Re: 2.5.2-pre7 still missing bits of kdev_t

Date: Thu Jan 03 2002 - 20:47:52 EST

    From Fri Jan 4 01:13:34 2002

    reiserfs is blindly storing the kernel's kdev_t value raw to disk.

    AFAICS this will need a policy decision not just cleanup, before it
    works in 2.5.2 properly. If we switch the kernel to 12:20 major:minor
    numbers, suddenly the reiserfs disk format changes based on kernel
    version, and earlier kernels see corrupted major:minor numbers.

No, not really. For how to do this, see a fragment of example code
that Linus removed from kdev_t.h in pre6, it went something like
(adapted for 12+20 instead of 16+16):

int major(dev_t dev) {
        int ma;

        ma = (dev >> 20);
        if (!ma)
                ma = (dev >> 8);
        return ma;

int minor(dev_t dev) {
        if (dev >> 20)
                return (dev & 0xfffff);
                return (dev & 0xff);

dev_t mkdev(int ma, int mi) {
        if (mi & ~0xff)
                return ((ma << 20) + mi);
                return ((ma << 8) + mi);

(with the correctness conditions that ma is 12-bit,
mi is 20-bit, and major 0 has only 8-bit minors).

You see that the representation of old values does not change.
No disk corruption.


[I didnt spell it out, but you understand: the dev_t is the on-disk
format, the conversion finds the major and minor, and these are
combined again into a kdev_t for use by the kernel]

[Similar code occurs is isofs/rock.c, where a 64-bit dev
must be converted.]

[I don't know whether reiserfs is a Linux-only filesystem.
If it is not, and has a disk format that is OS-independent,
a third struct stat_data might be needed, since the 12+20
is not universal, so 32+32 would be the better choice.]
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

This archive was generated by hypermail 2b29 : Mon Jan 07 2002 - 21:00:23 EST