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 at 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 at redhat.com >>> > > <mailto:david at 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
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 at 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 at redhat.com > >>> > > <mailto:david at 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.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
There were apparently good reasons for moving away from OOM notifier callback: https://lkml.org/lkml/2018/7/12/314 https://lkml.org/lkml/2018/8/2/322 In particular the OOM notifier is worse than the shrinker because: 1. It is last-resort, which means the system has already gone through heroics to prevent OOM. Those heroic reclaim efforts are expensive and impact application performance. 2. It lacks understanding of NUMA or other OOM constraints. 3. It has a higher potential for bugs due to the subtlety of the callback context. Given the above, I think the shrinker API certainly makes the most sense _if_ the balloon size is static. In that case memory should be reclaimed from the balloon early and proportionally to balloon size, which the shrinker API achieves. However, if the balloon is inflating and intentionally causing memory pressure then this results in the inefficiency pointed out earlier. If the balloon is inflating but not causing memory pressure then there is no problem with either API. This suggests another route: rather than cause memory pressure to shrink the page cache, the balloon could issue the equivalent of "echo 3 > /proc/sys/vm/drop_caches". Of course ideally, we want to be more fine grained than "drop everything". We really want an API that says "drop everything that hasn't been accessed in the last 5 minutes". This would eliminate the need for the balloon to cause memory pressure at all which avoids the inefficiency in question. Furthermore, this pairs nicely with the FREE_PAGE_HINT feature. On Mon, Feb 3, 2020 at 9:04 AM Michael S. Tsirkin <mst at redhat.com> wrote:> 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 at 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 at redhat.com > > >>> > > <mailto:david at 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. > > 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 > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20200203/6c3740dd/attachment-0001.html>
> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david at 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 at 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 at redhat.com >>>>>> <mailto:david at 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
On Mon, Feb 3, 2020 at 1:22 PM Alexander Duyck < alexander.h.duyck at linux.intel.com> wrote:> On Mon, 2020-02-03 at 12:32 -0800, Tyler Sanderson wrote: > > There were apparently good reasons for moving away from OOM notifier > > callback: > > https://lkml.org/lkml/2018/7/12/314 > > https://lkml.org/lkml/2018/8/2/322 > > > > In particular the OOM notifier is worse than the shrinker because: > > It is last-resort, which means the system has already gone through > > heroics to prevent OOM. Those heroic reclaim efforts are expensive and > > impact application performance. > > It lacks understanding of NUMA or other OOM constraints. > > It has a higher potential for bugs due to the subtlety of the callback > > context. > > Given the above, I think the shrinker API certainly makes the most sense > > _if_ the balloon size is static. In that case memory should be reclaimed > > from the balloon early and proportionally to balloon size, which the > > shrinker API achieves. > > The problem is the shrinker doesn't have any concept of tiering or > priority. I suspect he reason for using the OOM notification is because in > practice it should be the last thing we are pulling memory out of with > things like page cache and slab caches being first. Once we have pages > that are leaked out of the balloon by the shrinker it will trigger the > balloon wanting to reinflate.Deciding whether to trade IO performance (page cache) for memory-usage efficiency (balloon) seems use-case dependent. Deciding when to re-inflate is a similar policy choice. If the balloon's shrinker priority is hard-coded to "last-resort" then there would be no way to implement a policy where page cache growth could shrink the balloon. The current balloon implementation allows the host to implement this policy and tune the tradeoff between balloon and page cache.> Ideally if the shrinker is running we > shouldn't be able to reinflate the balloon, and if we are reinflating the > balloon we shouldn't need to run the shrinker. The fact that we can do > both at the same time is problematic. >I agree that this is inefficient.> > > However, if the balloon is inflating and intentionally causing memory > > pressure then this results in the inefficiency pointed out earlier. > > > > If the balloon is inflating but not causing memory pressure then there > > is no problem with either API. > > The entire point of the balloon is to cause memory pressure. Otherwise > essentially all we are really doing is hinting since the guest doesn't > need the memory and isn't going to use it any time soon. >Causing memory pressure is just a mechanism to achieve increased reclaim. If there was a better mechanism (like the fine-grained-cache-shrinking one discussed below) then I think the balloon device would be perfectly justified in using that instead (and maybe "balloon" becomes a misnomer. Oh well).> > > This suggests another route: rather than cause memory pressure to shrink > > the page cache, the balloon could issue the equivalent of "echo 3 > > > /proc/sys/vm/drop_caches". > > Of course ideally, we want to be more fine grained than "drop > > everything". We really want an API that says "drop everything that > > hasn't been accessed in the last 5 minutes". > > > > This would eliminate the need for the balloon to cause memory pressure > > at all which avoids the inefficiency in question. Furthermore, this > > pairs nicely with the FREE_PAGE_HINT feature. > > Something similar was brought up in the discussion we had about this in my > patch set. The problem is, by trying to use a value like "5 minutes" it > implies that we are going to need to track some extra state somewhere to > determine that value. > > An alternative is to essentially just slowly shrink memory for the guest. > We had some discussion about this in another thread, and the following > code example was brought up as a way to go about doing that: > > https://github.com/Conan-Kudo/omv-kernel-rc/blob/master/0154-sysctl-vm-Fine-grained-cache-shrinking.patch > > The idea is you essentially just slowly bleed the memory from the guest by > specifying some amount of MB of cache to be freed on some regular > interval. >Makes sense. Whatever API is settled on, I'd just propose that we allow the host to invoke it via the balloon device since the host has a host-global view of memory and can make decisions that an individual guest cannot. Alex, what is the status of your fine-grained-cache-shrinking patch? It seems like a really good idea.> Thanks. > > - Alex > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20200203/bcb47daf/attachment.html>
>>> 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.s/VIRTIO_BALLOON_F_MUST_TELL_HOST/VIRTIO_BALLOON_F_DEFLATE_ON_OOM/ :)>> >> (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.Will do. It all sounds sub-optimal to me at this point ... but I prefer the old variant where a simple "drop_slab()" won't deflate the balloon. That looks broken to me. -- Thanks, David / dhildenb
> On Feb 3, 2020, at 2:50 PM, Nadav Amit <namit at vmware.com> wrote: > >> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david at 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 at 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 at redhat.com >>>>>>> <mailto:david at 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.Adding some information for the record (if someone googles this thread): In the VMware balloon driver, the shrinker is disabled by default since we encountered a performance degradation in testing. I tried to avoid rapid inflation/shrinker-deflation cycles by adding a timeout, but apparently it did not help in avoiding the performance regression. So there is no such issue in VMware balloon driver, unless someone intentionally enables the shrinker through a module parameter.