gup.c

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/*
 * Lockless get_user_pages_fast for powerpc
 *
 * Copyright (C) 2008 Nick Piggin
 * Copyright (C) 2008 Novell Inc.
 */
#undef DEBUG

#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/vmstat.h>
#include <linux/pagemap.h>
#include <linux/rwsem.h>
#include <asm/pgtable.h>

#ifdef __HAVE_ARCH_PTE_SPECIAL

/*
 * 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, result;
    pte_t *ptep;

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

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

        if ((pte_val(pte) & mask) != result)
            return 0;
        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
        page = pte_page(pte);
        if (!page_cache_get_speculative(page))
            return 0;
        if (unlikely(pte_val(pte) != pte_val(*ptep))) {
            put_page(page);
            return 0;
        }
        pages[*nr] = page;
        (*nr)++;

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

    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);
        if (pmd_none(pmd))
            return 0;
        if (is_hugepd(pmdp)) {
            if (!gup_hugepd((hugepd_t *)pmdp, PMD_SHIFT,
                    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 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 (is_hugepd(pudp)) {
            if (!gup_hugepd((hugepd_t *)pudp, PUD_SHIFT,
                    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;
}

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;

    pr_devel("%s(%lx,%x,%s)\n", __func__, start, nr_pages, write ? "write" : "read");

    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)))
        goto slow_irqon;

    pr_devel("  aligned: %lx .. %lx\n", start, end);

    /*
     * 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 from being freed on powerpc.
     *
     * So long as we atomically load page table pointers versus teardown,
     * 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;

        pr_devel("  %016lx: normal pgd %p\n", addr,
             (void *)pgd_val(pgd));
        next = pgd_addr_end(addr, end);
        if (pgd_none(pgd))
            goto slow;
        if (is_hugepd(pgdp)) {
            if (!gup_hugepd((hugepd_t *)pgdp, PGDIR_SHIFT,
                    addr, next, write, pages, &nr))
                goto slow;
        } else 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:
        pr_devel("  slow path ! nr = %d\n", nr);

        /* 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;
    }
}

#endif /* __HAVE_ARCH_PTE_SPECIAL */