The simplest way to map a set of pages from the kernel in your mmap method is to use the fault handler to map the pages. Basically you end up with something like:
static int my_mmap(struct file *filp, struct vm_area_struct *vma)
{
vma->vm_ops = &my_vm_ops;
return 0;
}
static const struct file_operations my_fops = {
.owner = THIS_MODULE,
.open = nonseekable_open,
.mmap = my_mmap,
.llseek = no_llseek,
};
(where the other file operations are whatever your module needs). Also in my_mmap
you do whatever range checking etc. is needed to validate the mmap parameters.
Then the vm_ops
look like:
static int my_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
vmf->page = my_page_at_index(vmf->pgoff);
get_page(vmf->page);
return 0;
}
static const struct vm_operations_struct my_vm_ops = {
.fault = my_fault
}
where you just need to figure out for a given vma / vmf passed to your fault function which page to map into userspace. This depends on exactly how your module works. For example, if you did
my_buf = vmalloc_user(MY_BUF_SIZE);
then the page you use would be something like
vmalloc_to_page(my_buf + (vmf->pgoff << PAGE_SHIFT));
But you could easily create an array and allocate a page for each entry, use kmalloc, whatever.
[just noticed that my_fault
is a slightly amusing name for a function]