powerpc/mm: Lockless get_user_pages_fast() for 64-bit (v3)

Implement lockless get_user_pages_fast for 64-bit powerpc.

Page table existence is guaranteed with RCU, and speculative page references
are used to take a reference to the pages without having a prior existence
guarantee on them.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 280 insertions, 0 deletions

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
new file mode 100644
index 000000000000..9fdf4d6335e4
--- /dev/null
+++ b/arch/powerpc/mm/gup.c
@@ -0,0 +1,280 @@
+/*
+ * 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>
+
+/*
+ * 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 != *ptep)) {
+                        put_page(page);
+                        return 0;
+                }
+                pages[*nr] = page;
+                (*nr)++;
+
+        } while (ptep++, addr += PAGE_SIZE, addr != end);
+
+        return 1;
+}
+
+#ifdef CONFIG_HUGETLB_PAGE
+static noinline int gup_huge_pte(pte_t *ptep, struct hstate *hstate,
+                                 unsigned long *addr, unsigned long end,
+                                 int write, struct page **pages, int *nr)
+{
+        unsigned long mask;
+        unsigned long pte_end;
+        struct page *head, *page;
+        pte_t pte;
+        int refs;
+
+        pte_end = (*addr + huge_page_size(hstate)) & huge_page_mask(hstate);
+        if (pte_end < end)
+                end = pte_end;
+
+        pte = *ptep;
+        mask = _PAGE_PRESENT|_PAGE_USER;
+        if (write)
+                mask |= _PAGE_RW;
+        if ((pte_val(pte) & mask) != mask)
+                return 0;
+        /* hugepages are never "special" */
+        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+
+        refs = 0;
+        head = pte_page(pte);
+        page = head + ((*addr & ~huge_page_mask(hstate)) >> PAGE_SHIFT);
+        do {
+                VM_BUG_ON(compound_head(page) != head);
+                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(pte != *ptep)) {
+                /* Could be optimized better */
+                while (*nr) {
+                        put_page(page);
+                        (*nr)--;
+                }
+        }
+
+        return 1;
+}
+#endif /* CONFIG_HUGETLB_PAGE */
+
+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 (!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 (!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 psize, nr = 0;
+        unsigned int shift;
+
+        pr_debug("%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_debug("  aligned: %lx .. %lx\n", start, end);
+
+#ifdef CONFIG_HUGETLB_PAGE
+        /* We bail out on slice boundary crossing when hugetlb is
+         * enabled in order to not have to deal with two different
+         * page table formats
+         */
+        if (addr < SLICE_LOW_TOP) {
+                if (end > SLICE_LOW_TOP)
+                        goto slow_irqon;
+
+                if (unlikely(GET_LOW_SLICE_INDEX(addr) !=
+                             GET_LOW_SLICE_INDEX(end - 1)))
+                        goto slow_irqon;
+        } else {
+                if (unlikely(GET_HIGH_SLICE_INDEX(addr) !=
+                             GET_HIGH_SLICE_INDEX(end - 1)))
+                        goto slow_irqon;
+        }
+#endif /* CONFIG_HUGETLB_PAGE */
+
+        /*
+         * 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();
+
+        psize = get_slice_psize(mm, addr);
+        shift = mmu_psize_defs[psize].shift;
+
+#ifdef CONFIG_HUGETLB_PAGE
+        if (unlikely(mmu_huge_psizes[psize])) {
+                pte_t *ptep;
+                unsigned long a = addr;
+                unsigned long sz = ((1UL) << shift);
+                struct hstate *hstate = size_to_hstate(sz);
+
+                BUG_ON(!hstate);
+                /*
+                 * XXX: could be optimized to avoid hstate
+                 * lookup entirely (just use shift)
+                 */
+
+                do {
+                        VM_BUG_ON(shift != mmu_psize_defs[get_slice_psize(mm, a)].shift);
+                        ptep = huge_pte_offset(mm, a);
+                        pr_debug(" %016lx: huge ptep %p\n", a, ptep);
+                        if (!ptep || !gup_huge_pte(ptep, hstate, &a, end, write, pages,
+                                                   &nr))
+                                goto slow;
+                } while (a != end);
+        } else
+#endif /* CONFIG_HUGETLB_PAGE */
+        {
+                pgdp = pgd_offset(mm, addr);
+                do {
+                        pgd_t pgd = *pgdp;
+
+                        VM_BUG_ON(shift != mmu_psize_defs[get_slice_psize(mm, addr)].shift);
+                        pr_debug("  %016lx: normal pgd %p\n", addr, (void *)pgd);
+                        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:
+                pr_debug("  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;
+        }
+}