Re: Balloon pressuring page cache

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

On Mon, Feb 03, 2020 at 05:34:20PM +0100, David Hildenbrand wrote:
> On 03.02.20 17:18, Alexander Duyck wrote:
> > On Mon, 2020-02-03 at 08:11 -0500, Michael S. Tsirkin wrote:
> >> On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote:
> >>>
> >>> On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W <wei.w.wang@intel.com> wrote:
> >>>
> >>>     On Thursday, January 30, 2020 11:03 PM, David Hildenbrand wrote:
> >>>     > On 29.01.20 20:11, Tyler Sanderson wrote:
> >>>     > >
> >>>     > >
> >>>     > > On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <david@redhat.com
> >>>     > > <mailto:david@redhat.com>> wrote:
> >>>     > >
> >>>     > >     On 29.01.20 01:22, Tyler Sanderson via Virtualization wrote:
> >>>     > >     > A primary advantage of virtio balloon over other memory reclaim
> >>>     > >     > mechanisms is that it can pressure the guest's page cache into
> >>>     > >     shrinking.
> >>>     > >     >
> >>>     > >     > However, since the balloon driver changed to using the shrinker
> >>>     API
> >>>     > >     >
> >>>     > >
> >>>     > <https://github.com/torvalds/linux/commit/71994620bb25a8b109388fefa9
> >>>     > e99a28e355255a#diff-fd202acf694d9eba19c8c64da3e480c9> this
> >>>     > >     > use case has become a bit more tricky. I'm wondering what the
> >>>     > intended
> >>>     > >     > device implementation is.
> >>>     > >     >
> >>>     > >     > When inflating the balloon against page cache (i.e. no free
> >>>     memory
> >>>     > >     > remains) vmscan.c will both shrink page cache, but also invoke
> >>>     the
> >>>     > >     > shrinkers -- including the balloon's shrinker. So the balloon
> >>>     driver
> >>>     > >     > allocates memory which requires reclaim, vmscan gets this memory
> >>>     > by
> >>>     > >     > shrinking the balloon, and then the driver adds the memory back
> >>>     to
> >>>     > the
> >>>     > >     > balloon. Basically a busy no-op.
> >>>
> >>>     Per my understanding, the balloon allocation won’t invoke shrinker as
> >>>     __GFP_DIRECT_RECLAIM isn't set, no?
> >>>
> >>> I could be wrong about the mechanism, but the device sees lots of activity on
> >>> the deflate queue. The balloon is being shrunk. And this only starts once all
> >>> free memory is depleted and we're inflating into page cache.
> >>
> >> So given this looks like a regression, maybe we should revert the
> >> patch in question 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> >> Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT
> >> shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't nice
> >> at all.
> >>
> >> So it looks like all this rework introduced more issues than it
> >> addressed ...
> >>
> >> I also CC Alex Duyck for an opinion on this.
> >> Alex, what do you use to put pressure on page cache?
> > 
> > I would say reverting probably makes sense. I'm not sure there is much
> > value to having a shrinker running deflation when you are actively trying
> > to increase the balloon. It would make more sense to wait until you are
> > actually about to start hitting oom.
> 
> I think the shrinker makes sense for free page hinting feature
> (everything on free_page_list).
> 
> So instead of only reverting, I think we should split it up and always
> register the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT and the OOM
> notifier (as before) for VIRTIO_BALLOON_F_MUST_TELL_HOST.

OK ... I guess that means we need to fix shrinker to take
VIRTIO_BALLOON_F_MUST_TELL_HOST into account correctly.
Hosts ignore it at the moment but it's a fragile thing
to do what it does and ignore used buffers.

> (Of course, adapting what is being done in the shrinker and in the OOM
> notifier)
> 
> -- 
> Thanks,
> 
> David / dhildenb