[PATCH v25 0/2] Virtio-balloon: support free page reporting

[Wei Wang]

This patch series is separated from the previous "Virtio-balloon
Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,  
implemented by this series enables the virtio-balloon driver to report
hints of guest free pages to the host. It can be used to accelerate live
migration of VMs. Here is an introduction of this usage:

Live migration needs to transfer the VM's memory from the source machine
to the destination round by round. For the 1st round, all the VM's memory
is transferred. From the 2nd round, only the pieces of memory that were
written by the guest (after the 1st round) are transferred. One method
that is popularly used by the hypervisor to track which part of memory is
written is to write-protect all the guest memory.

The second feature enables the optimization of the 1st round memory
transfer - the hypervisor can skip the transfer of guest free pages in the
1st round. It is not concerned that the memory pages are used after they
are given to the hypervisor as a hint of the free pages, because they will
be tracked by the hypervisor and transferred in the next round if they are
used and written.

ChangeLog:
v24->v25:
    - mm: change walk_free_mem_block to return 0 (instead of true) on
          completing the report, and return a non-zero value from the
          callabck, which stops the reporting.
    - virtio-balloon:
        - use enum instead of define for VIRTIO_BALLOON_VQ_INFLATE etc.
        - avoid __virtio_clear_bit when bailing out;
        - a new method to avoid reporting the some cmd id to host twice
        - destroy_workqueue can cancel free page work when the feature is
          negotiated;
        - fail probe when the free page vq size is less than 2.
v23->v24:
    - change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
      VIRTIO_BALLOON_F_FREE_PAGE_HINT
    - kick when vq->num_free < half full, instead of "= half full"
    - replace BUG_ON with bailing out
    - check vb->balloon_wq in probe(), if null, bail out
    - add a new feature bit for page poisoning
    - solve the corner case that one cmd id being sent to host twice
v22->v23:
    - change to kick the device when the vq is half-way full;
    - open-code batch_free_page_sg into add_one_sg;
    - change cmd_id from "uint32_t" to "__virtio32";
    - reserver one entry in the vq for teh driver to send cmd_id, instead
      of busywaiting for an available entry;
    - add "stop_update" check before queue_work for prudence purpose for
      now, will have a separate patch to discuss this flag check later;
    - init_vqs: change to put some variables on stack to have simpler
      implementation;
    - add destroy_workqueue(vb->balloon_wq);

v21->v22:
    - add_one_sg: some code and comment re-arrangement
    - send_cmd_id: handle a cornercase

For previous ChangeLog, please reference
https://lwn.net/Articles/743660/

Wei Wang (2):
  mm: support reporting free page blocks
  virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

 drivers/virtio/virtio_balloon.c     | 251 ++++++++++++++++++++++++++++++------
 include/linux/mm.h                  |   6 +
 include/uapi/linux/virtio_balloon.h |   7 +
 mm/page_alloc.c                     |  96 ++++++++++++++
 4 files changed, 324 insertions(+), 36 deletions(-)

-- 
2.7.4