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    SubjectRe: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS
    On Wed, 13 Apr 2022 14:52:01 +0530 Jagdish Gediya <jvgediya@linux.ibm.com> wrote:

    > Current implementation to find the demotion targets works
    > based on node state N_MEMORY, however some systems may have
    > dram only memory numa node which are N_MEMORY but not the
    > right choices as demotion targets.

    Why are they not the right choice? Please describe this fully so we
    can understand the motivation and end-user benefit of the proposed
    change. And please more fully describe the end-user benefits of this
    change.

    > This patch series introduces the new node state
    > N_DEMOTION_TARGETS, which is used to distinguish the nodes which
    > can be used as demotion targets, node_states[N_DEMOTION_TARGETS]
    > is used to hold the list of nodes which can be used as demotion
    > targets, support is also added to set the demotion target
    > list from user space so that default behavior can be overridden.

    Permanently extending the kernel ABI is a fairly big deal. Please
    fully explain the end-user value, usage scenarios, etc.

    What would go wrong if we simply omitted this interface?

    > node state N_DEMOTION_TARGETS is also set from the dax kmem
    > driver, certain type of memory which registers through dax kmem
    > (e.g. HBM) may not be the right choices for demotion so in future
    > they should be distinguished based on certain attributes and dax
    > kmem driver should avoid setting them as N_DEMOTION_TARGETS,
    > however current implementation also doesn't distinguish any
    > such memory and it considers all N_MEMORY as demotion targets
    > so this patch series doesn't modify the current behavior.
    >
    > Current code which sets migration targets is modified in
    > this patch series to avoid some of the limitations on the demotion
    > target sharing and to use N_DEMOTION_TARGETS only nodes while
    > finding demotion targets.
    >
    > Changelog
    > ----------
    >
    > v2:
    > In v1, only 1st patch of this patch series was sent, which was
    > implemented to avoid some of the limitations on the demotion
    > target sharing, however for certain numa topology, the demotion
    > targets found by that patch was not most optimal, so 1st patch
    > in this series is modified according to suggestions from Huang
    > and Baolin. Different examples of demotion list comparasion
    > between existing implementation and changed implementation can
    > be found in the commit message of 1st patch.
    >
    > Jagdish Gediya (5):
    > mm: demotion: Set demotion list differently
    > mm: demotion: Add new node state N_DEMOTION_TARGETS
    > mm: demotion: Add support to set targets from userspace
    > device-dax/kmem: Set node state as N_DEMOTION_TARGETS
    > mm: demotion: Build demotion list based on N_DEMOTION_TARGETS
    >
    > .../ABI/testing/sysfs-kernel-mm-numa | 12 ++++

    This description is rather brief. Some additional user-facing material
    under Documentation/ would help. Describe the format for writing to the
    file, what is seen when reading from it, provide a bit of help to the
    user so they can understand how to use it, what effects they might see,
    etc.

    > drivers/base/node.c | 4 ++
    > drivers/dax/kmem.c | 2 +
    > include/linux/nodemask.h | 1 +
    > mm/migrate.c | 67 +++++++++++++++----
    > 5 files changed, 72 insertions(+), 14 deletions(-)

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