linux-kernel - Re: virt_to_page for userspace pointers

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Message-ID: <87tyg6utdl.fsf@inspiron.ap.columbia.edu>
Date:	Mon, 14 Feb 2011 17:46:30 -0500
From:	Nikolaus Rath <Nikolaus@...h.org>
To:	linux-kernel@...r.kernel.org
Subject: Re: virt_to_page for userspace pointers

Nikolaus Rath <Nikolaus@...h.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

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