Copy-on-write with vmalloc

Copy-on-write with vmalloc

[Eric Wheeler]

Hello all,

I would like to clone a virtual address space so that the address spaces 
share physical pages until a write happens, at which point it would copy 
to a new physical page.  I've looked around and haven't found any 
documentation. Certainly fork() already does this, but is there already 
simple way to do it with a virtual address space?

That is, does anything already implement the hypothetical vmalloc_clone 
in this example (4k pages):

	v = vmalloc(1024*1024);

	v[0] = 1;
	v[4096] = 2;

	v_copy = vmalloc_clone(v);

	v_copy[0] = 3;     /* copy-on-write */

And then the following expressions would still be true:

	/* different pages */
	v[0] == 1;
	v_copy[0] == 3; 

	/* shared page */
	v[4096] == 2;
	v_copy[4096] == 2; 

Thank you for your help!

--
Eric Wheeler

Re: Copy-on-write with vmalloc

[Christopher Lameter]

On Mon, 7 May 2018, Eric Wheeler wrote:

> I would like to clone a virtual address space so that the address spaces
> share physical pages until a write happens, at which point it would copy
> to a new physical page.  I've looked around and haven't found any
> documentation. Certainly fork() already does this, but is there already
> simple way to do it with a virtual address space?

The clone() syscall does it (since it is the underlying basis for fork).

The same effect can also be had by using mmap with
MAP_PRIVATE on a shared memory segment.