mounting fs from memory

mounting fs from memory

[Andrew Frezell]

Hello All,

I have a bootloader that that is reading segments out of flash and into 
memory.  The segments are 3 compressed ext2 filesystems and the linux 
kernel.  After a signature check of each of the sections in memory the 
bootloader jumps to the kernel.

I would like to mount one filesystem section in RAM as the root 
filesystem.  I think it's easy enough to specify initrd as the offset 
and size of the section in RAM.  But I would also like to mount the 
remaining two filesystems in RAM when linux starts up.  This is where 
I'm having some trouble.  I have two questions:

1.  Is there some way to protect the memory regions in RAM from linux 
just trashing it?  I saw a function add_memory_region in 
arch/mips/kernel/setup.c that seems to do something, does anyone know 
what exactly this does?

2.  How do you mount an area of memory that you know has a filesystem 
already there under linux?  Is there some mount command where you can 
pass the address and size, and mount does the right thing?

Thank you in advance,

Drew Frezell

Re: mounting fs from memory

[Ralf Baechle]

On Sun, Mar 21, 2004 at 10:18:37PM -0500, Andrew Frezell wrote:

> 1.  Is there some way to protect the memory regions in RAM from linux 
> just trashing it?  I saw a function add_memory_region in 
> arch/mips/kernel/setup.c that seems to do something, does anyone know 
> what exactly this does?

The kernel won't touch any memory below the kernel itself.  I consider that
a bug so will change that for now that's a save region to place something.
add_memory_region takes a third argument which can be BOOT_MEM_RAM,
BOOT_MEM_ROM_DATA or BOOT_MEM_RESERVED.  You should pass BOOT_MEM_RESERVED
for to tell the kernel that a certain region should not be considered
usable memory.  For completeness sake BOOT_MEM_RAM is free memory and
BOOT_MEM_ROM_DATA will be free at the end of kernel initialization so is
usually used to describe free memory regions which hold firmware data that
becomes useless after initialization.

  Ralf