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    SubjectRe: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness
    On Fri, 6 Nov 2015, H. Peter Anvin wrote:
    > On 11/06/15 15:17, Dan Williams wrote:
    > >>
    > >> Is it really required to do that on all cpus?
    > >
    > > I believe it is, but I'll double check.
    > >
    >
    > It's required on all CPUs on which the DAX memory may have been dirtied.
    > This is similar to the way we flush TLBs.

    Right. And that's exactly the problem: "may have been dirtied"

    If DAX is used on 50% of the CPUs and the other 50% are plumming away
    happily in user space or run low latency RT tasks w/o ever touching
    it, then having an unconditional flush on ALL CPUs is just wrong
    because you penalize the uninvolved cores with a completely pointless
    SMP function call and drain their caches.

    Thanks,

    tglx


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