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    SubjectRe: [PATCH 7/9] mm, page_alloc: remove stop_machine from build_all_zonelists
    On Fri, Jul 14, 2017 at 10:00:04AM +0200, Michal Hocko wrote:
    > From: Michal Hocko <mhocko@suse.com>
    >
    > build_all_zonelists has been (ab)using stop_machine to make sure that
    > zonelists do not change while somebody is looking at them. This is
    > is just a gross hack because a) it complicates the context from which
    > we can call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug:
    > switch locking to a percpu rwsem")) and b) is is not really necessary
    > especially after "mm, page_alloc: simplify zonelist initialization".
    >
    > Updates of the zonelists happen very seldom, basically only when a zone
    > becomes populated during memory online or when it loses all the memory
    > during offline. A racing iteration over zonelists could either miss a
    > zone or try to work on one zone twice. Both of these are something we
    > can live with occasionally because there will always be at least one
    > zone visible so we are not likely to fail allocation too easily for
    > example.
    >
    > Signed-off-by: Michal Hocko <mhocko@suse.com>

    This patch is contingent on the last patch which updates in place
    instead of zeroing the early part of the zonelist first but needs to fix
    the stack usage issues. I think it's also worth pointing out in the
    changelog that stop_machine never gave the guarantees it claimed as a
    process iterating through the zonelist can be stopped so when it resumes
    the zonelist has changed underneath it. Doing it online is roughly
    equivalent in terms of safety.

    --
    Mel Gorman
    SUSE Labs

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