fb: defio nopage

works:

- userspace app like Xfbdev mmaps framebuffer

- deferred IO and driver sets up nopage and page_mkwrite handlers

- deferred IO and driver sets up fault and page_mkwrite handlers

- userspace app tries to write to mmaped vaddress

- we get pagefault and reach nopage handler

- nopage handler finds and returns physical page

- we get pagefault and reach fault handler

- fault handler finds and returns physical page

- we get page_mkwrite where we add this page to a list

- schedule a workqueue task to be run after a delay

- app continues writing to that page with no additional cost. this is

#include <linux/pagemap.h>

/* this is to find and return the vmalloc-ed fb pages */

static struct page* fb_deferred_io_nopage(struct vm_area_struct *vma,

					unsigned long vaddr, int *type)

static int fb_deferred_io_fault(struct vm_area_struct *vma,

				struct vm_fault *vmf)

{

	unsigned long offset;

	struct page *page;

	/* info->screen_base is in System RAM */

	void *screen_base = (void __force *) info->screen_base;

	offset = (vaddr - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);

	offset = vmf->pgoff << PAGE_SHIFT;

	if (offset >= info->fix.smem_len)

		return NOPAGE_SIGBUS;

		return VM_FAULT_SIGBUS;

	page = vmalloc_to_page(screen_base + offset);

	if (!page)

		return NOPAGE_OOM;

		return VM_FAULT_SIGBUS;

	get_page(page);

	if (type)

		*type = VM_FAULT_MINOR;

	return page;

	vmf->page = page;

	return 0;

}

int fb_deferred_io_fsync(struct file *file, struct dentry *dentry, int datasync)

}

static struct vm_operations_struct fb_deferred_io_vm_ops = {

	.nopage   	= fb_deferred_io_nopage,

	.fault		= fb_deferred_io_fault,

	.page_mkwrite	= fb_deferred_io_mkwrite,

};