Re: Balloon pressuring page cache

[Nadav Amit <namit@vmware.com>]

> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@redhat.com> 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.
> 
> (Of course, adapting what is being done in the shrinker and in the OOM
> notifier)

David,

Please keep me posted. I decided to adapt the same solution as the virtio
balloon for the VMware balloon. If the verdict is that this is damaging and
the OOM notifier should be used instead, I will submit patches to move to
OOM notifier as well.

Regards,
Nadav