linux-kernel - RE: Why get_user

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Message-ID: <6B24EEDBA38D764293B27C04FE414CC64E4091DF15@G1W0491.americas.hpqcorp.net>
Date:	Mon, 30 Jun 2008 20:45:47 +0000
From:	"Pan, Weimin" <weimin.pan@...com>
To:	Jonathan Corbet <corbet@....net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Why  get_user_pages fails?

Yes, I totally agree with you that the best solution is to bypass the Linux VM.
The reason to continue using get_user_pages() is that we can leverage the existing kernel driver - the SCST.

Do I have a way to map a range of phy memory to user virtual address and have the page structures built for that memory region?
I know remap_pfn_range() doesn't work.

Weimin

-----Original Message-----
From: Jonathan Corbet [mailto:corbet@....net]
Sent: Monday, June 30, 2008 2:52 PM
To: Pan, Weimin
Cc: 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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