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

From: Ross Dickson
Date: Fri Apr 16 2004 - 23:41:47 EST

On Fri, 16 Apr 2004, Ross Biro wrote:

> On Fri, 16 Apr 2004, Richard B. Johnson 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.
> -

This is all most enlightening. If I am understanding correctly then every
device driver that the author specifies to use a "mem=" command to
reserve some memory for said drivers use at the upper part of physical
memory is stuffed by design.

I thought it was a valid technique? I never questioned it because there is
a history of its use -I think the early bttv driver was written this way.

I have been debugging an oops on a system which uses the open source
driver for the Matrox MeteorII multichannel available from,
This driver uses the technique and I am getting a corrupted slab free list.

Ross B, could I please have details of your mem bios hack please so I can try
it as a workaround.

Ross Dickson

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