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    SubjectRe: [BUG] set_mempolicy(MPOL_INTERLEAV) cause kernel panic
    On Fri, 17 Jul 2009 11:07:09 +0900 (JST)
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    > > On Fri, 17 Jul 2009 09:04:46 +0900 (JST)
    > > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
    > >
    > > > > On Wed, 15 Jul 2009, Lee Schermerhorn wrote:
    > > > >
    > > > > > Interestingly, on ia64, the top cpuset mems_allowed gets set to all
    > > > > > possible nodes, while on x86_64, it gets set to on-line nodes [or nodes
    > > > > > with memory]. Maybe this is a to support hot-plug?
    > > > > >
    > > > >
    > > > > numactl --interleave=all simply passes a nodemask with all bits set, so if
    > > > > cpuset_current_mems_allowed includes offline nodes from node_possible_map,
    > > > > then mpol_set_nodemask() doesn't mask them off.
    > > > >
    > > > > Seems like we could handle this strictly in mempolicies without worrying
    > > > > about top_cpuset like in the following?
    > > >
    > > > This patch seems band-aid patch. it will change memory-hotplug behavior.
    > > > Please imazine following scenario:
    > > >
    > > > 1. numactl interleave=all process-A
    > > > 2. memory hot-add
    > > >
    > > > before 2.6.30:
    > > > -> process-A can use hot-added memory
    > > >
    > > > your proposal patch:
    > > > -> process-A can't use hot-added memory
    > > >
    > >
    > > IMHO, the application itseld should be notifed to change its mempolicy by
    > > hot-plug script on the host. While an application uses interleave, a new node
    > > hot-added is just a noise. I think "How pages are interleaved" should not be
    > > changed implicitly. Then, checking at set_mempolicy() seems sane. If notified,
    > > application can do page migration and rebuild his mapping in ideal way.
    >
    > Do you really want ABI change?
    >
    No ;_

    Hmm, IIUC, current handling of nodemask of mempolicy is below.
    There should be 3 masks.
    - systems's N_HIGH_MEMORY
    - the mask user specified via mempolicy() (remembered only when MPOL_F_RELATIVE
    - cpusets's one

    And pol->v.nodes is just a _cache_ of logical-and of aboves.
    Synchronization with cpusets is guaranteed by cpuset's generation.
    Synchronization with N_HIGH_MEMORY should be guaranteed by memory hotplug
    notifier, but this is not implemented yet.

    Then, what I can tell here is...
    - remember what's user requested. (only when MPOL_F_RELATIVE_NODES ?)
    - add notifiers for memory hot-add. (only when MPOL_F_RELATIVE_NODES ?)
    - add notifiers for memory hot-remove (both MPOL_F_STATIC/RELATIVE_NODES ?)

    IMHO, for cpusets, don't calculate v.nodes again if MPOL_F_STATIC is good.
    But for N_HIGH_MEMORY, v.nodes should be caluculated even if MPOL_F_STATIC is set.

    Then, I think the mask user passed should be remembered even if MPOL_F_STATIC is
    set and v.nodes should work as cache and should be updated in appropriate way.

    Thanks,
    -Kame














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