gup.c

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/*
 * Lockless get_user_pages_fast for x86
 *
 * Copyright (C) 2008 Nick Piggin
 * Copyright (C) 2008 Novell Inc.
 */
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/vmstat.h>
#include <linux/highmem.h>
#include <linux/swap.h>

#include <asm/pgtable.h>

static inline pte_t gup_get_pte(pte_t *ptep)
{
#ifndef CONFIG_X86_PAE
    return ACCESS_ONCE(*ptep);
#else
    /*
     * With get_user_pages_fast, we walk down the pagetables without taking
     * any locks.  For this we would like to load the pointers atomically,
     * but that is not possible (without expensive cmpxchg8b) on PAE.  What
     * we do have is the guarantee that a pte will only either go from not
     * present to present, or present to not present or both -- it will not
     * switch to a completely different present page without a TLB flush in
     * between; something that we are blocking by holding interrupts off.
     *
     * Setting ptes from not present to present goes:
     * ptep->pte_high = h;
     * smp_wmb();
     * ptep->pte_low = l;
     *
     * And present to not present goes:
     * ptep->pte_low = 0;
     * smp_wmb();
     * ptep->pte_high = 0;
     *
     * We must ensure here that the load of pte_low sees l iff pte_high
     * sees h. We load pte_high *after* loading pte_low, which ensures we
     * don't see an older value of pte_high.  *Then* we recheck pte_low,
     * which ensures that we haven't picked up a changed pte high. We might
     * have got rubbish values from pte_low and pte_high, but we are
     * guaranteed that pte_low will not have the present bit set *unless*
     * it is 'l'. And get_user_pages_fast only operates on present ptes, so
     * we're safe.
     *
     * gup_get_pte should not be used or copied outside gup.c without being
     * very careful -- it does not atomically load the pte or anything that
     * is likely to be useful for you.
     */
    pte_t pte;

retry:
    pte.pte_low = ptep->pte_low;
    smp_rmb();
    pte.pte_high = ptep->pte_high;
    smp_rmb();
    if (unlikely(pte.pte_low != ptep->pte_low))
        goto retry;

    return pte;
#endif
}

/*
 * The performance critical leaf functions are made noinline otherwise gcc
 * inlines everything into a single function which results in too much
 * register pressure.
 */
static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
        unsigned long end, int write, struct page **pages, int *nr)
{
    unsigned long mask;
    pte_t *ptep;

    mask = _PAGE_PRESENT|_PAGE_USER;
    if (write)
        mask |= _PAGE_RW;

    ptep = pte_offset_map(&pmd, addr);
    do {
        pte_t pte = gup_get_pte(ptep);
        struct page *page;

        if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
            pte_unmap(ptep);
            return 0;
        }
        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
        page = pte_page(pte);
        get_page(page);
        SetPageReferenced(page);
        pages[*nr] = page;
        (*nr)++;

    } while (ptep++, addr += PAGE_SIZE, addr != end);
    pte_unmap(ptep - 1);

    return 1;
}

static inline void get_head_page_multiple(struct page *page, int nr)
{
    VM_BUG_ON(page != compound_head(page));
    VM_BUG_ON(page_count(page) == 0);
    atomic_add(nr, &page->_count);
    SetPageReferenced(page);
}

static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
        unsigned long end, int write, struct page **pages, int *nr)
{
    unsigned long mask;
    pte_t pte = *(pte_t *)&pmd;
    struct page *head, *page;
    int refs;

    mask = _PAGE_PRESENT|_PAGE_USER;
    if (write)
        mask |= _PAGE_RW;
    if ((pte_flags(pte) & mask) != mask)
        return 0;
    /* hugepages are never "special" */
    VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
    VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

    refs = 0;
    head = pte_page(pte);
    page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
    do {
        VM_BUG_ON(compound_head(page) != head);
        pages[*nr] = page;
        if (PageTail(page))
            get_huge_page_tail(page);
        (*nr)++;
        page++;
        refs++;
    } while (addr += PAGE_SIZE, addr != end);
    get_head_page_multiple(head, refs);

    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 = *pmdp;

        next = pmd_addr_end(addr, end);
        /*
         * The pmd_trans_splitting() check below explains why
         * pmdp_splitting_flush has to flush the tlb, to stop
         * this gup-fast code from running while we set the
         * splitting bit in the pmd. Returning zero will take
         * the slow path that will call wait_split_huge_page()
         * if the pmd is still in splitting state. gup-fast
         * can't because it has irq disabled and
         * wait_split_huge_page() would never return as the
         * tlb flush IPI wouldn't run.
         */
        if (pmd_none(pmd) || pmd_trans_splitting(pmd))
            return 0;
        if (unlikely(pmd_large(pmd))) {
            if (!gup_huge_pmd(pmd, addr, 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 noinline int gup_huge_pud(pud_t pud, unsigned long addr,
        unsigned long end, int write, struct page **pages, int *nr)
{
    unsigned long mask;
    pte_t pte = *(pte_t *)&pud;
    struct page *head, *page;
    int refs;

    mask = _PAGE_PRESENT|_PAGE_USER;
    if (write)
        mask |= _PAGE_RW;
    if ((pte_flags(pte) & mask) != mask)
        return 0;
    /* hugepages are never "special" */
    VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
    VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

    refs = 0;
    head = pte_page(pte);
    page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
    do {
        VM_BUG_ON(compound_head(page) != head);
        pages[*nr] = page;
        if (PageTail(page))
            get_huge_page_tail(page);
        (*nr)++;
        page++;
        refs++;
    } while (addr += PAGE_SIZE, addr != end);
    get_head_page_multiple(head, refs);

    return 1;
}

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

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

        next = pud_addr_end(addr, end);
        if (pud_none(pud))
            return 0;
        if (unlikely(pud_large(pud))) {
            if (!gup_huge_pud(pud, addr, 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;
}

/*
 * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 * back to the regular GUP.
 */
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;
    unsigned long 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,
                    (void __user *)start, len)))
        return 0;

    /*
     * XXX: batch / limit 'nr', to avoid large irq off latency
     * needs some instrumenting to determine the common sizes used by
     * important workloads (eg. DB2), and whether limiting the batch size
     * will decrease performance.
     *
     * It seems like we're in the clear for the moment. Direct-IO is
     * the main guy that batches up lots of get_user_pages, and even
     * they are limited to 64-at-a-time which is not so many.
     */
    /*
     * This doesn't prevent pagetable teardown, but does prevent
     * the pagetables and pages from being freed on x86.
     *
     * So long as we atomically load page table pointers versus teardown
     * (which we do on x86, with the above PAE exception), we can follow the
     * address down to the the page and take a ref on it.
     */
    local_irq_save(flags);
    pgdp = pgd_offset(mm, addr);
    do {
        pgd_t pgd = *pgdp;

        next = pgd_addr_end(addr, end);
        if (pgd_none(pgd))
            break;
        if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
            break;
    } while (pgdp++, addr = next, addr != end);
    local_irq_restore(flags);

    return nr;
}

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;
    pgd_t *pgdp;
    int nr = 0;

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

    end = start + len;
    if (end < start)
        goto slow_irqon;

#ifdef CONFIG_X86_64
    if (end >> __VIRTUAL_MASK_SHIFT)
        goto slow_irqon;
#endif

    /*
     * XXX: batch / limit 'nr', to avoid large irq off latency
     * needs some instrumenting to determine the common sizes used by
     * important workloads (eg. DB2), and whether limiting the batch size
     * will decrease performance.
     *
     * It seems like we're in the clear for the moment. Direct-IO is
     * the main guy that batches up lots of get_user_pages, and even
     * they are limited to 64-at-a-time which is not so many.
     */
    /*
     * This doesn't prevent pagetable teardown, but does prevent
     * the pagetables and pages from being freed on x86.
     *
     * So long as we atomically load page table pointers versus teardown
     * (which we do on x86, with the above PAE exception), we can follow the
     * address down to the the page and take a ref on it.
     */
    local_irq_disable();
    pgdp = pgd_offset(mm, addr);
    do {
        pgd_t pgd = *pgdp;

        next = pgd_addr_end(addr, end);
        if (pgd_none(pgd))
            goto slow;
        if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
            goto slow;
    } while (pgdp++, addr = next, addr != end);
    local_irq_enable();

    VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
    return nr;

    {
        int ret;

slow:
        local_irq_enable();
slow_irqon:
        /* Try to get the remaining pages with get_user_pages */
        start += nr << PAGE_SHIFT;
        pages += nr;

        down_read(&mm->mmap_sem);
        ret = get_user_pages(current, mm, start,
            (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
        up_read(&mm->mmap_sem);

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

        return ret;
    }
}