Messages in this thread | | | From | Torrey Hoffman <> | Subject | RE: ramdisk corruption problems - was: RE: pivot_root and initrd kern el panic woes | Date | Tue, 18 Dec 2001 12:14:49 -0800 |
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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
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