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    SubjectRe: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

    * Dave Hansen <dave@linux.vnet.ibm.com> wrote:

    > On Thu, 2008-10-09 at 14:46 +0200, Ingo Molnar wrote:
    > > * Oren Laadan <orenl@cs.columbia.edu> wrote:
    > >
    > > > These patches implement basic checkpoint-restart [CR]. This version
    > > > (v6) supports basic tasks with simple private memory, and open files
    > > > (regular files and directories only). Changes mainly cleanups. See
    > > > original announcements below.
    > >
    > > i'm wondering about the following productization aspect: it would be
    > > very useful to applications and users if they knew whether it is safe to
    > > checkpoint a given app. I.e. whether that app has any state that cannot
    > > be stored/restored yet.
    >
    > Absolutely!
    >
    > My first inclination was to do this at checkpoint time: detect and
    > tell users why an app or container can't actually be checkpointed.
    > But, if I get you right, you're talking about something that happens
    > more during the runtime of the app than during the checkpoint. This
    > sounds like a wonderful approach to me, and much better than what I
    > was thinking of.
    >
    > What kind of mechanism do you have in mind?
    >
    > int sys_remap_file_pages(...)
    > {
    > ...
    > oh_crap_we_dont_support_this_yet(current);
    > }
    >
    > Then the oh_crap..() function sets a task flag or something?

    yeah, something like that. A key aspect of it is that is has to be very
    low-key on the source code level - we dont want to sprinkle the kernel
    with anything ugly. Perhaps something pretty explicit:

    current->flags |= PF_NOCR;

    as we do the same thing today for certain facilities:

    current->flags |= PF_NOFREEZE;

    you probably want to hide it behind:

    set_current_nocr();

    and have a set_task_nocr() as well, in case there's some proxy state
    installed by another task.

    Via such wrappers there's no overhead at all in the
    !CONFIG_CHECKPOINT_RESTART case.

    Plus you could drive the debug mechanism via it as well, by using a
    trivial extension of the facility:

    set_current_nocr("CR: sys_remap_file_pages not supported yet.");
    ...
    set_task_nocr(t, "CR: PI futexes not supported yet.");

    Ingo


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