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SubjectRe: [RFC][PATCH 2/2] first callers of process_deny_checkpoint()

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

> On Thu, 2008-10-09 at 14:43 -0500, Serge E. Hallyn wrote:
> > Hmm, I don't know too much about aio, but is it possible to succeed with
> > io_getevents if we didn't first do a submit? It looks like the contexts
> > are looked up out of current->mm, so I don't think we need this call
> > here.
> >
> > Otherwise, this is neat.
>
> Good question. I know nothing, either. :)
>
> My thought was that any process *trying* to do aio stuff of any kind
> is going to be really confused if it gets checkpointed. Or, it might
> try to submit an aio right after it checks the list of them. I
> thought it best to be cautious and say, if you screw with aio, no
> checkpointing for you!

as long as there's total transparency and the transition from CR-capable
to CR-disabled state is absolutely safe and race-free, that should be
fine.

I expect users to quickly cause enough pressure to reduce the NOCR areas
of the kernel significantly ;-)

In the long run, could we expect a (experimental) version of hibernation
that would just use this checkpointing facility to hibernate? That would
be way cool for users and for testing: we could do transparent kernel
upgrades/downgrades via this form of hibernation, between CR-compatible
kernels (!).

distros could mark certain kernels as 'safe fallback' kernels, and if
say a WARN_ON() or app breakage hits that is suspected to be kernel
related, the user could hit a 'switch back to safe kernel' button -
which would switch back _without losing any app state_.

People could even try new versions of the kernel which would just fall
back to the known-workin safe kernel if the bootup fails for example.

Pie in the sky for sure, but way cool: it could propel Linux kernel
testing to completely new areas - new kernels could be tried
non-intrusively. (as long as a new kernel does not corrupt the CR data
structures - so some good consistency and redundancy checking would be
nice in the format!)

Ingo


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