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    SubjectRe: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev
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    On Mon, 2009-08-10 at 08:39 -0700, Greg KH wrote:
    > On Sat, Aug 08, 2009 at 07:07:56PM -0400, David Dillow wrote:
    > > I followed up with Greg offline to make sure I was looking at the right
    > > thing, but the upshot is that Moblin does not use an initrd and devtmpfs
    > > does not in any way reduce the amount of code for those distros that
    > > want to support more than the root=/dev/blah syntax.
    >
    > Like Scott points out, this option now allows distros to use blkid
    > directly instead of udev if they want to implement things like this
    > without udev.

    But they already have that functionality -- RedHat has been doing it for
    ages, without devtmpfs. No new kernel code needed.

    > > Given Eric and Arjan's numbers about the time it takes to populate /dev
    > > from /sys -- pointing the speed problem squarely at udev or the ruleset
    > > -- I don't see much of a win for devtmpfs other than avoiding a
    > > static /dev to make init=/bin/sh work.
    >
    > That's a very big win right there, don't you think?

    No, not really. It isn't hard to make a static /dev, or a rescue initrd
    for the cases with dynamic device numbers.

    > Also, embedded and
    > rescue disks also would like it for this very reason.

    As a former embedded developer, I'd rather not waste kernel space on
    something I can throw away when I've booted. I know where my root is,
    and can statically populate /dev. Now, perhaps more embedded devices are
    moving to use USB for their root, but then it would seem to make more
    sense to use Eric/Arjan's program to populate /dev and do blk_id if I
    need to find a label or UUID, and then I can reuse the memory when I'm
    done. Perhaps I can find it by path and avoid the blk_id, but I still
    can do that without more kernel code.

    And I realize that we're not talking about a lot of kernel memory here,
    so it depends on one's definition of waste.

    > > You cannot rely on it for a hotplug disk such as USB, as it doesn't do
    > > the root=LABEL=usbroot that you'd want to do since the location is not
    > > stable.
    >
    > Again, not all the world requires such labels.

    Sure. But it seems the world can get what the best of both worlds from a
    simple program to populate /dev from sysfs.

    An interesting experiment may be time a boot to initial user space with
    and without devtmpfs -- and compare the difference to Eric/Arjan's
    program on the same hardware. I wonder how much is really saved.



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