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) -- Thanks, David / dhildenb
