On Mon, 16 Aug 2010 16:40:20 +0530
Prabhu nath <gprabhunath@xxxxxxxxx> wrote:
> On Mon, Aug 16, 2010 at 1:30 PM, Prabhu nath <gprabhunath@xxxxxxxxx> wrote:
>
> > Dear All,
> >
> > In linux kernel, for all memory allocation done by *vmalloc*,
> > kernel maintains memory region descriptor *(vm_struct)* which stores
> > information about the linear virtual address range, no. of physical page
> > frames allocated... as a linked list headed by *vmlist *symbol.
> > Can you please give me information about how does kernel maintain
> > information about memory allocation done by malloc/calloc invoked by a user
> > application ?
> >
> > I understand that kernel maintains a descriptor (*vm_area_struct*) per
> > application to hold information about the virtual address space allocated
> > for the heap region.
> >
>
> I would like to elicit my question. Here is a simple code sample
>
> int *b;
> int main()
> {
> b = malloc(4);
> printf ("Address of b = 0x%08x \n", b);
> getchar();
> return 0;
> }
>
> If I pause the program and inspect the maps file </proc/<pid>/maps>, I get
> to see a whooping 132K of linear virtual address allocated and Address of b
> = 0x0804a008
> Here is a fragment of the maps file.
>
> 08048000-08049000 r-xp 00000000 fd:00 16845753 cSamples/cTests/a.out
> 08049000-0804a000 rw-p 00000000 fd:00 16845753 cSamples/cTests/a.out
> *0804a000-0806b000 rw-p 0804a000 00:00 0 [heap]*
> b7f3d000-b7f3e000 rw-p b7f3d000 00:00 0
> b7f4a000-b7f4d000 rw-p b7f4a000 00:00 0
>
> Suppose if have to extend my code as
> c = malloc (34);
> d = malloc(12);
> e = malloc(1024);
> free(e);
> f = malloc(1845);
>
> Assume (c, d, e, f are all declared globally)
>
> Now, when free(e) is executed, kernel should have some information about the
> size of address space allocated to symbol 'e' so that it will only free that
> virtual address region in the heap space that was allocated to e.
>
> Q. Where and how does the kernel maintain information about the size of
> address space allocated to symbol 'e'
Memory management at malloc/free level is done in userspace by the libc.
The libc ask the kernel for extension of the virtual space of the
current process when the available memory is exhausted.
The kernel only allocate pages to processes. In those pages the libc
uses its own internal data structures to manage malloc allocation.
--
Guillaume Knispel
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