RE: ramdisk corruption problems - was: RE: pivot_root and initrd

[Torrey Hoffman]

More information!  And a workaround!

I conjecture that the ramdisk driver (post 2.4.9) only grabs
VM pages properly if it is accessed directly, as a dd to 
/dev/ram0 does.  I further conjecture that accessing the 
ramdisk through a mounted filesystem does not grab pages 
properly.

The reason I believe this is that removing the call to 
"freeramdisk" from my original script avoids corruption.  

Another way to avoid ramdisk corruption is to 
"dd if=/dev/zero of=/dev/ram0 bs=1k count=4000" 
immediately after the call to freeramdisk.

If my conjecture is right, then the corruption is caused 
because mke2fs on a "freed" /dev/ram0 doesn't touch every 
block of the fs, leaving "holes" where pages are not properly 
grabbed from the VM. The resulting filesystem appears to work, 
but dd'ing from /dev/ram0 gets a broken filesystem image.

Note that "freeramdisk /dev/ram0" is pretty much just:
#define FLKFLSBUF  _IO(0x12,97) /* flush buffer cache */
f = open("/dev/ram0", O_RDWR);
ioctl(f, BLKFLSBUF);

To experiment for yourself, stick the following script in
a subdirectory which also contains a "testdir" directory
with about 3 MB of data.

- - - - - - - -
#!/bin/bash

# to tickle the bug, do the freeramdisk but not the
# dd from /dev/zero to /dev/ram0.  

freeramdisk /dev/ram0
#dd if=/dev/zero of=/dev/ram0 bs=1k count=4000

mke2fs -m0 /dev/ram0 4000
mount -t ext2 /dev/ram0 /mnt/ramdisk
rm -rf /mnt/ramdisk/*

cp -a ./testdir /mnt/ramdisk
umount /dev/ram0

dd if=/dev/ram0 of=ram0.img bs=1k count=4000
dd if=ram0.img of=/dev/ram0 bs=1k count=4000

mount -t ext2 /dev/ram0 /mnt/ramdisk
diff -q -r ./testdir /mnt/ramdisk/testdir

# If diff reports mismatches, you saw the bug.

umount /dev/ram0
- - - - - - - -

If the gods of the VM and VFS don't bother to look at 
it, I might take a peek at the relevant kernel code myself.  
Might take two months of study before I know enough though.

Torrey