Linux Kernel Architecture

Chapter 4: Virtual Process Memory

struct vm_area_struct * vma;

unsigned long address;

unsigned long page;

int write, si_code;

int fault;

...

/* get the address */

address = read_cr2();

...

Table 4-1: Meaning of Page Fault Error Codes on IA-32

Bit Set (1) Not set (0)

0 No page present in RAM Protection fault (insufficient access permission)

1 Read access Write access

2 Privileged kernel mode User mode

Once a large number of variables have been declared for subsequent use, the kernel stores the address of

the location that triggered the fault inaddress.^18

arch/i386/mm/fault.c

tsk = current;

si_code = SEGV_MAPERR;

/*

* We fault-in kernel-space virtual memory on-demand. The

* ’reference’ page table is init_mm.pgd.

*

* NOTE! We MUST NOT take any locks for this case. We may

* be in an interrupt or a critical region, and should

* only copy the information from the master page table,

* nothing more.

*

* This verifies that the fault happens in kernel space

* (error_code & 4) == 0, and that the fault was not a

* protection error (error_code & 9) == 0.

*/

if (unlikely(address >= TASK_SIZE)) {

if (!(error_code & 0x0000000d) && vmalloc_fault(address) >= 0)

return;

/*

* Don’t take the mm semaphore here. If we fixup a prefetch

* fault we could otherwise deadlock.

(^18) On IA-32 processors, the address is held in registerCR2, whose contents are copied toaddressbyread_cr2. The processor-
specific details are of no interest.