Re: [Etherboot-developers] [Patch] E820 Memory support....

[<ebi...@ln...>]

Vasil Vasilev <vas...@sy...> writes:

> On 6 Dec 2001, Eric W. Biederman wrote:
> 
> > When this has been previously discussed it was decided to drop
> > it because applications can get their own memory size.  But...
> > I just ran into a machine with 6GiB of ram where the legacy
> > routines tell me I only have 64MiB of ram.  Which is painful.
> >
> > The immediate problem is that at 64MiB and older kernels will
> > over write the ramdisk.
> 
> How old kernels. 

Anything before 2.4.10 + enough memory, where enough memory
is I think 2GB.   With 2.2 + 1GB I have seen a ramdisk at 32MB
overwritten.   After 2.4.10 a ramdisk above 8MB should be safe,
and it is useable if it is under I think 890MB, because as you noted
highmem isn't supported.

Ouch I'm starting to get evil ideas about what you could put in
a ramdisk that would need highmem support.  For a diskless machine on
gigabit ethernet it might even be practical to take 10s to load an
image and have an unabridged linux filesystem system on a ramdisk.   

> I am using 2.2.16 on machine with 256MB but the BIOS
> reporting 66559KB (65535+1024) and mknbi rounding it down to 65556KB. It even
> works when the kernel is forced by a boot option to think there is
> 256MB.

Nasty case.  With only 256MB of ram you are unlikely to hit the
struct page array with a ramdisk at even 32MB.  So I wouldn't worry
about this specific case.


 > > So I have update etherboot so that it will query the E820 memory
> > map so it has a true picture of what the memory layout is.  Doing
> > this I have more freedom to locate ramdisk etc.
> 
> I was looking at kernel 2.4 and it appeared that the ramdisk cannot be in the
> upper memory (above 1GB or so).

Correct assesment.

Eric