kernel vaddr -> struct page

kernel vaddr -> struct page

[Eric Barton]

Hi,

I'm trying to turn a kernel virtual address into a struct page that I can
then pass to tcp_sendpage().  

The kernel virtual address could be {kmalloc(),vmalloc(),kmap()}-ed memory,
and I guarantee that this memory will not be {kfree(),vfree(),kunmap()}-ed
until the socket has done with the page (i.e. all the data has been acked).

I'd have thought that vmalloc_to_page(kvaddr) should give me a page I could
use, since it is walking the page tables to find the pte for 'kvaddr', and
checking that the physical page is present.

However I find I'm sending garbage when I use this method.

Can anyone help me understand?

-- 

                Cheers,
                        Eric

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|Eric Barton        Barton Software                |
|9 York Gardens     Tel:    +44 (117) 330 1575     |
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----------------------------------------------------

Re: kernel vaddr -> struct page

[Brian Gerst]

Eric Barton wrote:
> Hi,
> 
> I'm trying to turn a kernel virtual address into a struct page that I can
> then pass to tcp_sendpage().  
> 
> The kernel virtual address could be {kmalloc(),vmalloc(),kmap()}-ed memory,
> and I guarantee that this memory will not be {kfree(),vfree(),kunmap()}-ed
> until the socket has done with the page (i.e. all the data has been acked).
> 
> I'd have thought that vmalloc_to_page(kvaddr) should give me a page I could
> use, since it is walking the page tables to find the pte for 'kvaddr', and
> checking that the physical page is present.
> 
> However I find I'm sending garbage when I use this method.
> 
> Can anyone help me understand?
> 

virt_to_page()

--
				Brian Gerst

Re: kernel vaddr -> struct page

[David S. Miller]

vmalloc_to_page() does not work on kmalloc/kmap'd memory.

On many system kmalloc memory is not even represented in the
page tables because the mapping goes through a direct bypass
window.

You need to know where the object came from to convert it
into a page properly.  You should be allocating and passing
around pages internally anyways.

Re: kernel vaddr -> struct page

[David S. Miller]

   From: Brian Gerst <bgerst@didntduck.org>
   Date: Thu, 17 Oct 2002 15:24:49 -0400

   virt_to_page()
   
Doesn't work on vmalloc() data, he said that was a possibility for his
case.