simple_alloc space tramples initrd

[dwalker at fifo99.com]

On Mon, Jan 11, 2016 at 02:09:34PM +1100, Michael Ellerman wrote:
> On Fri, 2016-01-08 at 09:45 -0800, dwalker at fifo99.com wrote:
> > Hi,
> > 
> > A powerpc machine I'm working on has this problem where the
> > simple_alloc_init() area is trampling the initrd. The two are placed fairly
> > close together.
> 
> Which machine / platform?

It's not upstream yet. I'm still putting the patches together, that's when this
issue came up. I can send an RFC if you want to look at the patches.
 
> > I have a fix for this proposed to add a section into
> > arch/powerpc/boot/zImage.lds.S called "mallocarea" to account for this space,
> > but not all powerpc platforms use simple_alloc_init(). So for those platforms
> > it's a waste.
> 
> Yeah I don't really like the sound of that. We could do it if it was behind a
> CONFIG option, but hopefully there is a better solution.
> 
> > Another alternative is to alter the bootloader to place more space between
> > the kernel image and initrd image.
> >
> > I wanted to get some feedback on the right way to fix this. It seems like it
> > could be a generic issue on powerpc, or it's possibly already fixed someplace
> > and I just haven't noticed.
> 
> I don't really know that code very well. But ideally either the boot loader
> gives you space, or the platform boot code is smart enough to detect that there
> is insufficient room and puts the heap somewhere else.

It seems like the kernel should be able to handle it. I believe the bootloader passes
the initrd location , but I don't think it's evaluated till later in the boot up. For
simple_alloc_init() it seems all platforms just assume the space is empty without checking.

Daniel