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 READ_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                 /* Similar to the PMD case, NUMA hinting must take slow path */
  87                 if (pte_numa(pte)) {
  88                         pte_unmap(ptep);
  89                         return 0;
  90                 }
  91
  92                 if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
  93                         pte_unmap(ptep);
  94                         return 0;
  95                 }
  96                 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
  97                 page = pte_page(pte);
  98                 get_page(page);
  99                 SetPageReferenced(page);
 100                 pages[*nr] = page;
 101                 (*nr)++;
 102
 103         } while (ptep++, addr += PAGE_SIZE, addr != end);
 104         pte_unmap(ptep - 1);
 105
 106         return 1;
 107 }
 108
 109 static inline void get_head_page_multiple(struct page *page, int nr)
 110 {
 111         VM_BUG_ON_PAGE(page != compound_head(page), page);
 112         VM_BUG_ON_PAGE(page_count(page) == 0, page);
 113         atomic_add(nr, &page->_count);
 114         SetPageReferenced(page);
 115 }
 116
 117 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
 118                 unsigned long end, int write, struct page **pages, int *nr)
 119 {
 120         unsigned long mask;
 121         pte_t pte = *(pte_t *)&pmd;
 122         struct page *head, *page;
 123         int refs;
 124
 125         mask = _PAGE_PRESENT|_PAGE_USER;
 126         if (write)
 127                 mask |= _PAGE_RW;
 128         if ((pte_flags(pte) & mask) != mask)
 129                 return 0;
 130         /* hugepages are never "special" */
 131         VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 132         VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 133
 134         refs = 0;
 135         head = pte_page(pte);
 136         page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 137         do {
 138                 VM_BUG_ON_PAGE(compound_head(page) != head, page);
 139                 pages[*nr] = page;
 140                 if (PageTail(page))
 141                         get_huge_page_tail(page);
 142                 (*nr)++;
 143                 page++;
 144                 refs++;
 145         } while (addr += PAGE_SIZE, addr != end);
 146         get_head_page_multiple(head, refs);
 147
 148         return 1;
 149 }
 150
 151 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 152                 int write, struct page **pages, int *nr)
 153 {
 154         unsigned long next;
 155         pmd_t *pmdp;
 156
 157         pmdp = pmd_offset(&pud, addr);
 158         do {
 159                 pmd_t pmd = *pmdp;
 160
 161                 next = pmd_addr_end(addr, end);
 162                 /*
 163                  * The pmd_trans_splitting() check below explains why
 164                  * pmdp_splitting_flush has to flush the tlb, to stop
 165                  * this gup-fast code from running while we set the
 166                  * splitting bit in the pmd. Returning zero will take
 167                  * the slow path that will call wait_split_huge_page()
 168                  * if the pmd is still in splitting state. gup-fast
 169                  * can't because it has irq disabled and
 170                  * wait_split_huge_page() would never return as the
 171                  * tlb flush IPI wouldn't run.
 172                  */
 173                 if (pmd_none(pmd) || pmd_trans_splitting(pmd))
 174                         return 0;
 175                 if (unlikely(pmd_large(pmd))) {
 176                         /*
 177                          * NUMA hinting faults need to be handled in the GUP
 178                          * slowpath for accounting purposes and so that they
 179                          * can be serialised against THP migration.
 180                          */
 181                         if (pmd_numa(pmd))
 182                                 return 0;
 183                         if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
 184                                 return 0;
 185                 } else {
 186                         if (!gup_pte_range(pmd, addr, next, write, pages, nr))
 187                                 return 0;
 188                 }
 189         } while (pmdp++, addr = next, addr != end);
 190
 191         return 1;
 192 }
 193
 194 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
 195                 unsigned long end, int write, struct page **pages, int *nr)
 196 {
 197         unsigned long mask;
 198         pte_t pte = *(pte_t *)&pud;
 199         struct page *head, *page;
 200         int refs;
 201
 202         mask = _PAGE_PRESENT|_PAGE_USER;
 203         if (write)
 204                 mask |= _PAGE_RW;
 205         if ((pte_flags(pte) & mask) != mask)
 206                 return 0;
 207         /* hugepages are never "special" */
 208         VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
 209         VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 210
 211         refs = 0;
 212         head = pte_page(pte);
 213         page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 214         do {
 215                 VM_BUG_ON_PAGE(compound_head(page) != head, page);
 216                 pages[*nr] = page;
 217                 if (PageTail(page))
 218                         get_huge_page_tail(page);
 219                 (*nr)++;
 220                 page++;
 221                 refs++;
 222         } while (addr += PAGE_SIZE, addr != end);
 223         get_head_page_multiple(head, refs);
 224
 225         return 1;
 226 }
 227
 228 static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 229                         int write, struct page **pages, int *nr)
 230 {
 231         unsigned long next;
 232         pud_t *pudp;
 233
 234         pudp = pud_offset(&pgd, addr);
 235         do {
 236                 pud_t pud = *pudp;
 237
 238                 next = pud_addr_end(addr, end);
 239                 if (pud_none(pud))
 240                         return 0;
 241                 if (unlikely(pud_large(pud))) {
 242                         if (!gup_huge_pud(pud, addr, next, write, pages, nr))
 243                                 return 0;
 244                 } else {
 245                         if (!gup_pmd_range(pud, addr, next, write, pages, nr))
 246                                 return 0;
 247                 }
 248         } while (pudp++, addr = next, addr != end);
 249
 250         return 1;
 251 }
 252
 253 /*
 254  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 255  * back to the regular GUP.
 256  */
 257 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 258                           struct page **pages)
 259 {
 260         struct mm_struct *mm = current->mm;
 261         unsigned long addr, len, end;
 262         unsigned long next;
 263         unsigned long flags;
 264         pgd_t *pgdp;
 265         int nr = 0;
 266
 267         start &= PAGE_MASK;
 268         addr = start;
 269         len = (unsigned long) nr_pages << PAGE_SHIFT;
 270         end = start + len;
 271         if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 272                                         (void __user *)start, len)))
 273                 return 0;
 274
 275         /*
 276          * XXX: batch / limit 'nr', to avoid large irq off latency
 277          * needs some instrumenting to determine the common sizes used by
 278          * important workloads (eg. DB2), and whether limiting the batch size
 279          * will decrease performance.
 280          *
 281          * It seems like we're in the clear for the moment. Direct-IO is
 282          * the main guy that batches up lots of get_user_pages, and even
 283          * they are limited to 64-at-a-time which is not so many.
 284          */
 285         /*
 286          * This doesn't prevent pagetable teardown, but does prevent
 287          * the pagetables and pages from being freed on x86.
 288          *
 289          * So long as we atomically load page table pointers versus teardown
 290          * (which we do on x86, with the above PAE exception), we can follow the
 291          * address down to the the page and take a ref on it.
 292          */
 293         local_irq_save(flags);
 294         pgdp = pgd_offset(mm, addr);
 295         do {
 296                 pgd_t pgd = *pgdp;
 297
 298                 next = pgd_addr_end(addr, end);
 299                 if (pgd_none(pgd))
 300                         break;
 301                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 302                         break;
 303         } while (pgdp++, addr = next, addr != end);
 304         local_irq_restore(flags);
 305
 306         return nr;
 307 }
 308
 309 /**
 310  * get_user_pages_fast() - pin user pages in memory
 311  * @start:      starting user address
 312  * @nr_pages:   number of pages from start to pin
 313  * @write:      whether pages will be written to
 314  * @pages:      array that receives pointers to the pages pinned.
 315  *              Should be at least nr_pages long.
 316  *
 317  * Attempt to pin user pages in memory without taking mm->mmap_sem.
 318  * If not successful, it will fall back to taking the lock and
 319  * calling get_user_pages().
 320  *
 321  * Returns number of pages pinned. This may be fewer than the number
 322  * requested. If nr_pages is 0 or negative, returns 0. If no pages
 323  * were pinned, returns -errno.
 324  */
 325 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 326                         struct page **pages)
 327 {
 328         struct mm_struct *mm = current->mm;
 329         unsigned long addr, len, end;
 330         unsigned long next;
 331         pgd_t *pgdp;
 332         int nr = 0;
 333
 334         start &= PAGE_MASK;
 335         addr = start;
 336         len = (unsigned long) nr_pages << PAGE_SHIFT;
 337
 338         end = start + len;
 339         if (end < start)
 340                 goto slow_irqon;
 341
 342 #ifdef CONFIG_X86_64
 343         if (end >> __VIRTUAL_MASK_SHIFT)
 344                 goto slow_irqon;
 345 #endif
 346
 347         /*
 348          * XXX: batch / limit 'nr', to avoid large irq off latency
 349          * needs some instrumenting to determine the common sizes used by
 350          * important workloads (eg. DB2), and whether limiting the batch size
 351          * will decrease performance.
 352          *
 353          * It seems like we're in the clear for the moment. Direct-IO is
 354          * the main guy that batches up lots of get_user_pages, and even
 355          * they are limited to 64-at-a-time which is not so many.
 356          */
 357         /*
 358          * This doesn't prevent pagetable teardown, but does prevent
 359          * the pagetables and pages from being freed on x86.
 360          *
 361          * So long as we atomically load page table pointers versus teardown
 362          * (which we do on x86, with the above PAE exception), we can follow the
 363          * address down to the the page and take a ref on it.
 364          */
 365         local_irq_disable();
 366         pgdp = pgd_offset(mm, addr);
 367         do {
 368                 pgd_t pgd = *pgdp;
 369
 370                 next = pgd_addr_end(addr, end);
 371                 if (pgd_none(pgd))
 372                         goto slow;
 373                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 374                         goto slow;
 375         } while (pgdp++, addr = next, addr != end);
 376         local_irq_enable();
 377
 378         VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
 379         return nr;
 380
 381         {
 382                 int ret;
 383
 384 slow:
 385                 local_irq_enable();
 386 slow_irqon:
 387                 /* Try to get the remaining pages with get_user_pages */
 388                 start += nr << PAGE_SHIFT;
 389                 pages += nr;
 390
 391                 down_read(&mm->mmap_sem);
 392                 ret = get_user_pages(current, mm, start,
 393                         (end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
 394                 up_read(&mm->mmap_sem);
 395
 396                 /* Have to be a bit careful with return values */
 397                 if (nr > 0) {
 398                         if (ret < 0)
 399                                 ret = nr;
 400                         else
 401                                 ret += nr;
 402                 }
 403
 404                 return ret;
 405         }
 406 }