how to reused the initrd ram?

[zhuzhenhua <zzh.hust@gmail.com>]

my board has 64M sdram, when i boot nfs root, it show Mem total about 62M
and if i boot initrd and then switch to nfs root, it show mem total
about 54M,and freeramdisk does nothing to it.

my kernel commandline is
root=/dev/nfs init=/linuxrc rd_start=0x80a00000 rd_size=0x260000
ramdisk_size=8192 ip=bootp

and in my initrd, the linuxrc is
#!/bin/nash
mount -t proc /proc /proc
echo Mounting root filesystem
mount -o nolock,rsize=1024,wsize=1024 --ro -t nfs
10.10.10.2:/root/nfsroot/rootfs /sysroot
pivot_root /sysroot /sysroot/initrd
umount /initrd/proc

and in my nfs root, i type
$umount initrd
$freeramdisk /dev/ram0

it only flushed bufferd mem, and do nothing to the mem total.
i wonder how to reused the initrd mem?

BTW, if i change the kernel comandline to root=/dev/ram0 ...
it will show messages as
"Failed to execute /linuxrc.  Attempting defaults...
Kernel panic - not syncing: No init found.  Try passing init= option to kernel"


Thanks for any hints
Best Regards

zhuzhenhua