Chapter 4: Virtual Process Memory
The heap is a contiguous memory area that grows from bottom to top when expanded. Themm_struct
structure already mentioned includes the start and the current end address (start_brkandbrk)ofthe
heap in virtual address space.
<mm_types.h>
struct mm_struct {
...
unsigned long start_brk, brk, start_stack;
...
};
Thebrksystem call expects just a single parameter to specify the new end address of the heap in virtual
address space (it can, of course, be smaller than the previous value if the heap is to be shrunk).
As usual, the entry point for the implementation of thebrksystem call is thesys_brkfunction, whose
code flow diagram is shown in Figure 4-16.
Check resource limits
sys_brk
find_vma_intersection
do_brk
do_munmap
Align brk value per page
Increase of brk value?
Return new brk value
Return new brk value
No
Yes
Figure 4-16: Code flow diagram forsys_brk.
Thebrkmechanism is not another independent kernel concept but is implemented on the basis of anony-
mous mappings to reduce internal overhead. Many of the functions to manage memory mappings
discussed in the preceding sections can therefore be reused to implementsys_brk.
After it has been checked that the new desired address forbrkis actually within the heap limits, the first
important action ofsys_brkis to align the request to page size.
mm/mmap.c
asmlinkage unsigned long sys_brk(unsigned long brk)
{
unsigned long rlim, retval;
unsigned long newbrk, oldbrk;
struct mm_struct *mm = current->mm;
