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    SubjectRe: [PATCH] mm: make PR_SET_THP_DISABLE immediately active
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    On 06/02/2017 10:40 PM, Andrew Morton wrote:
    > On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka <vbabka@suse.cz> wrote:
    >>> Perhaps we should be adding new prctl modes to select this new
    >>> behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is?
    >>
    >> I think we can reasonably assume that most users of the prctl do just
    >> the fork() & exec() thing, so they will be unaffected.
    >
    > That sounds optimistic. Perhaps people are using the current behaviour
    > to set on particular mapping to MMF_DISABLE_THP, with
    >
    > prctl(PR_SET_THP_DISABLE)
    > mmap()
    > prctl(PR_CLR_THP_DISABLE)
    >
    > ?
    >
    > Seems a reasonable thing to do.

    Using madvise(MADV_NOHUGEPAGE) seems reasonabler to me, with the same
    effect. And it's older (2.6.38).

    > But who knows - people do all sorts of
    > inventive things.

    Yeah :( but we can hope they don't even know that the prctl currently
    behaves they way it does - man page doesn't suggest it would, and most
    of us in this thread found it surprising.

    >> And as usual, if
    >> somebody does complain in the end, we revert and try the other way?
    >
    > But by then it's too late - the new behaviour will be out in the field.

    Revert in stable then?
    But I don't think this patch should go to stable. I understand right
    that CRIU will switch to the UFFDIO_COPY approach and doesn't need the
    prctl change/new madvise anymore?

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