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    SubjectRe: Back to the future.


    On Thu, 26 Apr 2007, Linus Torvalds wrote:
    >
    > Once you have that snapshot image in user space you can do anything you
    > want.

    Side note: the exception, of course, is page out more. The swap device has
    to be read-only.

    We actually have support for that mode (it's how "swapoff" works: it marks
    swap devices as not accepting _new_ entries, even though old entries are
    still valid). So you can have a fully running system, with 99% of memory
    swapped out, and still guarantee that you won't swap out anything *more*
    (which would destroy the swap image, which you don't want, since it's
    where a lot of the memory may end up being, in order to make the snapshot
    itself as small as possible)!

    Anybody who cares can look at the code that messes with the the
    SWP_WRITEOK flag. You'd basically swap out enough to make the snapshot
    image fit comfortably in memory, and then you'd clear SWP_WRITEOK on all
    swap devices and return to user space. Or something very close to that.

    But the point here is that we should actually really be able to have a
    fully working system, even _after_ we created the snapshot. I don't even
    think you should need any "initrd only" kind of situation.

    If somebody can do that, with just those two system calls, I'll remove
    every other suspend-to-disk wannabe from the kernel in a heartbeat. I may
    have missed something subtle, of course, but I really *think* it should be
    doable.

    Linus
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