1 files changed, 662 insertions, 0 deletions

diff --git a/mm/gup.c b/mm/gup.c
new file mode 100644
index 000000000000..cc5a9e7adea7
--- /dev/null
+++ b/mm/gup.c
@@ -0,0 +1,662 @@
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/err.h>
+#include <linux/spinlock.h>
+
+#include <linux/hugetlb.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <linux/swapops.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 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 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) || pte_file(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_numa(pte))
+                goto no_page;
+        if ((flags & FOLL_WRITE) && !pte_write(pte)) {
+                pte_unmap_unlock(ptep, ptl);
+                return NULL;
+        }
+
+        page = vm_normal_page(vma, address, pte);
+        if (unlikely(!page)) {
+                if ((flags & FOLL_DUMP) ||
+                    !is_zero_pfn(pte_pfn(pte)))
+                        goto bad_page;
+                page = pte_page(pte);
+        }
+
+        if (flags & FOLL_GET)
+                get_page_foll(page);
+        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)) {
+                /*
+                 * 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);
+                }
+        }
+        pte_unmap_unlock(ptep, ptl);
+        return page;
+bad_page:
+        pte_unmap_unlock(ptep, ptl);
+        return ERR_PTR(-EFAULT);
+
+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;
+        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);
+
+        pud = pud_offset(pgd, address);
+        if (pud_none(*pud))
+                return no_page_table(vma, flags);
+        if (pud_huge(*pud) && vma->vm_flags & VM_HUGETLB) {
+                if (flags & FOLL_GET)
+                        return NULL;
+                page = follow_huge_pud(mm, address, pud, flags & FOLL_WRITE);
+                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 & FOLL_WRITE);
+                if (flags & FOLL_GET) {
+                        /*
+                         * Refcount on tail pages are not well-defined and
+                         * shouldn't be taken. The caller should handle a NULL
+                         * return when trying to follow tail pages.
+                         */
+                        if (PageHead(page))
+                                get_page(page);
+                        else
+                                page = NULL;
+                }
+                return page;
+        }
+        if ((flags & FOLL_NUMA) && pmd_numa(*pmd))
+                return no_page_table(vma, flags);
+        if (pmd_trans_huge(*pmd)) {
+                if (flags & FOLL_SPLIT) {
+                        split_huge_page_pmd(vma, address, pmd);
+                        return follow_page_pte(vma, address, pmd, flags);
+                }
+                ptl = pmd_lock(mm, pmd);
+                if (likely(pmd_trans_huge(*pmd))) {
+                        if (unlikely(pmd_trans_splitting(*pmd))) {
+                                spin_unlock(ptl);
+                                wait_split_huge_page(vma->anon_vma, pmd);
+                        } else {
+                                page = follow_trans_huge_pmd(vma, address,
+                                                             pmd, flags);
+                                spin_unlock(ptl);
+                                *page_mask = HPAGE_PMD_NR - 1;
+                                return page;
+                        }
+                } else
+                        spin_unlock(ptl);
+        }
+        return follow_page_pte(vma, address, pmd, flags);
+}
+
+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;
+        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));
+        pud = pud_offset(pgd, 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;
+}
+
+static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
+                unsigned long address, unsigned int *flags, int *nonblocking)
+{
+        struct mm_struct *mm = vma->vm_mm;
+        unsigned int fault_flags = 0;
+        int ret;
+
+        /* For mlock, just skip the stack guard page. */
+        if ((*flags & FOLL_MLOCK) &&
+                        (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 (nonblocking)
+                fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+        if (*flags & FOLL_NOWAIT)
+                fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
+
+        ret = handle_mm_fault(mm, 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)
+                        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_WRITE;
+        return 0;
+}
+
+static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
+{
+        vm_flags_t vm_flags = vma->vm_flags;
+
+        if (vm_flags & (VM_IO | VM_PFNMAP))
+                return -EFAULT;
+
+        if (gup_flags & FOLL_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)) {
+                                WARN_ON_ONCE(vm_flags & VM_MAYWRITE);
+                                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;
+        }
+        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 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.
+ *
+ * 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.
+ *
+ * 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.
+ */
+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);
+                                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();
+                }
+                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;
+}
+EXPORT_SYMBOL(__get_user_pages);
+
+/*
+ * 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()
+ *
+ * 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
+ * handle_mm_fault() 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 should be called with the mm_sem held for read.
+ */
+int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
+                     unsigned long address, unsigned int fault_flags)
+{
+        struct vm_area_struct *vma;
+        vm_flags_t vm_flags;
+        int ret;
+
+        vma = find_extend_vma(mm, address);
+        if (!vma || address < vma->vm_start)
+                return -EFAULT;
+
+        vm_flags = (fault_flags & FAULT_FLAG_WRITE) ? VM_WRITE : VM_READ;
+        if (!(vm_flags & vma->vm_flags))
+                return -EFAULT;
+
+        ret = handle_mm_fault(mm, 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 -EHWPOISON;
+                if (ret & VM_FAULT_SIGBUS)
+                        return -EFAULT;
+                BUG();
+        }
+        if (tsk) {
+                if (ret & VM_FAULT_MAJOR)
+                        tsk->maj_flt++;
+                else
+                        tsk->min_flt++;
+        }
+        return 0;
+}
+
+/*
+ * get_user_pages() - 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
+ * @write:      whether pages will be written to by the caller
+ * @force:      whether to force access even when user mapping is currently
+ *              protected (but never forces write access to shared mapping).
+ * @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.
+ *
+ * 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 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.
+ */
+long get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
+                unsigned long start, unsigned long nr_pages, int write,
+                int force, struct page **pages, struct vm_area_struct **vmas)
+{
+        int flags = FOLL_TOUCH;
+
+        if (pages)
+                flags |= FOLL_GET;
+        if (write)
+                flags |= FOLL_WRITE;
+        if (force)
+                flags |= FOLL_FORCE;
+
+        return __get_user_pages(tsk, mm, start, nr_pages, flags, pages, vmas,
+                                NULL);
+}
+EXPORT_SYMBOL(get_user_pages);
+
+/**
+ * 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 page_cache_release() or 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 */