Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker

[Wei Wang]

On 07/30/2018 05:00 PM, Michal Hocko wrote:
> On Fri 27-07-18 17:24:55, Wei Wang wrote:
> > The OOM notifier is getting deprecated to use for the reasons mentioned
> > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
> >
> > This patch replaces the virtio-balloon oom notifier with a shrinker
> > to release balloon pages on memory pressure.
> It would be great to document the replacement. This is not a small
> change...

OK. I plan to document the following to the commit log:

  The OOM notifier is getting deprecated to use for the reasons:
    - As a callout from the oom context, it is too subtle and easy to
      generate bugs and corner cases which are hard to track;
    - It is called too late (after the reclaiming has been performed).
      Drivers with large amuont of reclaimable memory is expected to be
      released them at an early age of memory pressure;
    - The notifier callback isn't aware of the oom contrains;
    Link: https://lkml.org/lkml/2018/7/12/314

    This patch replaces the virtio-balloon oom notifier with a shrinker
    to release balloon pages on memory pressure. Users can set the 
amount of
    memory pages to release each time a shrinker_scan is called via the
    module parameter balloon_pages_to_shrink, and the default amount is 256
    pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has
    been used to release balloon pages on OOM. We continue to use this
    feature bit for the shrinker, so the shrinker is only registered when
    this feature bit has been negotiated with host.

    In addition, the bug in the replaced virtballoon_oom_notify that only
    VIRTIO_BALLOON_ARRAY_PFNS_MAX (i.e 256) balloon pages can be freed
    though the user has specified more than that number is fixed in the
    shrinker_scan function.


Best,
Wei