> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxx >>>>>> <mailto:david@xxxxxxxxxx>> 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
