Re: [RFC PATCH 00/11] mm / virtio: Provide support for paravirtual waste page treatment

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 30, 2019 at 02:53:34PM -0700, Alexander Duyck wrote:
> This series provides an asynchronous means of hinting to a hypervisor
> that a guest page is no longer in use and can have the data associated
> with it dropped. To do this I have implemented functionality that allows
> for what I am referring to as "waste page treatment".
> 
> I have based many of the terms and functionality off of waste water
> treatment, the idea for the similarity occured to me after I had reached
> the point of referring to the hints as "bubbles", as the hints used the
> same approach as the balloon functionality but would disappear if they
> were touched, as a result I started to think of the virtio device as an
> aerator. The general idea with all of this is that the guest should be
> treating the unused pages so that when they end up heading "downstream"
> to either another guest, or back at the host they will not need to be
> written to swap.

A lovely analogy.

> So for a bit of background for the treatment process, it is based on a
> sequencing batch reactor (SBR)[1]. The treatment process itself has five
> stages. The first stage is the fill, with this we take the raw pages and
> add them to the reactor. The second stage is react, in this stage we hand
> the pages off to the Virtio Balloon driver to have hints attached to them
> and for those hints to be sent to the hypervisor. The third stage is
> settle, in this stage we are waiting for the hypervisor to process the
> pages, and we should receive an interrupt when it is completed. The fourth
> stage is to decant, or drain the reactor of pages. Finally we have the
> idle stage which we will go into if the reference count for the reactor
> gets down to 0 after a drain, or if a fill operation fails to obtain any
> pages and the reference count has hit 0. Otherwise we return to the first
> state and start the cycle over again.

will review the patchset closely shortly.

> This patch set is still far more intrusive then I would really like for
> what it has to do. Currently I am splitting the nr_free_pages into two
> values and having to add a pointer and an index to track where we area in
> the treatment process for a given free_area. I'm also not sure I have
> covered all possible corner cases where pages can get into the free_area
> or move from one migratetype to another.
> 
> Also I am still leaving a number of things hard-coded such as limiting the
> lowest order processed to PAGEBLOCK_ORDER, and have left it up to the
> guest to determine what size of reactor it wants to allocate to process
> the hints.
> 
> Another consideration I am still debating is if I really want to process
> the aerator_cycle() function in interrupt context or if I should have it
> running in a thread somewhere else.
> 
> [1]: https://en.wikipedia.org/wiki/Sequencing_batch_reactor
> 
> ---
> 
> Alexander Duyck (11):
>       mm: Move MAX_ORDER definition closer to pageblock_order
>       mm: Adjust shuffle code to allow for future coalescing
>       mm: Add support for Treated Buddy pages
>       mm: Split nr_free into nr_free_raw and nr_free_treated
>       mm: Propogate Treated bit when splitting
>       mm: Add membrane to free area to use as divider between treated and raw pages
>       mm: Add support for acquiring first free "raw" or "untreated" page in zone
>       mm: Add support for creating memory aeration
>       mm: Count isolated pages as "treated"
>       virtio-balloon: Add support for aerating memory via bubble hinting
>       mm: Add free page notification hook
> 
> 
>  arch/x86/include/asm/page.h         |   11 +
>  drivers/virtio/Kconfig              |    1 
>  drivers/virtio/virtio_balloon.c     |   89 ++++++++++
>  include/linux/gfp.h                 |   10 +
>  include/linux/memory_aeration.h     |   54 ++++++
>  include/linux/mmzone.h              |  100 +++++++++--
>  include/linux/page-flags.h          |   32 +++
>  include/linux/pageblock-flags.h     |    8 +
>  include/uapi/linux/virtio_balloon.h |    1 
>  mm/Kconfig                          |    5 +
>  mm/Makefile                         |    1 
>  mm/aeration.c                       |  324 +++++++++++++++++++++++++++++++++++
>  mm/compaction.c                     |    4 
>  mm/page_alloc.c                     |  220 ++++++++++++++++++++----
>  mm/shuffle.c                        |   24 ---
>  mm/shuffle.h                        |   35 ++++
>  mm/vmstat.c                         |    5 -
>  17 files changed, 838 insertions(+), 86 deletions(-)
>  create mode 100644 include/linux/memory_aeration.h
>  create mode 100644 mm/aeration.c
> 
> --