Re: Balloon pressuring page cache

[David Hildenbrand <david@redhat.com>]

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.

(Of course, adapting what is being done in the shrinker and in the OOM
notifier)

-- 
Thanks,

David / dhildenb