
                         * pmd format and THP pmd format
                         */
                        if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
                                         PMD_SHIFT, next, write, pages, nr))
                                return 0;
                } else if (!gup_pte_range(pmd, addr, next, write, pages, nr))
                                return 0;
        } while (pmdp++, addr = next, addr != end);

        return 1;
}

static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
                         int write, struct page **pages, int *nr)
{
        unsigned long next;
        pud_t *pudp;

        pudp = pud_offset(&p4d, addr);
        do {
                pud_t pud = READ_ONCE(*pudp);

                next = pud_addr_end(addr, end);
                if (pud_none(pud))
                        return 0;
                if (unlikely(pud_huge(pud))) {
                        if (!gup_huge_pud(pud, pudp, addr, next, write,
                                          pages, nr))
                                return 0;
                } else if (unlikely(is_hugepd(__hugepd(pud_val(pud))))) {
                        if (!gup_huge_pd(__hugepd(pud_val(pud)), addr,
                                         PUD_SHIFT, next, write, pages, nr))
                                return 0;
                } else if (!gup_pmd_range(pud, addr, next, write, pages, nr))
                        return 0;
        } while (pudp++, addr = next, addr != end);

        return 1;
}

static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
                         int write, struct page **pages, int *nr)
{
        unsigned long next;
        p4d_t *p4dp;

        p4dp = p4d_offset(&pgd, addr);
        do {
                p4d_t p4d = READ_ONCE(*p4dp);

                next = p4d_addr_end(addr, end);
                if (p4d_none(p4d))
                        return 0;
                BUILD_BUG_ON(p4d_huge(p4d));
                if (unlikely(is_hugepd(__hugepd(p4d_val(p4d))))) {
                        if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr,
                                         P4D_SHIFT, next, write, pages, nr))
                                return 0;
                } else if (!gup_pud_range(p4d, addr, next, write, pages, nr))
                        return 0;
        } while (p4dp++, addr = next, addr != end);

        return 1;
}

/*
 * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
 * the regular GUP. It will only return non-negative values.
 */
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
                          struct page **pages)
{
        struct mm_struct *mm = current->mm;
        unsigned long addr, len, end;
        unsigned long next, flags;
        pgd_t *pgdp;
        int nr = 0;

        start &= PAGE_MASK;
        addr = start;
        len = (unsigned long) nr_pages << PAGE_SHIFT;
        end = start + len;

        if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
                                        start, len)))
                return 0;

        /*
         * Disable interrupts.  We use the nested form as we can already have
         * interrupts disabled by get_futex_key.
         *
         * With interrupts disabled, we block page table pages from being
         * freed from under us. See mmu_gather_tlb in asm-generic/tlb.h
         * for more details.
         *
         * We do not adopt an rcu_read_lock(.) here as we also want to
         * block IPIs that come from THPs splitting.
         */

        local_irq_save(flags);
        pgdp = pgd_offset(mm, addr);
        do {
                pgd_t pgd = READ_ONCE(*pgdp);

                next = pgd_addr_end(addr, end);
                if (pgd_none(pgd))
                        break;
                if (unlikely(pgd_huge(pgd))) {
                        if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
                                          pages, &nr))
                                break;
                } else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
                        if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
                                         PGDIR_SHIFT, next, write, pages, &nr))
                                break;
                } else if (!gup_p4d_range(pgd, addr, next, write, pages, &nr))
                        break;
        } while (pgdp++, addr = next, addr != end);
        local_irq_restore(flags);

        return nr;
}

/**
 * get_user_pages_fast() - pin user pages in memory
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @write:      whether pages will be written to
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 *
 * Attempt to pin user pages in memory without taking mm->mmap_sem.
 * If not successful, it will fall back to taking the lock and
 * calling get_user_pages().
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno.
 */
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
                        struct page **pages)
{
        int nr, ret;

        start &= PAGE_MASK;
        nr = __get_user_pages_fast(start, nr_pages, write, pages);
        ret = nr;

        if (nr < nr_pages) {
                /* Try to get the remaining pages with get_user_pages */
                start += nr << PAGE_SHIFT;
                pages += nr;

                ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
                                write ? FOLL_WRITE : 0);

                /* Have to be a bit careful with return values */
                if (nr > 0) {
                        if (ret < 0)
                                ret = nr;
                        else
                                ret += nr;
                }
        }

        return ret;
}

#endif /* CONFIG_HAVE_GENERIC_RCU_GUP */