get_user_pages() still broken in 2.6

get_user_pages() still broken in 2.6

[Timur Tabi]

I was hoping that this bug would be fixed in the 2.6 kernels, but 
apparently it hasn't been.

Function get_user_pages() is supposed to lock user memory.  However, 
under extreme memory constraints, the kernel will swap out the "locked" 
memory.

I have a test app which does this:

1) Calls our driver, which issues a get_user_pages() call for one page.
2) Calls our driver again to get the physical address of that page (the 
driver uses pgd/pmd/pte_offset).
3) Tries allocate 1GB of memory (this system has 1GB of physical RAM).
4) Tries to get the physical address again.

In step 4, the physical address is usually zero, which means either 
pgd_offset or pmd_offset failed.  This indicates the page was swapped out.

I don't understand how this bug can continue to exist after all this 
time.  get_user_pages() is supposed to lock the memory, because drivers 
use it for DMA'ing directly into user memory.

-- 
Timur Tabi
Staff Software Engineer
timur.tabi@ammasso.com
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Re: get_user_pages() still broken in 2.6

[Christoph Hellwig]

On Tue, Sep 28, 2004 at 05:40:26PM -0500, Timur Tabi wrote:
> I was hoping that this bug would be fixed in the 2.6 kernels, but 
> apparently it hasn't been.
> 
> Function get_user_pages() is supposed to lock user memory.  However, 
> under extreme memory constraints, the kernel will swap out the "locked" 
> memory.
> 
> I have a test app which does this:
> 
> 1) Calls our driver, which issues a get_user_pages() call for one page.
> 2) Calls our driver again to get the physical address of that page (the 
> driver uses pgd/pmd/pte_offset).
> 3) Tries allocate 1GB of memory (this system has 1GB of physical RAM).
> 4) Tries to get the physical address again.
> 
> In step 4, the physical address is usually zero, which means either 
> pgd_offset or pmd_offset failed.  This indicates the page was swapped out.
> 
> I don't understand how this bug can continue to exist after all this 
> time.  get_user_pages() is supposed to lock the memory, because drivers 
> use it for DMA'ing directly into user memory.

get_user_pages locks the page in memory.  It doesn't do anything about ptes.

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Re: get_user_pages() still broken in 2.6

[Dave Hansen]

On Tue, 2004-09-28 at 16:03, Christoph Hellwig wrote:
> get_user_pages locks the page in memory.  It doesn't do anything about ptes.

You probably want mlock(2) to keep the kernel from messing with the ptes
at all.  But, you should probably really be thinking about why you're
accessing the page tables at all.  I count *ONE* instance in drivers/
where page tables are accessed directly.

-- Dave

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Re: get_user_pages() still broken in 2.6

[Timur Tabi]

Dave Hansen wrote:

> You probably want mlock(2) to keep the kernel from messing with the ptes
> at all.

mlock() can only be called via sys_mlock(), which is a user-space call. 
  Not only that, but only root can call sys_mlock().  This is not 
compatible with our needs.

 >  But, you should probably really be thinking about why you're
> accessing the page tables at all.  I count *ONE* instance in drivers/
> where page tables are accessed directly.

I access PTEs to get the physical addresses of a user-space buffer, so 
that we can DMA to/from it directly.

-- 
Timur Tabi
Staff Software Engineer
timur.tabi@ammasso.com
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Re: get_user_pages() still broken in 2.6

[Timur Tabi]

Christoph Hellwig wrote:

> get_user_pages locks the page in memory.  It doesn't do anything about ptes.

I don't understand the difference.  I thought a locked page is one that 
stays in memory (i.e. isn't swapped out) and whose physical address 
never changes.  Is that wrong?  All I need to do is keep a page in 
memory at the same physical address until I'm done with it.

-- 
Timur Tabi
Staff Software Engineer
timur.tabi@ammasso.com
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Re: get_user_pages() still broken in 2.6

[Christoph Hellwig]

On Wed, Sep 29, 2004 at 09:48:09AM -0500, Timur Tabi wrote:
> Christoph Hellwig wrote:
> 
> > get_user_pages locks the page in memory.  It doesn't do anything about ptes.
> 
> I don't understand the difference.  I thought a locked page is one that 
> stays in memory (i.e. isn't swapped out) and whose physical address 
> never changes.  Is that wrong?

Yes.  But if you're walking ptes you're looking at virtual addresses
somehow.  Can you send me a pointer to your code please?  I suspect
it's doing something terribly stupid.

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opening a file inside the kernel module

[Rakesh Jagota]

Hi all,
I am working in linux, i would like to know abt whether can I open a file
inside the kernel module without using any application. If so how how the
files_struct will be maintained. Does a kernel module has this struct?

Waiting for any suggestion from the list.

Thanks in advance,
rakesh

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Re: opening a file inside the kernel module

[Jeff Garzik]

Rakesh Jagota wrote:
> Hi all,
> I am working in linux, i would like to know abt whether can I open a file
> inside the kernel module without using any application. If so how how the
> files_struct will be maintained. Does a kernel module has this struct?

Don't do this.  It's incompatible with namespaces.

Instead, figure out some way to pass the file contents to the kernel module.

	Jeff


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Re: opening a file inside the kernel module

[Rakesh Jagota]

Hi,
Thnx.

I want to implement socket from the module. I won't be having any user
process running to handle the descriptors coming from socket. Could you pl
tell me how to handle the socket descriptor from the kernel module.

Thanks,
rakesh
----- Original Message -----
From: "Jeff Garzik" <jgarzik@pobox.com>
To: "Rakesh Jagota" <j.rakesh@gdatech.co.in>
Cc: <linux-mm@kvack.org>; <linux-kernel@vger.kernel.org>;
<kernelnewbies@nl.linux.org>
Sent: Wednesday, September 29, 2004 10:30 AM
Subject: Re: opening a file inside the kernel module


> Rakesh Jagota wrote:
> > Hi all,
> > I am working in linux, i would like to know abt whether can I open a
file
> > inside the kernel module without using any application. If so how how
the
> > files_struct will be maintained. Does a kernel module has this struct?
>
> Don't do this.  It's incompatible with namespaces.
>
> Instead, figure out some way to pass the file contents to the kernel
module.
>
> Jeff
>
>
>
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
> Archive:       http://mail.nl.linux.org/kernelnewbies/
> FAQ:           http://kernelnewbies.org/faq/
>
>

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RE: opening a file inside the kernel module

[Stuart MacDonald, linux-kernel-owner]

> I want to implement socket from the module. I won't be having any user
> process running to handle the descriptors coming from socket. 
> Could you pl
> tell me how to handle the socket descriptor from the kernel module.

Check out fs/smbfs/sock.c.

..Stu

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