Re: Kernel writes to RAM it doesn't own on 2.4.24

From: Ross Biro
Date: Fri Apr 16 2004 - 12:09:19 EST

On Fri, 16 Apr 2004 12:55:33 -0400 (EDT), Richard B. Johnson
<root@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 16 Apr 2004, Ross Biro wrote:
> > mem= isn't there to tell the kernel what ram it owns and what ram it
> > doesn't own. It's there to tell the kernel what ram is in the system.
> > Since you told the system it only has 500m, it assumes the rest of
> > the 3.5G of address space is available for things like memory mapped
> > i/o. If you cat /proc/iomem, you'll probably see something has
> > reserved the memory range in question.
> >
> No! This is address space, not RAM. Whether or not a PCI device
> or whatever has internal RAM that's mapped makes no difference.
> I told the kernel that it has 500m of RAM. It better not assume
> I don't know what I'm talking about. I might have reserved that
> RAM because it's bad or I may have something else important to
> do with that RAM (which I do).

The problem is that the kernel does assume you know what you are
talking about, and you don't. You are abusing the mem= parameter.
That's fine, but then you have to tell the kernel what you really
mean. What you really want to say is there is memory above 500M and I
don't want you to touch it. There may be a way to do that via the
fancy mem=@ parameters.

What mem= tells the kernel is that there is RAM in a certain spot an
no where else. Since you told the kernel there is no ram about 500M,
that means that address space is free to be used for memory mapped
I/O. Since the kernel trusts you, it started using the memory above
500m for memory mapped i/o. Since you LIED to the kernel, you are
getting results you do not like. The solution I settled on was to
tell the kernel that people LIE to it and only use memory for I/O if
both the BIOS and the USER agree that it's available. You have to
find a way to tell the kernel the TRUTH, or you will never get the
results you want.
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