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    Subject[RFC PATCH 0/4] kvm: Report unused guest pages to host
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    This patch set provides a mechanism by which guests can notify the host of
    pages that are not currently in use. Using this data a KVM host can more
    easily balance memory workloads between guests and improve overall system
    performance by avoiding unnecessary writing of unused pages to swap.

    In order to support this I have added a new hypercall to provided unused
    page hints and made use of mechanisms currently used by PowerPC and s390
    architectures to provide those hints. To reduce the overhead of this call
    I am only using it per huge page instead of of doing a notification per 4K
    page. By doing this we can avoid the expense of fragmenting higher order
    pages, and reduce overall cost for the hypercall as it will only be
    performed once per huge page.

    Because we are limiting this to huge pages it was necessary to add a
    secondary location where we make the call as the buddy allocator can merge
    smaller pages into a higher order huge page.

    This approach is not usable in all cases. Specifically, when KVM direct
    device assignment is used, the memory for a guest is permanently assigned
    to physical pages in order to support DMA from the assigned device. In
    this case we cannot give the pages back, so the hypercall is disabled by
    the host.

    Another situation that can lead to issues is if the page were accessed
    immediately after free. For example, if page poisoning is enabled the
    guest will populate the page *after* freeing it. In this case it does not
    make sense to provide a hint about the page being freed so we do not
    perform the hypercalls from the guest if this functionality is enabled.

    My testing up till now has consisted of setting up 4 8GB VMs on a system
    with 32GB of memory and 4GB of swap. To stress the memory on the system I
    would run "memhog 8G" sequentially on each of the guests and observe how
    long it took to complete the run. The observed behavior is that on the
    systems with these patches applied in both the guest and on the host I was
    able to complete the test with a time of 5 to 7 seconds per guest. On a
    system without these patches the time ranged from 7 to 49 seconds per
    guest. I am assuming the variability is due to time being spent writing
    pages out to disk in order to free up space for the guest.

    ---

    Alexander Duyck (4):
    madvise: Expose ability to set dontneed from kernel
    kvm: Add host side support for free memory hints
    kvm: Add guest side support for free memory hints
    mm: Add merge page notifier


    Documentation/virtual/kvm/cpuid.txt | 4 ++
    Documentation/virtual/kvm/hypercalls.txt | 14 ++++++++
    arch/x86/include/asm/page.h | 25 +++++++++++++++
    arch/x86/include/uapi/asm/kvm_para.h | 3 ++
    arch/x86/kernel/kvm.c | 51 ++++++++++++++++++++++++++++++
    arch/x86/kvm/cpuid.c | 6 +++-
    arch/x86/kvm/x86.c | 35 +++++++++++++++++++++
    include/linux/gfp.h | 4 ++
    include/linux/mm.h | 2 +
    include/uapi/linux/kvm_para.h | 1 +
    mm/madvise.c | 13 +++++++-
    mm/page_alloc.c | 2 +
    12 files changed, 158 insertions(+), 2 deletions(-)

    --

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