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    SubjectRe: [PATCH 00/22] HWPOISON: Intro (v5)
    On Mon, Jun 15, 2009 at 08:25:28PM +0800, Nick Piggin wrote:
    > On Mon, Jun 15, 2009 at 08:10:01PM +0800, Wu Fengguang wrote:
    > > On Mon, Jun 15, 2009 at 03:19:07PM +0800, Nick Piggin wrote:
    > > > > For KVM you need early kill, for the others it remains to be seen.
    > > >
    > > > Right. It's almost like you need to do a per-process thing, and
    > > > those that can handle things (such as the new SIGBUS or the new
    > > > EIO) could get those, and others could be killed.
    > >
    > > To send early SIGBUS kills to processes who has called
    > > sigaction(SIGBUS, ...)? KVM will sure do that. For other apps we
    > > don't mind they can understand that signal at all.
    > For apps that hook into SIGBUS for some other means and

    Yes I was referring to the sigaction(SIGBUS) apps, others will
    be late killed anyway.

    > do not understand the new type of SIGBUS signal? What about
    > those?

    We introduced two new SIGBUS codes:
    BUS_MCEERR_AO=5 for early kill
    BUS_MCEERR_AR=4 for late kill
    I'd assume a legacy application will handle them in the same way (both
    are unexpected code to the application).

    We don't care whether the application can be killed by BUS_MCEERR_AO
    or BUS_MCEERR_AR depending on its SIGBUS handler implementation.
    But (in the rare case) if the handler
    - refused to die on BUS_MCEERR_AR, it may create a busy loop and
    flooding of SIGBUS signals, which is a bug of the application.
    BUS_MCEERR_AO is one time and won't lead to busy loops.
    - does something that hurts itself (ie. data safety) on BUS_MCEERR_AO,
    it may well hurt the same way on BUS_MCEERR_AR. The latter one is
    unavoidable, so the application must be fixed anyway.

    > > > Early-kill for KVM does seem like reasonable justification on the
    > > > surface, but when I think more about it, I wonder does the guest
    > > > actually stand any better chance to correct the error if it is
    > > > reported at time T rather than T+delta? (who knows what the page
    > > > will be used for at any given time).
    > >
    > > Early kill makes a lot difference for KVM. Think about the vast
    > > amount of clean page cache pages. With early kill the page can be
    > > trivially isolated. With late kill the whole virtual machine dies
    > > hard.
    > Why? In both cases it will enter the exception handler and
    > attempt to do something about it... in both cases I would
    > have thought there is some chance that the page error is not
    > recoverable and some chance it is recoverable. Or am I
    > missing something?

    The early kill / late kill to KVM from the POV of host kernel matches
    the MCE AO/AR events inside the KVM guest kernel. The key difference
    between AO/AR is, whether the page is _being_ consumed.

    It's a lot harder (if possible) to try to stop an active consumer.
    For example, the clean cache pages can be consumed in many ways:
    - be accessed by read()/write() or mapped read/write
    - be reclaimed and then allocated for whatever new usage, for example,
    be zeroed by __GFP_ZERO, or be insert into another file and start
    read/write IO and be accessed by disk driver via DMA, or even be
    allocated for kernel slabs..
    Frankly speaking I don't know how to stop all the above consumers.
    We now simply die on AR events.

    > Anyway, I would like to see a basic analysis of those probabilities
    > to justify early kill. Not saying there is no justification, but
    > it would be helpful to see why.

    That's fine. I'd be glad if the above explanation paves way to
    solutions for AR events :)


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