arch/x86_64/mm/fault.c

   1 /*
   2  *  linux/arch/x86-64/mm/fault.c
   3  *
   4  *  Copyright (C) 1995  Linus Torvalds
   5  *  Copyright (C) 2001,2002 Andi Kleen, SuSE Labs.
   6  */
   7
   8 #include <linux/signal.h>
   9 #include <linux/sched.h>
  10 #include <linux/kernel.h>
  11 #include <linux/errno.h>
  12 #include <linux/string.h>
  13 #include <linux/types.h>
  14 #include <linux/ptrace.h>
  15 #include <linux/mman.h>
  16 #include <linux/mm.h>
  17 #include <linux/smp.h>
  18 #include <linux/smp_lock.h>
  19 #include <linux/interrupt.h>
  20 #include <linux/init.h>
  21 #include <linux/tty.h>
  22 #include <linux/vt_kern.h>              /* For unblank_screen() */
  23 #include <linux/compiler.h>
  24 #include <linux/module.h>
  25 #include <linux/kprobes.h>
  26
  27 #include <asm/system.h>
  28 #include <asm/uaccess.h>
  29 #include <asm/pgalloc.h>
  30 #include <asm/smp.h>
  31 #include <asm/tlbflush.h>
  32 #include <asm/proto.h>
  33 #include <asm/kdebug.h>
  34 #include <asm-generic/sections.h>
  35
  36 /* Page fault error code bits */
  37 #define PF_PROT (1<<0)          /* or no page found */
  38 #define PF_WRITE        (1<<1)
  39 #define PF_USER (1<<2)
  40 #define PF_RSVD (1<<3)
  41 #define PF_INSTR        (1<<4)
  42
  43 static ATOMIC_NOTIFIER_HEAD(notify_page_fault_chain);
  44
  45 /* Hook to register for page fault notifications */
  46 int register_page_fault_notifier(struct notifier_block *nb)
  47 {
  48         vmalloc_sync_all();
  49         return atomic_notifier_chain_register(&notify_page_fault_chain, nb);
  50 }
  51 EXPORT_SYMBOL_GPL(register_page_fault_notifier);
  52
  53 int unregister_page_fault_notifier(struct notifier_block *nb)
  54 {
  55         return atomic_notifier_chain_unregister(&notify_page_fault_chain, nb);
  56 }
  57 EXPORT_SYMBOL_GPL(unregister_page_fault_notifier);
  58
  59 static inline int notify_page_fault(enum die_val val, const char *str,
  60                         struct pt_regs *regs, long err, int trap, int sig)
  61 {
  62         struct die_args args = {
  63                 .regs = regs,
  64                 .str = str,
  65                 .err = err,
  66                 .trapnr = trap,
  67                 .signr = sig
  68         };
  69         return atomic_notifier_call_chain(&notify_page_fault_chain, val, &args);
  70 }
  71
  72 void bust_spinlocks(int yes)
  73 {
  74         int loglevel_save = console_loglevel;
  75         if (yes) {
  76                 oops_in_progress = 1;
  77         } else {
  78 #ifdef CONFIG_VT
  79                 unblank_screen();
  80 #endif
  81                 oops_in_progress = 0;
  82                 /*
  83                  * OK, the message is on the console.  Now we call printk()
  84                  * without oops_in_progress set so that printk will give klogd
  85                  * a poke.  Hold onto your hats...
  86                  */
  87                 console_loglevel = 15;          /* NMI oopser may have shut the console up */
  88                 printk(" ");
  89                 console_loglevel = loglevel_save;
  90         }
  91 }
  92
  93 /* Sometimes the CPU reports invalid exceptions on prefetch.
  94    Check that here and ignore.
  95    Opcode checker based on code by Richard Brunner */
  96 static noinline int is_prefetch(struct pt_regs *regs, unsigned long addr,
  97                                 unsigned long error_code)
  98 {
  99         unsigned char __user *instr;
 100         int scan_more = 1;
 101         int prefetch = 0;
 102         unsigned char *max_instr;
 103
 104         /* If it was a exec fault ignore */
 105         if (error_code & PF_INSTR)
 106                 return 0;
 107
 108         instr = (unsigned char __user *)convert_rip_to_linear(current, regs);
 109         max_instr = instr + 15;
 110
 111         if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE)
 112                 return 0;
 113
 114         while (scan_more && instr < max_instr) {
 115                 unsigned char opcode;
 116                 unsigned char instr_hi;
 117                 unsigned char instr_lo;
 118
 119                 if (__get_user(opcode, (char __user *)instr))
 120                         break;
 121
 122                 instr_hi = opcode & 0xf0;
 123                 instr_lo = opcode & 0x0f;
 124                 instr++;
 125
 126                 switch (instr_hi) {
 127                 case 0x20:
 128                 case 0x30:
 129                         /* Values 0x26,0x2E,0x36,0x3E are valid x86
 130                            prefixes.  In long mode, the CPU will signal
 131                            invalid opcode if some of these prefixes are
 132                            present so we will never get here anyway */
 133                         scan_more = ((instr_lo & 7) == 0x6);
 134                         break;
 135
 136                 case 0x40:
 137                         /* In AMD64 long mode, 0x40 to 0x4F are valid REX prefixes
 138                            Need to figure out under what instruction mode the
 139                            instruction was issued ... */
 140                         /* Could check the LDT for lm, but for now it's good
 141                            enough to assume that long mode only uses well known
 142                            segments or kernel. */
 143                         scan_more = (!user_mode(regs)) || (regs->cs == __USER_CS);
 144                         break;
 145
 146                 case 0x60:
 147                         /* 0x64 thru 0x67 are valid prefixes in all modes. */
 148                         scan_more = (instr_lo & 0xC) == 0x4;
 149                         break;
 150                 case 0xF0:
 151                         /* 0xF0, 0xF2, and 0xF3 are valid prefixes in all modes. */
 152                         scan_more = !instr_lo || (instr_lo>>1) == 1;
 153                         break;
 154                 case 0x00:
 155                         /* Prefetch instruction is 0x0F0D or 0x0F18 */
 156                         scan_more = 0;
 157                         if (__get_user(opcode, (char __user *)instr))
 158                                 break;
 159                         prefetch = (instr_lo == 0xF) &&
 160                                 (opcode == 0x0D || opcode == 0x18);
 161                         break;
 162                 default:
 163                         scan_more = 0;
 164                         break;
 165                 }
 166         }
 167         return prefetch;
 168 }
 169
 170 static int bad_address(void *p)
 171 {
 172         unsigned long dummy;
 173         return __get_user(dummy, (unsigned long __user *)p);
 174 }
 175
 176 void dump_pagetable(unsigned long address)
 177 {
 178         pgd_t *pgd;
 179         pud_t *pud;
 180         pmd_t *pmd;
 181         pte_t *pte;
 182
 183         asm("movq %%cr3,%0" : "=r" (pgd));
 184
 185         pgd = __va((unsigned long)pgd & PHYSICAL_PAGE_MASK);
 186         pgd += pgd_index(address);
 187         if (bad_address(pgd)) goto bad;
 188         printk("PGD %lx ", pgd_val(*pgd));
 189         if (!pgd_present(*pgd)) goto ret;
 190
 191         pud = pud_offset(pgd, address);
 192         if (bad_address(pud)) goto bad;
 193         printk("PUD %lx ", pud_val(*pud));
 194         if (!pud_present(*pud)) goto ret;
 195
 196         pmd = pmd_offset(pud, address);
 197         if (bad_address(pmd)) goto bad;
 198         printk("PMD %lx ", pmd_val(*pmd));
 199         if (!pmd_present(*pmd)) goto ret;
 200
 201         pte = pte_offset_kernel(pmd, address);
 202         if (bad_address(pte)) goto bad;
 203         printk("PTE %lx", pte_val(*pte));
 204 ret:
 205         printk("\n");
 206         return;
 207 bad:
 208         printk("BAD\n");
 209 }
 210
 211 static const char errata93_warning[] =
 212 KERN_ERR "******* Your BIOS seems to not contain a fix for K8 errata #93\n"
 213 KERN_ERR "******* Working around it, but it may cause SEGVs or burn power.\n"
 214 KERN_ERR "******* Please consider a BIOS update.\n"
 215 KERN_ERR "******* Disabling USB legacy in the BIOS may also help.\n";
 216
 217 /* Workaround for K8 erratum #93 & buggy BIOS.
 218    BIOS SMM functions are required to use a specific workaround
 219    to avoid corruption of the 64bit RIP register on C stepping K8.
 220    A lot of BIOS that didn't get tested properly miss this.
 221    The OS sees this as a page fault with the upper 32bits of RIP cleared.
 222    Try to work around it here.
 223    Note we only handle faults in kernel here. */
 224
 225 static int is_errata93(struct pt_regs *regs, unsigned long address)
 226 {
 227         static int warned;
 228         if (address != regs->rip)
 229                 return 0;
 230         if ((address >> 32) != 0)
 231                 return 0;
 232         address |= 0xffffffffUL << 32;
 233         if ((address >= (u64)_stext && address <= (u64)_etext) ||
 234             (address >= MODULES_VADDR && address <= MODULES_END)) {
 235                 if (!warned) {
 236                         printk(errata93_warning);
 237                         warned = 1;
 238                 }
 239                 regs->rip = address;
 240                 return 1;
 241         }
 242         return 0;
 243 }
 244
 245 int unhandled_signal(struct task_struct *tsk, int sig)
 246 {
 247         if (is_init(tsk))
 248                 return 1;
 249         if (tsk->ptrace & PT_PTRACED)
 250                 return 0;
 251         return (tsk->sighand->action[sig-1].sa.sa_handler == SIG_IGN) ||
 252                 (tsk->sighand->action[sig-1].sa.sa_handler == SIG_DFL);
 253 }
 254
 255 static noinline void pgtable_bad(unsigned long address, struct pt_regs *regs,
 256                                  unsigned long error_code)
 257 {
 258         unsigned long flags = oops_begin();
 259         struct task_struct *tsk;
 260
 261         printk(KERN_ALERT "%s: Corrupted page table at address %lx\n",
 262                current->comm, address);
 263         dump_pagetable(address);
 264         tsk = current;
 265         tsk->thread.cr2 = address;
 266         tsk->thread.trap_no = 14;
 267         tsk->thread.error_code = error_code;
 268         __die("Bad pagetable", regs, error_code);
 269         oops_end(flags);
 270         do_exit(SIGKILL);
 271 }
 272
 273 /*
 274  * Handle a fault on the vmalloc area
 275  *
 276  * This assumes no large pages in there.
 277  */
 278 static int vmalloc_fault(unsigned long address)
 279 {
 280         pgd_t *pgd, *pgd_ref;
 281         pud_t *pud, *pud_ref;
 282         pmd_t *pmd, *pmd_ref;
 283         pte_t *pte, *pte_ref;
 284
 285         /* Copy kernel mappings over when needed. This can also
 286            happen within a race in page table update. In the later
 287            case just flush. */
 288
 289         pgd = pgd_offset(current->mm ?: &init_mm, address);
 290         pgd_ref = pgd_offset_k(address);
 291         if (pgd_none(*pgd_ref))
 292                 return -1;
 293         if (pgd_none(*pgd))
 294                 set_pgd(pgd, *pgd_ref);
 295         else
 296                 BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
 297
 298         /* Below here mismatches are bugs because these lower tables
 299            are shared */
 300
 301         pud = pud_offset(pgd, address);
 302         pud_ref = pud_offset(pgd_ref, address);
 303         if (pud_none(*pud_ref))
 304                 return -1;
 305         if (pud_none(*pud) || pud_page_vaddr(*pud) != pud_page_vaddr(*pud_ref))
 306                 BUG();
 307         pmd = pmd_offset(pud, address);
 308         pmd_ref = pmd_offset(pud_ref, address);
 309         if (pmd_none(*pmd_ref))
 310                 return -1;
 311         if (pmd_none(*pmd) || pmd_page(*pmd) != pmd_page(*pmd_ref))
 312                 BUG();
 313         pte_ref = pte_offset_kernel(pmd_ref, address);
 314         if (!pte_present(*pte_ref))
 315                 return -1;
 316         pte = pte_offset_kernel(pmd, address);
 317         /* Don't use pte_page here, because the mappings can point
 318            outside mem_map, and the NUMA hash lookup cannot handle
 319            that. */
 320         if (!pte_present(*pte) || pte_pfn(*pte) != pte_pfn(*pte_ref))
 321                 BUG();
 322         return 0;
 323 }
 324
 325 int page_fault_trace = 0;
 326 int exception_trace = 1;
 327
 328 /*
 329  * This routine handles page faults.  It determines the address,
 330  * and the problem, and then passes it off to one of the appropriate
 331  * routines.
 332  */
 333 asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 334                                         unsigned long error_code)
 335 {
 336         struct task_struct *tsk;
 337         struct mm_struct *mm;
 338         struct vm_area_struct * vma;
 339         unsigned long address;
 340         const struct exception_table_entry *fixup;
 341         int write;
 342         unsigned long flags;
 343         siginfo_t info;
 344
 345         tsk = current;
 346         mm = tsk->mm;
 347         prefetchw(&mm->mmap_sem);
 348
 349         /* get the address */
 350         __asm__("movq %%cr2,%0":"=r" (address));
 351
 352         info.si_code = SEGV_MAPERR;
 353
 354
 355         /*
 356          * We fault-in kernel-space virtual memory on-demand. The
 357          * 'reference' page table is init_mm.pgd.
 358          *
 359          * NOTE! We MUST NOT take any locks for this case. We may
 360          * be in an interrupt or a critical region, and should
 361          * only copy the information from the master page table,
 362          * nothing more.
 363          *
 364          * This verifies that the fault happens in kernel space
 365          * (error_code & 4) == 0, and that the fault was not a
 366          * protection error (error_code & 9) == 0.
 367          */
 368         if (unlikely(address >= TASK_SIZE64)) {
 369                 /*
 370                  * Don't check for the module range here: its PML4
 371                  * is always initialized because it's shared with the main
 372                  * kernel text. Only vmalloc may need PML4 syncups.
 373                  */
 374                 if (!(error_code & (PF_RSVD|PF_USER|PF_PROT)) &&
 375                       ((address >= VMALLOC_START && address < VMALLOC_END))) {
 376                         if (vmalloc_fault(address) >= 0)
 377                                 return;
 378                 }
 379                 if (notify_page_fault(DIE_PAGE_FAULT, "page fault", regs, error_code, 14,
 380                                                 SIGSEGV) == NOTIFY_STOP)
 381                         return;
 382                 /*
 383                  * Don't take the mm semaphore here. If we fixup a prefetch
 384                  * fault we could otherwise deadlock.
 385                  */
 386                 goto bad_area_nosemaphore;
 387         }
 388
 389         if (notify_page_fault(DIE_PAGE_FAULT, "page fault", regs, error_code, 14,
 390                                         SIGSEGV) == NOTIFY_STOP)
 391                 return;
 392
 393         if (likely(regs->eflags & X86_EFLAGS_IF))
 394                 local_irq_enable();
 395
 396         if (unlikely(page_fault_trace))
 397                 printk("pagefault rip:%lx rsp:%lx cs:%lu ss:%lu address %lx error %lx\n",
 398                        regs->rip,regs->rsp,regs->cs,regs->ss,address,error_code);
 399
 400         if (unlikely(error_code & PF_RSVD))
 401                 pgtable_bad(address, regs, error_code);
 402
 403         /*
 404          * If we're in an interrupt or have no user
 405          * context, we must not take the fault..
 406          */
 407         if (unlikely(in_atomic() || !mm))
 408                 goto bad_area_nosemaphore;
 409
 410  again:
 411         /* When running in the kernel we expect faults to occur only to
 412          * addresses in user space.  All other faults represent errors in the
 413          * kernel and should generate an OOPS.  Unfortunatly, in the case of an
 414          * erroneous fault occurring in a code path which already holds mmap_sem
 415          * we will deadlock attempting to validate the fault against the
 416          * address space.  Luckily the kernel only validly references user
 417          * space from well defined areas of code, which are listed in the
 418          * exceptions table.
 419          *
 420          * As the vast majority of faults will be valid we will only perform
 421          * the source reference check when there is a possibilty of a deadlock.
 422          * Attempt to lock the address space, if we cannot we then validate the
 423          * source.  If this is invalid we can skip the address space check,
 424          * thus avoiding the deadlock.
 425          */
 426         if (!down_read_trylock(&mm->mmap_sem)) {
 427                 if ((error_code & PF_USER) == 0 &&
 428                     !search_exception_tables(regs->rip))
 429                         goto bad_area_nosemaphore;
 430                 down_read(&mm->mmap_sem);
 431         }
 432
 433         vma = find_vma(mm, address);
 434         if (!vma)
 435                 goto bad_area;
 436         if (likely(vma->vm_start <= address))
 437                 goto good_area;
 438         if (!(vma->vm_flags & VM_GROWSDOWN))
 439                 goto bad_area;
 440         if (error_code & 4) {
 441                 /* Allow userspace just enough access below the stack pointer
 442                  * to let the 'enter' instruction work.
 443                  */
 444                 if (address + 65536 + 32 * sizeof(unsigned long) < regs->rsp)
 445                         goto bad_area;
 446         }
 447         if (expand_stack(vma, address))
 448                 goto bad_area;
 449 /*
 450  * Ok, we have a good vm_area for this memory access, so
 451  * we can handle it..
 452  */
 453 good_area:
 454         info.si_code = SEGV_ACCERR;
 455         write = 0;
 456         switch (error_code & (PF_PROT|PF_WRITE)) {
 457                 default:        /* 3: write, present */
 458                         /* fall through */
 459                 case PF_WRITE:          /* write, not present */
 460                         if (!(vma->vm_flags & VM_WRITE))
 461                                 goto bad_area;
 462                         write++;
 463                         break;
 464                 case PF_PROT:           /* read, present */
 465                         goto bad_area;
 466                 case 0:                 /* read, not present */
 467                         if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)))
 468                                 goto bad_area;
 469         }
 470
 471         /*
 472          * If for any reason at all we couldn't handle the fault,
 473          * make sure we exit gracefully rather than endlessly redo
 474          * the fault.
 475          */
 476         switch (handle_mm_fault(mm, vma, address, write)) {
 477         case VM_FAULT_MINOR:
 478                 tsk->min_flt++;
 479                 break;
 480         case VM_FAULT_MAJOR:
 481                 tsk->maj_flt++;
 482                 break;
 483         case VM_FAULT_SIGBUS:
 484                 goto do_sigbus;
 485         default:
 486                 goto out_of_memory;
 487         }
 488
 489         up_read(&mm->mmap_sem);
 490         return;
 491
 492 /*
 493  * Something tried to access memory that isn't in our memory map..
 494  * Fix it, but check if it's kernel or user first..
 495  */
 496 bad_area:
 497         up_read(&mm->mmap_sem);
 498
 499 bad_area_nosemaphore:
 500         /* User mode accesses just cause a SIGSEGV */
 501         if (error_code & PF_USER) {
 502                 if (is_prefetch(regs, address, error_code))
 503                         return;
 504
 505                 /* Work around K8 erratum #100 K8 in compat mode
 506                    occasionally jumps to illegal addresses >4GB.  We
 507                    catch this here in the page fault handler because
 508                    these addresses are not reachable. Just detect this
 509                    case and return.  Any code segment in LDT is
 510                    compatibility mode. */
 511                 if ((regs->cs == __USER32_CS || (regs->cs & (1<<2))) &&
 512                     (address >> 32))
 513                         return;
 514
 515                 if (exception_trace && unhandled_signal(tsk, SIGSEGV)) {
 516                         printk(
 517                        "%s%s[%d]: segfault at %016lx rip %016lx rsp %016lx error %lx\n",
 518                                         tsk->pid > 1 ? KERN_INFO : KERN_EMERG,
 519                                         tsk->comm, tsk->pid, address, regs->rip,
 520                                         regs->rsp, error_code);
 521                 }
 522
 523                 tsk->thread.cr2 = address;
 524                 /* Kernel addresses are always protection faults */
 525                 tsk->thread.error_code = error_code | (address >= TASK_SIZE);
 526                 tsk->thread.trap_no = 14;
 527                 info.si_signo = SIGSEGV;
 528                 info.si_errno = 0;
 529                 /* info.si_code has been set above */
 530                 info.si_addr = (void __user *)address;
 531                 force_sig_info(SIGSEGV, &info, tsk);
 532                 return;
 533         }
 534
 535 no_context:
 536
 537         /* Are we prepared to handle this kernel fault?  */
 538         fixup = search_exception_tables(regs->rip);
 539         if (fixup) {
 540                 regs->rip = fixup->fixup;
 541                 return;
 542         }
 543
 544         /*
 545          * Hall of shame of CPU/BIOS bugs.
 546          */
 547
 548         if (is_prefetch(regs, address, error_code))
 549                 return;
 550
 551         if (is_errata93(regs, address))
 552                 return;
 553
 554 /*
 555  * Oops. The kernel tried to access some bad page. We'll have to
 556  * terminate things with extreme prejudice.
 557  */
 558
 559         flags = oops_begin();
 560
 561         if (address < PAGE_SIZE)
 562                 printk(KERN_ALERT "Unable to handle kernel NULL pointer dereference");
 563         else
 564                 printk(KERN_ALERT "Unable to handle kernel paging request");
 565         printk(" at %016lx RIP: \n" KERN_ALERT,address);
 566         printk_address(regs->rip);
 567         dump_pagetable(address);
 568         tsk->thread.cr2 = address;
 569         tsk->thread.trap_no = 14;
 570         tsk->thread.error_code = error_code;
 571         __die("Oops", regs, error_code);
 572         /* Executive summary in case the body of the oops scrolled away */
 573         printk(KERN_EMERG "CR2: %016lx\n", address);
 574         oops_end(flags);
 575         do_exit(SIGKILL);
 576
 577 /*
 578  * We ran out of memory, or some other thing happened to us that made
 579  * us unable to handle the page fault gracefully.
 580  */
 581 out_of_memory:
 582         up_read(&mm->mmap_sem);
 583         if (is_init(current)) {
 584                 yield();
 585                 goto again;
 586         }
 587         printk("VM: killing process %s\n", tsk->comm);
 588         if (error_code & 4)
 589                 do_exit(SIGKILL);
 590         goto no_context;
 591
 592 do_sigbus:
 593         up_read(&mm->mmap_sem);
 594
 595         /* Kernel mode? Handle exceptions or die */
 596         if (!(error_code & PF_USER))
 597                 goto no_context;
 598
 599         tsk->thread.cr2 = address;
 600         tsk->thread.error_code = error_code;
 601         tsk->thread.trap_no = 14;
 602         info.si_signo = SIGBUS;
 603         info.si_errno = 0;
 604         info.si_code = BUS_ADRERR;
 605         info.si_addr = (void __user *)address;
 606         force_sig_info(SIGBUS, &info, tsk);
 607         return;
 608 }
 609
 610 DEFINE_SPINLOCK(pgd_lock);
 611 struct page *pgd_list;
 612
 613 void vmalloc_sync_all(void)
 614 {
 615         /* Note that races in the updates of insync and start aren't
 616            problematic:
 617            insync can only get set bits added, and updates to start are only
 618            improving performance (without affecting correctness if undone). */
 619         static DECLARE_BITMAP(insync, PTRS_PER_PGD);
 620         static unsigned long start = VMALLOC_START & PGDIR_MASK;
 621         unsigned long address;
 622
 623         for (address = start; address <= VMALLOC_END; address += PGDIR_SIZE) {
 624                 if (!test_bit(pgd_index(address), insync)) {
 625                         const pgd_t *pgd_ref = pgd_offset_k(address);
 626                         struct page *page;
 627
 628                         if (pgd_none(*pgd_ref))
 629                                 continue;
 630                         spin_lock(&pgd_lock);
 631                         for (page = pgd_list; page;
 632                              page = (struct page *)page->index) {
 633                                 pgd_t *pgd;
 634                                 pgd = (pgd_t *)page_address(page) + pgd_index(address);
 635                                 if (pgd_none(*pgd))
 636                                         set_pgd(pgd, *pgd_ref);
 637                                 else
 638                                         BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
 639                         }
 640                         spin_unlock(&pgd_lock);
 641                         set_bit(pgd_index(address), insync);
 642                 }
 643                 if (address == start)
 644                         start = address + PGDIR_SIZE;
 645         }
 646         /* Check that there is no need to do the same for the modules area. */
 647         BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
 648         BUILD_BUG_ON(!(((MODULES_END - 1) & PGDIR_MASK) ==
 649                                 (__START_KERNEL & PGDIR_MASK)));
 650 }
 651
 652 static int __init enable_pagefaulttrace(char *str)
 653 {
 654         page_fault_trace = 1;
 655         return 1;
 656 }
 657 __setup("pagefaulttrace", enable_pagefaulttrace);