linux-kernel - Re: Why get_user

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Message-ID: <20080630135139.2b53c93d@bike.lwn.net>
Date:	Mon, 30 Jun 2008 13:51:39 -0600
From:	Jonathan Corbet <corbet@....net>
To:	"Pan, Weimin" <weimin.pan@...com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Why  get_user_pages fails?

On Mon, 30 Jun 2008 19:32:41 +0000
"Pan, Weimin" <weimin.pan@...com> wrote:

> I booted the kernel and set "mem=512M" and then mmapped the physical
> memory above 512M to user space and got the user space virtual
> address for that memory. When I passed that address to my driver to
> do direct IO. The get_user_pages() in my driver fails. I found that
> this is due to the VM_IO and VM_PFNMAP are set. Does that mean the
> page structures are not built when I mmapped physical memory to user
> space (using remap_pfn_range())?
> 
> Do I have a way to solve this problem? How can I map the physical
> memory to user space virtual address and can pass this address to my
> driver to do direct IO by using get_user_pages() function?

In this situation, you already know where the memory sits in physical
space, and there's no point in trying to pin it.  So a call to
get_user_pages() is entirely unnecessary.  You're bypassing the Linux
VM, so bypass it entirely.

However, this whole scheme seems a little...baroque.  Why are you doing
things this way?  As always in such cases, posting your code can enable
others to help you find the right solution to your problem.

jon
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