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Subject: vm_ops.page_mkwrite() fails with vmalloc on 2.6.23
From: Stefani Seibold <stefani@seibold.net>
Reply-To: stefani@seibold.net
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Date: Mon, 22 Oct 2007 16:45:05 +0200
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Hi,

i have a problem with vmalloc() and vm_ops.page_mkwrite().

ReadOnly access works, but on a write access the VM will
endless invoke the vm_ops.page_mkwrite() handler.

I tracked down the problem to the
	struct page.mapping pointer,
which is NULL.

The problem original occurs with the fb_defio driver (driver/video/fb_defio.c).
This driver use the vm_ops.page_mkwrite() handler for tracking the modified pages,
which will be in an extra thread handled, to perform the IO and clean and
write protect all pages with page_clean().

I am not sure if the is a feature of the new rmap code or a bug.

Is there an way to get a similar functionality? Currently, i have no
idea
how to get the ptep from a page alloced with vmalloc().

Greetings,
Stefani

Here is a small sample driver:

#include <linux/module.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/fs.h>

#define	DEVICE_NAME	"mydrv"
#define	DEVICE_MAJOR	240

static u8 *mydrv_memory;
static const u_long mydrv_memory_size=1024*1024; // 1 Megabyte

#ifdef MODULE
static u_long vmas=0;
#endif

static void mydrv_vma_open(struct vm_area_struct *vma)
{
#ifdef MODULE
	if (vmas++==0)
		try_module_get(THIS_MODULE);
#endif
}

static void mydrv_vma_close(struct vm_area_struct *vma)
{
#ifdef MODULE
	if (--vmas==0)
		module_put(THIS_MODULE);
#endif
}

struct page *mydrv_vma_nopage(struct vm_area_struct *vma,unsigned long
address,int *type)
{
	unsigned long offset;
	struct page *page;

	offset = (address - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);
	printk("--> mydrv_vma_nopage:%lu(%08lx)\n",offset,offset);

	if (offset >= mydrv_memory_size)
		return NOPAGE_SIGBUS;

	page = vmalloc_to_page(mydrv_memory + offset);
	if (!page)
		return NOPAGE_OOM;

	get_page(page);
	if (type)
		*type = VM_FAULT_MINOR;
	return page;
}

int mydrv_mkwrite(struct vm_area_struct *vma,struct page *page)
{
	printk("--> mydrv_mkwrite:%p\n",page->mapping);

	return 0;
}

struct vm_operations_struct mydrv_vm_ops = {
	.open =		mydrv_vma_open,
	.close =	mydrv_vma_close,
	.nopage =	mydrv_vma_nopage,
	.page_mkwrite =	mydrv_mkwrite,
};

static int mydrv_mmap(struct file *file, struct vm_area_struct *vma)
{
	vma->vm_ops = &mydrv_vm_ops;
	vma->vm_flags |= ( VM_IO | VM_RESERVED | VM_DONTEXPAND );
	vma->vm_private_data = 0;
	mydrv_vma_open(vma);
	return 0;
}

int mydrv_open(struct inode *inode, struct file *file)
{
	int		minor;

	minor=MINOR(file->f_dentry->d_inode->i_rdev);
	printk("--> mydrv_open called for minor: %d\n", minor);

	if (minor>0) {
		printk("--> mydrv_open minor %d failed\n",minor);
		return -ENODEV;
	}

	printk("--> mydrv_open o.K.\n");
	return 0;
}


int mydrv_close(struct inode *inode, struct file *file)
{
	int		minor;

	minor=MINOR(file->f_dentry->d_inode->i_rdev);
	printk("--> mydrv_close for minor: %d\n", minor);
	return 0;
}

struct file_operations mydrv_fops = {
	.owner=THIS_MODULE,
	.mmap=mydrv_mmap,
	.open=mydrv_open,
	.release=mydrv_close,
};

int init_module(void)
{
	printk("--> init_module called\n");

	if (!(mydrv_memory = vmalloc(mydrv_memory_size)))
		return -ENOMEM;

	if (register_chrdev(DEVICE_MAJOR, DEVICE_NAME, &mydrv_fops) < 0) {
		printk("--> Error registering driver.\n");
		return -ENODEV;
	}

	memset(mydrv_memory, 0, mydrv_memory_size);
	printk("--> init_module done\n");
	return 0;
}


void cleanup_module(void)
{
	printk("--> cleanup_module called\n");
	unregister_chrdev(DEVICE_MAJOR,DEVICE_NAME);
	vfree(mydrv_memory);
	printk("--> cleanup_module done\n");
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Stefan Seibold <Stefani@Seibold.net>");
MODULE_DESCRIPTION("Test Driver");


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