Why don't we make mmap MAP_SHARED with /dev/zero possible?

[fxzhang <fxzhang@chpc.ict.ac.cn>]

and find this:
 /usr/src/linux/drivers/char/mem.c  
static int mmap_zero(struct file * file, struct vm_area_struct * vma)
{
        if (vma->vm_flags & VM_SHARED)
                return -EINVAL;

I don't understand why people don't implement it.Yes,in the source,I find something like
"the shared case is complex",Could someone tell me what's the difficulty?As it is a 
driver,I think it should not be too much to concern.At least I know in Solaris this works.
   I want to implement it but know I am not competent now,I am just beginning digging it:).
   
   Is there any good way to share memory between process at page granularity?That is,I can
share individual pages between them? Threads maybe a subtitue,but there are many things
that I don't want to share.

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