linux/arch/x86/mm/gup.c
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   1/*
   2 * Lockless get_user_pages_fast for x86
   3 *
   4 * Copyright (C) 2008 Nick Piggin
   5 * Copyright (C) 2008 Novell Inc.
   6 */
   7#include <linux/sched.h>
   8#include <linux/mm.h>
   9#include <linux/vmstat.h>
  10#include <linux/highmem.h>
  11
  12#include <asm/pgtable.h>
  13
  14static inline pte_t gup_get_pte(pte_t *ptep)
  15{
  16#ifndef CONFIG_X86_PAE
  17        return *ptep;
  18#else
  19        /*
  20         * With get_user_pages_fast, we walk down the pagetables without taking
  21         * any locks.  For this we would like to load the pointers atoimcally,
  22         * but that is not possible (without expensive cmpxchg8b) on PAE.  What
  23         * we do have is the guarantee that a pte will only either go from not
  24         * present to present, or present to not present or both -- it will not
  25         * switch to a completely different present page without a TLB flush in
  26         * between; something that we are blocking by holding interrupts off.
  27         *
  28         * Setting ptes from not present to present goes:
  29         * ptep->pte_high = h;
  30         * smp_wmb();
  31         * ptep->pte_low = l;
  32         *
  33         * And present to not present goes:
  34         * ptep->pte_low = 0;
  35         * smp_wmb();
  36         * ptep->pte_high = 0;
  37         *
  38         * We must ensure here that the load of pte_low sees l iff pte_high
  39         * sees h. We load pte_high *after* loading pte_low, which ensures we
  40         * don't see an older value of pte_high.  *Then* we recheck pte_low,
  41         * which ensures that we haven't picked up a changed pte high. We might
  42         * have got rubbish values from pte_low and pte_high, but we are
  43         * guaranteed that pte_low will not have the present bit set *unless*
  44         * it is 'l'. And get_user_pages_fast only operates on present ptes, so
  45         * we're safe.
  46         *
  47         * gup_get_pte should not be used or copied outside gup.c without being
  48         * very careful -- it does not atomically load the pte or anything that
  49         * is likely to be useful for you.
  50         */
  51        pte_t pte;
  52
  53retry:
  54        pte.pte_low = ptep->pte_low;
  55        smp_rmb();
  56        pte.pte_high = ptep->pte_high;
  57        smp_rmb();
  58        if (unlikely(pte.pte_low != ptep->pte_low))
  59                goto retry;
  60
  61        return pte;
  62#endif
  63}
  64
  65/*
  66 * The performance critical leaf functions are made noinline otherwise gcc
  67 * inlines everything into a single function which results in too much
  68 * register pressure.
  69 */
  70static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
  71                unsigned long end, int write, struct page **pages, int *nr)
  72{
  73        unsigned long mask;
  74        pte_t *ptep;
  75
  76        mask = _PAGE_PRESENT|_PAGE_USER;
  77        if (write)
  78                mask |= _PAGE_RW;
  79
  80        ptep = pte_offset_map(&pmd, addr);
  81        do {
  82                pte_t pte = gup_get_pte(ptep);
  83                struct page *page;
  84
  85                if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
  86                        pte_unmap(ptep);
  87                        return 0;
  88                }
  89                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
  90                page = pte_page(pte);
  91                get_page(page);
  92                pages[*nr] = page;
  93                (*nr)++;
  94
  95        } while (ptep++, addr += PAGE_SIZE, addr != end);
  96        pte_unmap(ptep - 1);
  97
  98        return 1;
  99}
 100
 101static inline void get_head_page_multiple(struct page *page, int nr)
 102{
 103        VM_BUG_ON(page != compound_head(page));
 104        VM_BUG_ON(page_count(page) == 0);
 105        atomic_add(nr, &page->_count);
 106}
 107
 108static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
 109                unsigned long end, int write, struct page **pages, int *nr)
 110{
 111        unsigned long mask;
 112        pte_t pte = *(pte_t *)&pmd;
 113        struct page *head, *page;
 114        int refs;
 115
 116        mask = _PAGE_PRESENT|_PAGE_USER;
 117        if (write)
 118                mask |= _PAGE_RW;
 119        if ((pte_flags(pte) & mask) != mask)
 120                return 0;
 121        /* hugepages are never "special" */
 122        VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 123        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 124
 125        refs = 0;
 126        head = pte_page(pte);
 127        page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 128        do {
 129                VM_BUG_ON(compound_head(page) != head);
 130                pages[*nr] = page;
 131                (*nr)++;
 132                page++;
 133                refs++;
 134        } while (addr += PAGE_SIZE, addr != end);
 135        get_head_page_multiple(head, refs);
 136
 137        return 1;
 138}
 139
 140static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 141                int write, struct page **pages, int *nr)
 142{
 143        unsigned long next;
 144        pmd_t *pmdp;
 145
 146        pmdp = pmd_offset(&pud, addr);
 147        do {
 148                pmd_t pmd = *pmdp;
 149
 150                next = pmd_addr_end(addr, end);
 151                if (pmd_none(pmd))
 152                        return 0;
 153                if (unlikely(pmd_large(pmd))) {
 154                        if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
 155                                return 0;
 156                } else {
 157                        if (!gup_pte_range(pmd, addr, next, write, pages, nr))
 158                                return 0;
 159                }
 160        } while (pmdp++, addr = next, addr != end);
 161
 162        return 1;
 163}
 164
 165static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
 166                unsigned long end, int write, struct page **pages, int *nr)
 167{
 168        unsigned long mask;
 169        pte_t pte = *(pte_t *)&pud;
 170        struct page *head, *page;
 171        int refs;
 172
 173        mask = _PAGE_PRESENT|_PAGE_USER;
 174        if (write)
 175                mask |= _PAGE_RW;
 176        if ((pte_flags(pte) & mask) != mask)
 177                return 0;
 178        /* hugepages are never "special" */
 179        VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 180        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 181
 182        refs = 0;
 183        head = pte_page(pte);
 184        page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 185        do {
 186                VM_BUG_ON(compound_head(page) != head);
 187                pages[*nr] = page;
 188                (*nr)++;
 189                page++;
 190                refs++;
 191        } while (addr += PAGE_SIZE, addr != end);
 192        get_head_page_multiple(head, refs);
 193
 194        return 1;
 195}
 196
 197static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 198                        int write, struct page **pages, int *nr)
 199{
 200        unsigned long next;
 201        pud_t *pudp;
 202
 203        pudp = pud_offset(&pgd, addr);
 204        do {
 205                pud_t pud = *pudp;
 206
 207                next = pud_addr_end(addr, end);
 208                if (pud_none(pud))
 209                        return 0;
 210                if (unlikely(pud_large(pud))) {
 211                        if (!gup_huge_pud(pud, addr, next, write, pages, nr))
 212                                return 0;
 213                } else {
 214                        if (!gup_pmd_range(pud, addr, next, write, pages, nr))
 215                                return 0;
 216                }
 217        } while (pudp++, addr = next, addr != end);
 218
 219        return 1;
 220}
 221
 222int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 223                        struct page **pages)
 224{
 225        struct mm_struct *mm = current->mm;
 226        unsigned long addr, len, end;
 227        unsigned long next;
 228        pgd_t *pgdp;
 229        int nr = 0;
 230
 231        start &= PAGE_MASK;
 232        addr = start;
 233        len = (unsigned long) nr_pages << PAGE_SHIFT;
 234        end = start + len;
 235        if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 236                                        (void __user *)start, len)))
 237                goto slow_irqon;
 238
 239        /*
 240         * XXX: batch / limit 'nr', to avoid large irq off latency
 241         * needs some instrumenting to determine the common sizes used by
 242         * important workloads (eg. DB2), and whether limiting the batch size
 243         * will decrease performance.
 244         *
 245         * It seems like we're in the clear for the moment. Direct-IO is
 246         * the main guy that batches up lots of get_user_pages, and even
 247         * they are limited to 64-at-a-time which is not so many.
 248         */
 249        /*
 250         * This doesn't prevent pagetable teardown, but does prevent
 251         * the pagetables and pages from being freed on x86.
 252         *
 253         * So long as we atomically load page table pointers versus teardown
 254         * (which we do on x86, with the above PAE exception), we can follow the
 255         * address down to the the page and take a ref on it.
 256         */
 257        local_irq_disable();
 258        pgdp = pgd_offset(mm, addr);
 259        do {
 260                pgd_t pgd = *pgdp;
 261
 262                next = pgd_addr_end(addr, end);
 263                if (pgd_none(pgd))
 264                        goto slow;
 265                if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 266                        goto slow;
 267        } while (pgdp++, addr = next, addr != end);
 268        local_irq_enable();
 269
 270        VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
 271        return nr;
 272
 273        {
 274                int ret;
 275
 276slow:
 277                local_irq_enable();
 278slow_irqon:
 279                /* Try to get the remaining pages with get_user_pages */
 280                start += nr << PAGE_SHIFT;
 281                pages += nr;
 282
 283                down_read(&mm->mmap_sem);
 284                ret = get_user_pages(current, mm, start,
 285                        (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
 286                up_read(&mm->mmap_sem);
 287
 288                /* Have to be a bit careful with return values */
 289                if (nr > 0) {
 290                        if (ret < 0)
 291                                ret = nr;
 292                        else
 293                                ret += nr;
 294                }
 295
 296                return ret;
 297        }
 298}
 299