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    SubjectRe: vmevent: question?
    On 04/30/2012 05:01 PM, Pekka Enberg wrote:

    > Hi Minchan,
    > On Mon, Apr 30, 2012 at 10:52 AM, Minchan Kim <> wrote:
    >>> It makes the userspace side simpler for "lowmem notification" use
    >>> case. I'm open to changing the ABI if it doesn't make the userspace
    >>> side too complex.
    >> Yes. I understand your point but if we still consider all of values,
    >> we don't have any way to capture exact values except triggered event value.
    >> I mean there is no lock to keep consistency.
    >> If stale data is okay, no problem but IMHO, it could make user very confusing.
    >> So let's return value for first matched event if various event match.
    >> Of course, let's write down it in ABI.
    >> If there is other idea for reporting all of item with consistent, I'm okay.
    > What kind of consistency guarantees do you mean? The data sent to
    > userspace is always a snapshot of the state and therefore can be stale
    > by the time it reaches userspace.

    Consistency between component of snapshot.
    let's assume following as

    1. User expect some events's value would be minus when event he expect happen.
    A : -3, B : -4, C : -5, D : -6
    2. Logically, it's not possible to mix plus and minus values for the events.
    A : -3, B : -4, C : -5, D : -6 ( O )
    A : -3, B : -4, C : 1, D : 2 ( X )

    But in current implementation, some of those could be minus and some of those could be plus.
    Which event could user believe?
    At least, we need a _captured_ value when event triggered so that user can ignore other values.

    > If your code needs stricter consistency guarantees, you probably want
    > to do it in the kernel.
    > Pekka
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    Kind regards,
    Minchan Kim

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