PROBLEM: kernel mount of initrd fails unless mke2fs uses 1024 byt e blocks

From: "Post, Mark K" <[email protected]>

[1.] One line summary of the problem:
Trying to use an initrd for an installation fails unless the mke2fs
used a blocksize of 1024 bytes.

[2.] Full description of the problem/report:
I'm trying to create a Linux for S/390 version 2.2.20 installation
kernel/ramdisk set. When I create the ramdisk, if I issue the mke2fs command with -b 2048 or -b 4096, it works fine. But, I try to boot the system, I get an "EXT2-fs: Magic mismatch, very weird !" error when the kernel tries to mount the ramdisk as the root file system. If I let the blocksize default, or specify -b 1024, everything works fine.

The comparison that seems to be failing is at line 500 of linux/fs/ext/super.c:
if (es->s_magic != le16_to_cpu(EXT2_SUPER_MAGIC)) {
printk ("EXT2-fs: Magic mismatch, very weird !\n"); goto failed_mount;

I answered:

I once submitted a patch for this, but apparently it is not in 2.2.20.

but was too hasty.
I recalled a buglet here, that was fixed in 2.4 and is still in 2.2, and indeed, there is such a buglet, but your problem is elsewhere.

The kernel does set_blocksize() to change the blocksize of your device. This set_blocksize() throws away all buffers with the now incorrect size. But your device is a ramdisk, and throwing out these buffers kills all your data.
So, after doing that, you have an empty ramdisk again. Hence mount fails.

Andries