HugeTLB: Driver example

[Alexey Korolev <akorolex@gmail.com>]

There is an example of simple driver which provides huge pages mapping
for user level applications. The  procedure for mapping of huge pages
to userspace by the driver is:

1. Create a hugetlb file on vfs mount of hugetlbfs (h_file)

2. File operations of /dev/hpage_map do the following:
In file open we  associate mappings of /dev/xxx with the file on
hugetlbfs (like it is done in ipc/shm.c)
       file->f_mapping = h_file->f_mapping;
In get_unmapped_area we should tell about addressing constraints in
case of huge pages by calling hugetlbfs procedures. (as in ipc/shm.c)
       return get_unmapped_area(h_file, addr, len, pgoff, flags);

3 In mmap get huge page in order to DMA or for something else
(hugetlb_get_user_page call).
..................
4 Remove hugetlbfs file

---
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/file.h>
#include <linux/pagemap.h>
#include <linux/hugetlb.h>
#include <linux/pagevec.h>
#include <linux/miscdevice.h>
#include <asm/io.h>
#include <asm/ioctl.h>

#define HFILE_SIZE 16UL*1024*1024
static struct file	*h_file;

static int hpage_map_mmap(struct file *file, struct vm_area_struct *vma)
{
	int ret;
	struct page *page;
	struct hstate *h;
	unsigned long addr = vma->vm_start;

	if ((ret = h_file->f_op->mmap(h_file, vma)))
	    return ret;

	h = hstate_file(h_file);
	
	while (addr < vma->vm_end) {
		page = hugetlb_get_user_page(vma, addr);
		if (IS_ERR(page))
		        return -EFAULT;
		addr += huge_page_size(h);
		/* Add code to configure DMA here */
	}
	return 0;
}

static unsigned long hpage_map_get_unmapped_area(struct file *file,
	unsigned long addr, unsigned long len, unsigned long pgoff,
	unsigned long flags)
{
	/* Tell about addressing constrains in case of huge pages,
	 * hugetlbfs knows how to do this */
	return get_unmapped_area(h_file, addr, len, pgoff, flags);
}
static int hpage_map_open(struct inode * inode, struct file * file)
{
	/* Associate mappings of /dev/xxx with the file on hugetlbfs
	 * like it is done in ipc/shm.c */
	file->f_mapping = h_file->f_mapping;
	return 0;
}

/*
 * The file operations for /dev/hpage_map
 */
static const struct file_operations hpage_map_fops = {
	.owner		= THIS_MODULE,
	.mmap		= hpage_map_mmap,
	.open 		= hpage_map_open,
	.get_unmapped_area	= hpage_map_get_unmapped_area,
};

static struct miscdevice hpage_map_dev = {
	MISC_DYNAMIC_MINOR,
	"hpage_map",
	&hpage_map_fops
};

static int __init
hpage_map_init(void)
{
	int ret;
	struct user_struct *user = NULL;

	/* Create the device in the /sys/class/misc directory. */
	if ((ret = misc_register(&hpage_map_dev)))
		return ret;

	/* Create file on hugetlbfs */
	h_file = hugetlb_file_setup("hpage_map_dev", HFILE_SIZE, 0, &user,
				HUGETLB_DEVBACK_INODE);
	if (IS_ERR(h_file)) {
		misc_deregister(&hpage_map_dev);
		ret = -ENOENT;
	}
	return ret;
}

module_init(hpage_map_init);

static void __exit
hpage_map_exit(void)
{
	fput(h_file);
	misc_deregister(&hpage_map_dev);
}

module_exit(hpage_map_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Alexey Korolev");
MODULE_DESCRIPTION("Example of driver with hugetlb mapping");
MODULE_VERSION("1.0");

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