arch/x86/mm/gup.c

   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 #include <linux/swap.h>
  12
  13 #include <asm/pgtable.h>
  14
  15 static inline pte_t gup_get_pte(pte_t *ptep)
  16 {
  17 #ifndef CONFIG_X86_PAE
  18         return ACCESS_ONCE(*ptep);
  19 #else
  20         /*
  21          * With get_user_pages_fast, we walk down the pagetables without taking
  22          * any locks.  For this we would like to load the pointers atomically,
  23          * but that is not possible (without expensive cmpxchg8b) on PAE.  What
  24          * we do have is the guarantee that a pte will only either go from not
  25          * present to present, or present to not present or both -- it will not
  26          * switch to a completely different present page without a TLB flush in
  27          * between; something that we are blocking by holding interrupts off.
  28          *
  29          * Setting ptes from not present to present goes:
  30          * ptep->pte_high = h;
  31          * smp_wmb();
  32          * ptep->pte_low = l;
  33          *
  34          * And present to not present goes:
  35          * ptep->pte_low = 0;
  36          * smp_wmb();
  37          * ptep->pte_high = 0;
  38          *
  39          * We must ensure here that the load of pte_low sees l iff pte_high
  40          * sees h. We load pte_high *after* loading pte_low, which ensures we
  41          * don't see an older value of pte_high.  *Then* we recheck pte_low,
  42          * which ensures that we haven't picked up a changed pte high. We might
  43          * have got rubbish values from pte_low and pte_high, but we are
  44          * guaranteed that pte_low will not have the present bit set *unless*
  45          * it is 'l'. And get_user_pages_fast only operates on present ptes, so
  46          * we're safe.
  47          *
  48          * gup_get_pte should not be used or copied outside gup.c without being
  49          * very careful -- it does not atomically load the pte or anything that
  50          * is likely to be useful for you.
  51          */
  52         pte_t pte;
  53
  54 retry:
  55         pte.pte_low = ptep->pte_low;
  56         smp_rmb();
  57         pte.pte_high = ptep->pte_high;
  58         smp_rmb();
  59         if (unlikely(pte.pte_low != ptep->pte_low))
  60                 goto retry;
  61
  62         return pte;
  63 #endif
  64 }
  65
  66 /*
  67  * The performance critical leaf functions are made noinline otherwise gcc
  68  * inlines everything into a single function which results in too much
  69  * register pressure.
  70  */
  71 static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
  72                 unsigned long end, int write, struct page **pages, int *nr)
  73 {
  74         unsigned long mask;
  75         pte_t *ptep;
  76
  77         mask = _PAGE_PRESENT|_PAGE_USER;
  78         if (write)
  79                 mask |= _PAGE_RW;
  80
  81         ptep = pte_offset_map(&pmd, addr);
  82         do {
  83                 pte_t pte = gup_get_pte(ptep);
  84                 struct page *page;
  85
  86                 if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
  87                         pte_unmap(ptep);
  88                         return 0;
  89                 }
  90                 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
  91                 page = pte_page(pte);
  92                 get_page(page);
  93                 SetPageReferenced(page);
  94                 pages[*nr] = page;
  95                 (*nr)++;
  96
  97         } while (ptep++, addr += PAGE_SIZE, addr != end);
  98         pte_unmap(ptep - 1);
  99
 100         return 1;
 101 }
 102
 103 static inline void get_head_page_multiple(struct page *page, int nr)
 104 {
 105         VM_BUG_ON(page != compound_head(page));
 106         VM_BUG_ON(page_count(page) == 0);
 107         atomic_add(nr, &page->_count);
 108         SetPageReferenced(page);
 109 }
 110
 111 static inline void get_huge_page_tail(struct page *page)
 112 {
 113         /*
 114          * __split_huge_page_refcount() cannot run
 115          * from under us.
 116          */
 117         VM_BUG_ON(atomic_read(&page->_count) < 0);
 118         atomic_inc(&page->_count);
 119 }
 120
 121 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
 122                 unsigned long end, int write, struct page **pages, int *nr)
 123 {
 124         unsigned long mask;
 125         pte_t pte = *(pte_t *)&pmd;
 126         struct page *head, *page;
 127         int refs;
 128
 129         mask = _PAGE_PRESENT|_PAGE_USER;
 130         if (write)
 131                 mask |= _PAGE_RW;
 132         if ((pte_flags(pte) & mask) != mask)
 133                 return 0;
 134         /* hugepages are never "special" */
 135         VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 136         VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 137
 138         refs = 0;
 139         head = pte_page(pte);
 140         page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 141         do {
 142                 VM_BUG_ON(compound_head(page) != head);
 143                 pages[*nr] = page;
 144                 if (PageTail(page))
 145                         get_huge_page_tail(page);
 146                 (*nr)++;
 147                 page++;
 148                 refs++;
 149         } while (addr += PAGE_SIZE, addr != end);
 150         get_head_page_multiple(head, refs);
 151
 152         return 1;
 153 }
 154
 155 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 156                 int write, struct page **pages, int *nr)
 157 {
 158         unsigned long next;
 159         pmd_t *pmdp;
 160
 161         pmdp = pmd_offset(&pud, addr);
 162         do {
 163                 pmd_t pmd = *pmdp;
 164
 165                 next = pmd_addr_end(addr, end);
 166                 /*
 167                  * The pmd_trans_splitting() check below explains why
 168                  * pmdp_splitting_flush has to flush the tlb, to stop
 169                  * this gup-fast code from running while we set the
 170                  * splitting bit in the pmd. Returning zero will take
 171                  * the slow path that will call wait_split_huge_page()
 172                  * if the pmd is still in splitting state. gup-fast
 173                  * can't because it has irq disabled and
 174                  * wait_split_huge_page() would never return as the
 175                  * tlb flush IPI wouldn't run.
 176                  */
 177                 if (pmd_none(pmd) || pmd_trans_splitting(pmd))
 178                         return 0;
 179                 if (unlikely(pmd_large(pmd))) {
 180                         if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
 181                                 return 0;
 182                 } else {
 183                         if (!gup_pte_range(pmd, addr, next, write, pages, nr))
 184                                 return 0;
 185                 }
 186         } while (pmdp++, addr = next, addr != end);
 187
 188         return 1;
 189 }
 190
 191 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
 192                 unsigned long end, int write, struct page **pages, int *nr)
 193 {
 194         unsigned long mask;
 195         pte_t pte = *(pte_t *)&pud;
 196         struct page *head, *page;
 197         int refs;
 198
 199         mask = _PAGE_PRESENT|_PAGE_USER;
 200         if (write)
 201                 mask |= _PAGE_RW;
 202         if ((pte_flags(pte) & mask) != mask)
 203                 return 0;
 204         /* hugepages are never "special" */
 205         VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 206         VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 207
 208         refs = 0;
 209         head = pte_page(pte);
 210         page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 211         do {
 212                 VM_BUG_ON(compound_head(page) != head);
 213                 pages[*nr] = page;
 214                 (*nr)++;
 215                 page++;
 216                 refs++;
 217         } while (addr += PAGE_SIZE, addr != end);
 218         get_head_page_multiple(head, refs);
 219
 220         return 1;
 221 }
 222
 223 static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 224                         int write, struct page **pages, int *nr)
 225 {
 226         unsigned long next;
 227         pud_t *pudp;
 228
 229         pudp = pud_offset(&pgd, addr);
 230         do {
 231                 pud_t pud = *pudp;
 232
 233                 next = pud_addr_end(addr, end);
 234                 if (pud_none(pud))
 235                         return 0;
 236                 if (unlikely(pud_large(pud))) {
 237                         if (!gup_huge_pud(pud, addr, next, write, pages, nr))
 238                                 return 0;
 239                 } else {
 240                         if (!gup_pmd_range(pud, addr, next, write, pages, nr))
 241                                 return 0;
 242                 }
 243         } while (pudp++, addr = next, addr != end);
 244
 245         return 1;
 246 }
 247
 248 /*
 249  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 250  * back to the regular GUP.
 251  */
 252 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 253                           struct page **pages)
 254 {
 255         struct mm_struct *mm = current->mm;
 256         unsigned long addr, len, end;
 257         unsigned long next;
 258         unsigned long flags;
 259         pgd_t *pgdp;
 260         int nr = 0;
 261
 262         start &= PAGE_MASK;
 263         addr = start;
 264         len = (unsigned long) nr_pages << PAGE_SHIFT;
 265         end = start + len;
 266         if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 267                                         (void __user *)start, len)))
 268                 return 0;
 269
 270         /*
 271          * XXX: batch / limit 'nr', to avoid large irq off latency
 272          * needs some instrumenting to determine the common sizes used by
 273          * important workloads (eg. DB2), and whether limiting the batch size
 274          * will decrease performance.
 275          *
 276          * It seems like we're in the clear for the moment. Direct-IO is
 277          * the main guy that batches up lots of get_user_pages, and even
 278          * they are limited to 64-at-a-time which is not so many.
 279          */
 280         /*
 281          * This doesn't prevent pagetable teardown, but does prevent
 282          * the pagetables and pages from being freed on x86.
 283          *
 284          * So long as we atomically load page table pointers versus teardown
 285          * (which we do on x86, with the above PAE exception), we can follow the
 286          * address down to the the page and take a ref on it.
 287          */
 288         local_irq_save(flags);
 289         pgdp = pgd_offset(mm, addr);
 290         do {
 291                 pgd_t pgd = *pgdp;
 292
 293                 next = pgd_addr_end(addr, end);
 294                 if (pgd_none(pgd))
 295                         break;
 296                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 297                         break;
 298         } while (pgdp++, addr = next, addr != end);
 299         local_irq_restore(flags);
 300
 301         return nr;
 302 }
 303
 304 /**
 305  * get_user_pages_fast() - pin user pages in memory
 306  * @start:      starting user address
 307  * @nr_pages:   number of pages from start to pin
 308  * @write:      whether pages will be written to
 309  * @pages:      array that receives pointers to the pages pinned.
 310  *              Should be at least nr_pages long.
 311  *
 312  * Attempt to pin user pages in memory without taking mm->mmap_sem.
 313  * If not successful, it will fall back to taking the lock and
 314  * calling get_user_pages().
 315  *
 316  * Returns number of pages pinned. This may be fewer than the number
 317  * requested. If nr_pages is 0 or negative, returns 0. If no pages
 318  * were pinned, returns -errno.
 319  */
 320 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 321                         struct page **pages)
 322 {
 323         struct mm_struct *mm = current->mm;
 324         unsigned long addr, len, end;
 325         unsigned long next;
 326         pgd_t *pgdp;
 327         int nr = 0;
 328
 329         start &= PAGE_MASK;
 330         addr = start;
 331         len = (unsigned long) nr_pages << PAGE_SHIFT;
 332
 333         end = start + len;
 334         if (end < start)
 335                 goto slow_irqon;
 336
 337 #ifdef CONFIG_X86_64
 338         if (end >> __VIRTUAL_MASK_SHIFT)
 339                 goto slow_irqon;
 340 #endif
 341
 342         /*
 343          * XXX: batch / limit 'nr', to avoid large irq off latency
 344          * needs some instrumenting to determine the common sizes used by
 345          * important workloads (eg. DB2), and whether limiting the batch size
 346          * will decrease performance.
 347          *
 348          * It seems like we're in the clear for the moment. Direct-IO is
 349          * the main guy that batches up lots of get_user_pages, and even
 350          * they are limited to 64-at-a-time which is not so many.
 351          */
 352         /*
 353          * This doesn't prevent pagetable teardown, but does prevent
 354          * the pagetables and pages from being freed on x86.
 355          *
 356          * So long as we atomically load page table pointers versus teardown
 357          * (which we do on x86, with the above PAE exception), we can follow the
 358          * address down to the the page and take a ref on it.
 359          */
 360         local_irq_disable();
 361         pgdp = pgd_offset(mm, addr);
 362         do {
 363                 pgd_t pgd = *pgdp;
 364
 365                 next = pgd_addr_end(addr, end);
 366                 if (pgd_none(pgd))
 367                         goto slow;
 368                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 369                         goto slow;
 370         } while (pgdp++, addr = next, addr != end);
 371         local_irq_enable();
 372
 373         VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
 374         return nr;
 375
 376         {
 377                 int ret;
 378
 379 slow:
 380                 local_irq_enable();
 381 slow_irqon:
 382                 /* Try to get the remaining pages with get_user_pages */
 383                 start += nr << PAGE_SHIFT;
 384                 pages += nr;
 385
 386                 down_read(&mm->mmap_sem);
 387                 ret = get_user_pages(current, mm, start,
 388                         (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
 389                 up_read(&mm->mmap_sem);
 390
 391                 /* Have to be a bit careful with return values */
 392                 if (nr > 0) {
 393                         if (ret < 0)
 394                                 ret = nr;
 395                         else
 396                                 ret += nr;
 397                 }
 398
 399                 return ret;
 400         }
 401 }