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@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. 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
