2.6.26.8-rt16 powerpc and mem=...

[Florian Boelstler <euphoria@arcor.de>]

Hi,

I'm currently running 2.6.26.8-rt16 on a custom MPC8540-based board.
I was wondering whether there is any experience reducing the available 
memory using the mem= kernel argument on linux-rt.
(Use case: A specific memory portion is reserved for configuration data, 
which is set by an external system attached over PCI)

It seems that there is a race condition, which is hit during extraction 
of the initrd only if the available memory is reduced.
(For vanilla 2.6.26.8 it works without any problems. The problem occurs 
in 2.6.24.7-rt24 as well)

[...]
RAMDISK: Compressed image found at block 0
crc error
VFS: Mounted root (ext2 filesystem).
Freeing unused kernel memory: 140k init
Kernel panic - not syncing: No init found.  Try passing init= option to 
kernel.
Call Trace:
[d7441e90] [c00072c4] show_stack+0x54/0x188 (unreliable)
[d7441ec0] [c0020d58] panic+0x98/0x160
[d7441f50] [c0001bf8] name_to_dev_t+0x0/0x25c
[d7441f60] [c03059e0] kernel_init+0x250/0x284
[d7441ff0] [c000d910] kernel_thread+0x44/0x60

Sometimes the output is slightly different, and init is actually called. 
But the file system is corrupt. E.g.:

[...]
RAMDISK: Compressed image found at block 0
RAMDISK: incomplete write (-28 != 7032) 33554432
crc error
VFS: Mounted root (ext2 filesystem).
Freeing unused kernel memory: 140k init
init started:  BusyBox v1.1.0 (2007.01.23-00:05+0000) multi-call binary
[...]
EXT2-fs error (device ram0): ext2_check_page: bad entry in directory 
#1594: : directory entry across blocks - offset=0, inode=909341804, 
rec_len=14392, name_len=55
Remounting filesystem read-only
[...]

In a single case Linux even booted without any problems.

Some details about my setup:
- 512MB RAM in total
- Memory available for Linux is reduced to 508M (mem=520192K)
- Linux is booted from U-Boot (v1.2.0), which places the initrd image in 
lowmem below 0x18000000 (initrd_high=0x18000000). This is done because 
Linux seems to not allow having the initrd in highmem. (Output from 
kernel regarding CAMs: "Memory CAM mapping: CAM0=256Mb, CAM1=64Mb, 
CAM2=64Mb residual: 124Mb")