linux-kernel - Re: virt_to_page for userspace pointers

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Message-ID: <loom.20110726T134944-182@post.gmane.org>
Date:	Tue, 26 Jul 2011 12:03:00 +0000 (UTC)
From:	Alexey Skidanov <alexey-s@...otech.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: virt_to_page for userspace pointers

Nikolaus Rath <Nikolaus <at> rath.org> writes:

> 
> Nikolaus Rath <Nikolaus <at> rath.org> writes:
> > On 02/05/2011 04:36 PM, Brice Goglin wrote:
> >>> Is there an equivalent function to virt_to_page (and virt_to_phys) that
> >>> works with userspace pointers? Or do I have to manually walk through the
> >>> page tables? In the later case, are there any examples of this kind of
> >>> search that I could use as a basis?
> >>>   
> >> 
> >> You probably want get_user_pages() (or get_user_pages_fast()).
> >
> > Thanks for the pointer, that already helped a lot!
> >
> > I would, however, prefer a more primitive function that just gives me
> > the struct page without locking it. And if the page is swapped out, I'd
> > rather get a NULL pointer than having a page frame allocated.  Is there
> > anything like that?
> 
> After studying the source for get_user_pages(), I believe the following
> should do what I want:
> 
>     vma = find_extend_vma(current->mm, (unsigned long) data.vi_buf);
>     if(vma == NULL) {
>         err("find_extend_vma failed for VI.");
>     }
>     acq_dev->vi_pg = follow_page(vma, (unsigned long) data.vi_buf, 0);
>     if(acq_dev->vi_pg == NULL) {
>         err("follow_page failed for VI.");
>     }
> 
> Unfortunately, I'm getting into trouble at link time when using these
> functions:
> 
>   MODPOST 1 modules
> WARNING: "follow_page" 
[/home/nikratio/control/acq196_hostdrv/acq200_hostdrv.ko] undefined!
> WARNING: "find_extend_vma" 
[/home/nikratio/control/acq196_hostdrv/acq200_hostdrv.ko] undefined!
> 
> Can they only be used in the kernel itself, but not in kernel modules?
> 
> Thanks,
> 
>    -Nikolaus
> 


Hi,
I had the same problem working with NVIDIA GPU driver. The NVIDIA driver mmap 
allocated memory using vm_insert_page() - allowing inserting single page into 
the vma. So you need to translate go over the whole vma and translate it page 
by page. I just implemented the function to translate the virtual address to 
pfn. It mostly the same as follow_page:

/**
 * This function walks through page tables and get the PFN of virtual address
 */
long get_pfn_of_virtual_address(struct vm_area_struct *vma, unsigned long 
address, unsigned long& pfn) {

    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;
    pte_t *ptep;
    spinlock_t *ptl;
    struct mm_struct *mm = vma->vm_mm;

    pgd = pgd_offset(mm, address);
    if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
        return -EFAULT;

    pud = pud_offset(pgd, address);
    if (pud_none(*pud) || unlikely(pud_bad(*pud)))
        return -EFAULT;

    pmd = pmd_offset(pud, address);
    if (pmd_none(*pmd))
        return -EFAULT;

    ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
    pfn =  pte_pfn(*ptep);
    pte_unmap_unlock(ptep, ptl);

    return 0;

} 

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