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 #include <linux/memremap.h>
  13
  14 #include <asm/mmu_context.h>
  15 #include <asm/pgtable.h>
  16
  17 static inline pte_t gup_get_pte(pte_t *ptep)
  18 {
  19 #ifndef CONFIG_X86_PAE
  20         return READ_ONCE(*ptep);
  21 #else
  22         /*
  23          * With get_user_pages_fast, we walk down the pagetables without taking
  24          * any locks.  For this we would like to load the pointers atomically,
  25          * but that is not possible (without expensive cmpxchg8b) on PAE.  What
  26          * we do have is the guarantee that a pte will only either go from not
  27          * present to present, or present to not present or both -- it will not
  28          * switch to a completely different present page without a TLB flush in
  29          * between; something that we are blocking by holding interrupts off.
  30          *
  31          * Setting ptes from not present to present goes:
  32          * ptep->pte_high = h;
  33          * smp_wmb();
  34          * ptep->pte_low = l;
  35          *
  36          * And present to not present goes:
  37          * ptep->pte_low = 0;
  38          * smp_wmb();
  39          * ptep->pte_high = 0;
  40          *
  41          * We must ensure here that the load of pte_low sees l iff pte_high
  42          * sees h. We load pte_high *after* loading pte_low, which ensures we
  43          * don't see an older value of pte_high.  *Then* we recheck pte_low,
  44          * which ensures that we haven't picked up a changed pte high. We might
  45          * have got rubbish values from pte_low and pte_high, but we are
  46          * guaranteed that pte_low will not have the present bit set *unless*
  47          * it is 'l'. And get_user_pages_fast only operates on present ptes, so
  48          * we're safe.
  49          *
  50          * gup_get_pte should not be used or copied outside gup.c without being
  51          * very careful -- it does not atomically load the pte or anything that
  52          * is likely to be useful for you.
  53          */
  54         pte_t pte;
  55
  56 retry:
  57         pte.pte_low = ptep->pte_low;
  58         smp_rmb();
  59         pte.pte_high = ptep->pte_high;
  60         smp_rmb();
  61         if (unlikely(pte.pte_low != ptep->pte_low))
  62                 goto retry;
  63
  64         return pte;
  65 #endif
  66 }
  67
  68 static void undo_dev_pagemap(int *nr, int nr_start, struct page **pages)
  69 {
  70         while ((*nr) - nr_start) {
  71                 struct page *page = pages[--(*nr)];
  72
  73                 ClearPageReferenced(page);
  74                 put_page(page);
  75         }
  76 }
  77
  78 /*
  79  * 'pteval' can come from a pte, pmd or pud.  We only check
  80  * _PAGE_PRESENT, _PAGE_USER, and _PAGE_RW in here which are the
  81  * same value on all 3 types.
  82  */
  83 static inline int pte_allows_gup(unsigned long pteval, int write)
  84 {
  85         unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
  86
  87         if (write)
  88                 need_pte_bits |= _PAGE_RW;
  89
  90         if ((pteval & need_pte_bits) != need_pte_bits)
  91                 return 0;
  92
  93         /* Check memory protection keys permissions. */
  94         if (!__pkru_allows_pkey(pte_flags_pkey(pteval), write))
  95                 return 0;
  96
  97         return 1;
  98 }
  99
 100 /*
 101  * The performance critical leaf functions are made noinline otherwise gcc
 102  * inlines everything into a single function which results in too much
 103  * register pressure.
 104  */
 105 static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 106                 unsigned long end, int write, struct page **pages, int *nr)
 107 {
 108         struct dev_pagemap *pgmap = NULL;
 109         int nr_start = *nr;
 110         pte_t *ptep;
 111
 112         ptep = pte_offset_map(&pmd, addr);
 113         do {
 114                 pte_t pte = gup_get_pte(ptep);
 115                 struct page *page;
 116
 117                 /* Similar to the PMD case, NUMA hinting must take slow path */
 118                 if (pte_protnone(pte)) {
 119                         pte_unmap(ptep);
 120                         return 0;
 121                 }
 122
 123                 if (pte_devmap(pte)) {
 124                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 125                         if (unlikely(!pgmap)) {
 126                                 undo_dev_pagemap(nr, nr_start, pages);
 127                                 pte_unmap(ptep);
 128                                 return 0;
 129                         }
 130                 } else if (!pte_allows_gup(pte_val(pte), write) ||
 131                            pte_special(pte)) {
 132                         pte_unmap(ptep);
 133                         return 0;
 134                 }
 135                 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 136                 page = pte_page(pte);
 137                 get_page(page);
 138                 put_dev_pagemap(pgmap);
 139                 SetPageReferenced(page);
 140                 pages[*nr] = page;
 141                 (*nr)++;
 142
 143         } while (ptep++, addr += PAGE_SIZE, addr != end);
 144         pte_unmap(ptep - 1);
 145
 146         return 1;
 147 }
 148
 149 static inline void get_head_page_multiple(struct page *page, int nr)
 150 {
 151         VM_BUG_ON_PAGE(page != compound_head(page), page);
 152         VM_BUG_ON_PAGE(page_count(page) == 0, page);
 153         page_ref_add(page, nr);
 154         SetPageReferenced(page);
 155 }
 156
 157 static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr,
 158                 unsigned long end, struct page **pages, int *nr)
 159 {
 160         int nr_start = *nr;
 161         unsigned long pfn = pmd_pfn(pmd);
 162         struct dev_pagemap *pgmap = NULL;
 163
 164         pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
 165         do {
 166                 struct page *page = pfn_to_page(pfn);
 167
 168                 pgmap = get_dev_pagemap(pfn, pgmap);
 169                 if (unlikely(!pgmap)) {
 170                         undo_dev_pagemap(nr, nr_start, pages);
 171                         return 0;
 172                 }
 173                 SetPageReferenced(page);
 174                 pages[*nr] = page;
 175                 get_page(page);
 176                 put_dev_pagemap(pgmap);
 177                 (*nr)++;
 178                 pfn++;
 179         } while (addr += PAGE_SIZE, addr != end);
 180         return 1;
 181 }
 182
 183 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
 184                 unsigned long end, int write, struct page **pages, int *nr)
 185 {
 186         struct page *head, *page;
 187         int refs;
 188
 189         if (!pte_allows_gup(pmd_val(pmd), write))
 190                 return 0;
 191
 192         VM_BUG_ON(!pfn_valid(pmd_pfn(pmd)));
 193         if (pmd_devmap(pmd))
 194                 return __gup_device_huge_pmd(pmd, addr, end, pages, nr);
 195
 196         /* hugepages are never "special" */
 197         VM_BUG_ON(pmd_flags(pmd) & _PAGE_SPECIAL);
 198
 199         refs = 0;
 200         head = pmd_page(pmd);
 201         page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 202         do {
 203                 VM_BUG_ON_PAGE(compound_head(page) != head, page);
 204                 pages[*nr] = page;
 205                 (*nr)++;
 206                 page++;
 207                 refs++;
 208         } while (addr += PAGE_SIZE, addr != end);
 209         get_head_page_multiple(head, refs);
 210
 211         return 1;
 212 }
 213
 214 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 215                 int write, struct page **pages, int *nr)
 216 {
 217         unsigned long next;
 218         pmd_t *pmdp;
 219
 220         pmdp = pmd_offset(&pud, addr);
 221         do {
 222                 pmd_t pmd = *pmdp;
 223
 224                 next = pmd_addr_end(addr, end);
 225                 if (pmd_none(pmd))
 226                         return 0;
 227                 if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
 228                         /*
 229                          * NUMA hinting faults need to be handled in the GUP
 230                          * slowpath for accounting purposes and so that they
 231                          * can be serialised against THP migration.
 232                          */
 233                         if (pmd_protnone(pmd))
 234                                 return 0;
 235                         if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
 236                                 return 0;
 237                 } else {
 238                         if (!gup_pte_range(pmd, addr, next, write, pages, nr))
 239                                 return 0;
 240                 }
 241         } while (pmdp++, addr = next, addr != end);
 242
 243         return 1;
 244 }
 245
 246 static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
 247                 unsigned long end, int write, struct page **pages, int *nr)
 248 {
 249         struct page *head, *page;
 250         int refs;
 251
 252         if (!pte_allows_gup(pud_val(pud), write))
 253                 return 0;
 254         /* hugepages are never "special" */
 255         VM_BUG_ON(pud_flags(pud) & _PAGE_SPECIAL);
 256         VM_BUG_ON(!pfn_valid(pud_pfn(pud)));
 257
 258         refs = 0;
 259         head = pud_page(pud);
 260         page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 261         do {
 262                 VM_BUG_ON_PAGE(compound_head(page) != head, page);
 263                 pages[*nr] = page;
 264                 (*nr)++;
 265                 page++;
 266                 refs++;
 267         } while (addr += PAGE_SIZE, addr != end);
 268         get_head_page_multiple(head, refs);
 269
 270         return 1;
 271 }
 272
 273 static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 274                         int write, struct page **pages, int *nr)
 275 {
 276         unsigned long next;
 277         pud_t *pudp;
 278
 279         pudp = pud_offset(&pgd, addr);
 280         do {
 281                 pud_t pud = *pudp;
 282
 283                 next = pud_addr_end(addr, end);
 284                 if (pud_none(pud))
 285                         return 0;
 286                 if (unlikely(pud_large(pud))) {
 287                         if (!gup_huge_pud(pud, addr, next, write, pages, nr))
 288                                 return 0;
 289                 } else {
 290                         if (!gup_pmd_range(pud, addr, next, write, pages, nr))
 291                                 return 0;
 292                 }
 293         } while (pudp++, addr = next, addr != end);
 294
 295         return 1;
 296 }
 297
 298 /*
 299  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
 300  * back to the regular GUP.
 301  */
 302 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 303                           struct page **pages)
 304 {
 305         struct mm_struct *mm = current->mm;
 306         unsigned long addr, len, end;
 307         unsigned long next;
 308         unsigned long flags;
 309         pgd_t *pgdp;
 310         int nr = 0;
 311
 312         start &= PAGE_MASK;
 313         addr = start;
 314         len = (unsigned long) nr_pages << PAGE_SHIFT;
 315         end = start + len;
 316         if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 317                                         (void __user *)start, len)))
 318                 return 0;
 319
 320         /*
 321          * XXX: batch / limit 'nr', to avoid large irq off latency
 322          * needs some instrumenting to determine the common sizes used by
 323          * important workloads (eg. DB2), and whether limiting the batch size
 324          * will decrease performance.
 325          *
 326          * It seems like we're in the clear for the moment. Direct-IO is
 327          * the main guy that batches up lots of get_user_pages, and even
 328          * they are limited to 64-at-a-time which is not so many.
 329          */
 330         /*
 331          * This doesn't prevent pagetable teardown, but does prevent
 332          * the pagetables and pages from being freed on x86.
 333          *
 334          * So long as we atomically load page table pointers versus teardown
 335          * (which we do on x86, with the above PAE exception), we can follow the
 336          * address down to the the page and take a ref on it.
 337          */
 338         local_irq_save(flags);
 339         pgdp = pgd_offset(mm, addr);
 340         do {
 341                 pgd_t pgd = *pgdp;
 342
 343                 next = pgd_addr_end(addr, end);
 344                 if (pgd_none(pgd))
 345                         break;
 346                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 347                         break;
 348         } while (pgdp++, addr = next, addr != end);
 349         local_irq_restore(flags);
 350
 351         return nr;
 352 }
 353
 354 /**
 355  * get_user_pages_fast() - pin user pages in memory
 356  * @start:      starting user address
 357  * @nr_pages:   number of pages from start to pin
 358  * @write:      whether pages will be written to
 359  * @pages:      array that receives pointers to the pages pinned.
 360  *              Should be at least nr_pages long.
 361  *
 362  * Attempt to pin user pages in memory without taking mm->mmap_sem.
 363  * If not successful, it will fall back to taking the lock and
 364  * calling get_user_pages().
 365  *
 366  * Returns number of pages pinned. This may be fewer than the number
 367  * requested. If nr_pages is 0 or negative, returns 0. If no pages
 368  * were pinned, returns -errno.
 369  */
 370 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 371                         struct page **pages)
 372 {
 373         struct mm_struct *mm = current->mm;
 374         unsigned long addr, len, end;
 375         unsigned long next;
 376         pgd_t *pgdp;
 377         int nr = 0;
 378
 379         start &= PAGE_MASK;
 380         addr = start;
 381         len = (unsigned long) nr_pages << PAGE_SHIFT;
 382
 383         end = start + len;
 384         if (end < start)
 385                 goto slow_irqon;
 386
 387 #ifdef CONFIG_X86_64
 388         if (end >> __VIRTUAL_MASK_SHIFT)
 389                 goto slow_irqon;
 390 #endif
 391
 392         /*
 393          * XXX: batch / limit 'nr', to avoid large irq off latency
 394          * needs some instrumenting to determine the common sizes used by
 395          * important workloads (eg. DB2), and whether limiting the batch size
 396          * will decrease performance.
 397          *
 398          * It seems like we're in the clear for the moment. Direct-IO is
 399          * the main guy that batches up lots of get_user_pages, and even
 400          * they are limited to 64-at-a-time which is not so many.
 401          */
 402         /*
 403          * This doesn't prevent pagetable teardown, but does prevent
 404          * the pagetables and pages from being freed on x86.
 405          *
 406          * So long as we atomically load page table pointers versus teardown
 407          * (which we do on x86, with the above PAE exception), we can follow the
 408          * address down to the the page and take a ref on it.
 409          */
 410         local_irq_disable();
 411         pgdp = pgd_offset(mm, addr);
 412         do {
 413                 pgd_t pgd = *pgdp;
 414
 415                 next = pgd_addr_end(addr, end);
 416                 if (pgd_none(pgd))
 417                         goto slow;
 418                 if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 419                         goto slow;
 420         } while (pgdp++, addr = next, addr != end);
 421         local_irq_enable();
 422
 423         VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
 424         return nr;
 425
 426         {
 427                 int ret;
 428
 429 slow:
 430                 local_irq_enable();
 431 slow_irqon:
 432                 /* Try to get the remaining pages with get_user_pages */
 433                 start += nr << PAGE_SHIFT;
 434                 pages += nr;
 435
 436                 ret = get_user_pages_unlocked(start,
 437                                               (end - start) >> PAGE_SHIFT,
 438                                               pages, write ? FOLL_WRITE : 0);
 439
 440                 /* Have to be a bit careful with return values */
 441                 if (nr > 0) {
 442                         if (ret < 0)
 443                                 ret = nr;
 444                         else
 445                                 ret += nr;
 446                 }
 447
 448                 return ret;
 449         }
 450 }