sh: lockless get_user_pages_fast()

Implement get_user_pages_fast without locking in the fastpath on sh.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

2 files changed, 274 insertions, 1 deletions

diff --git a/arch/sh/mm/Makefile b/arch/sh/mm/Makefile
index ab89ea4f9414..150aa326afff 100644
--- a/arch/sh/mm/Makefile
+++ b/arch/sh/mm/Makefile
@@ -15,7 +15,7 @@ cacheops-$(CONFIG_CPU_SHX3)		+= cache-shx3.o
 obj-y                   += $(cacheops-y)
 
 mmu-y                   := nommu.o extable_32.o
-mmu-$(CONFIG_MMU)       := extable_$(BITS).o fault_$(BITS).o \
+mmu-$(CONFIG_MMU)       := extable_$(BITS).o fault_$(BITS).o gup.o \
                            ioremap.o kmap.o pgtable.o tlbflush_$(BITS).o
 
 obj-y                   += $(mmu-y)
diff --git a/arch/sh/mm/gup.c b/arch/sh/mm/gup.c
new file mode 100644
index 000000000000..bf8daf9d9c9b
--- /dev/null
+++ b/arch/sh/mm/gup.c
@@ -0,0 +1,273 @@
+/*
+ * Lockless get_user_pages_fast for SuperH
+ *
+ * Copyright (C) 2009 - 2010  Paul Mundt
+ *
+ * Cloned from the x86 and PowerPC versions, by:
+ *
+ *      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 <asm/pgtable.h>
+
+static inline pte_t gup_get_pte(pte_t *ptep)
+{
+#ifndef CONFIG_X2TLB
+        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 with 64-bit PTEs.  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)
+{
+        u64 mask, result;
+        pte_t *ptep;
+
+#ifdef CONFIG_X2TLB
+        result = _PAGE_PRESENT | _PAGE_EXT(_PAGE_EXT_KERN_READ | _PAGE_EXT_USER_READ);
+        if (write)
+                result |= _PAGE_EXT(_PAGE_EXT_KERN_WRITE | _PAGE_EXT_USER_WRITE);
+#elif defined(CONFIG_SUPERH64)
+        result = _PAGE_PRESENT | _PAGE_USER | _PAGE_READ;
+        if (write)
+                result |= _PAGE_WRITE;
+#else
+        result = _PAGE_PRESENT | _PAGE_USER;
+        if (write)
+                result |= _PAGE_RW;
+#endif
+
+        mask = result | _PAGE_SPECIAL;
+
+        ptep = pte_offset_map(&pmd, addr);
+        do {
+                pte_t pte = gup_get_pte(ptep);
+                struct page *page;
+
+                if ((pte_val(pte) & mask) != result) {
+                        pte_unmap(ptep);
+                        return 0;
+                }
+                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+                page = pte_page(pte);
+                get_page(page);
+                pages[*nr] = page;
+                (*nr)++;
+
+        } while (ptep++, addr += PAGE_SIZE, addr != end);
+        pte_unmap(ptep - 1);
+
+        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 (!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;
+}
+
+/*
+ * 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;
+
+        /*
+         * This doesn't prevent pagetable teardown, but does prevent
+         * the pagetables and pages from being freed.
+         */
+        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;
+}
+
+/**
+ * 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)
+{
+        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;
+
+        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;
+        }
+}