Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 06, 2020 at 09:42:34AM +0100, David Hildenbrand wrote:
> On 06.02.20 08:40, Michael S. Tsirkin wrote:
> > On Wed, Feb 05, 2020 at 05:34:02PM +0100, David Hildenbrand wrote:
> >> Commit 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> >> changed the behavior when deflation happens automatically. Instead of
> >> deflating when called by the OOM handler, the shrinker is used.
> >>
> >> However, the balloon is not simply some slab cache that should be
> >> shrunk when under memory pressure. The shrinker does not have a concept of
> >> priorities, so this behavior cannot be configured.
> >>
> >> There was a report that this results in undesired side effects when
> >> inflating the balloon to shrink the page cache. [1]
> >> 	"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."
> >>
> >> The name "deflate on OOM" makes it pretty clear when deflation should
> >> happen - after other approaches to reclaim memory failed, not while
> >> reclaiming. This allows to minimize the footprint of a guest - memory
> >> will only be taken out of the balloon when really needed.
> >>
> >> Especially, a drop_slab() will result in the whole balloon getting
> >> deflated - undesired. While handling it via the OOM handler might not be
> >> perfect, it keeps existing behavior. If we want a different behavior, then
> >> we need a new feature bit and document it properly (although, there should
> >> be a clear use case and the intended effects should be well described).
> >>
> >> Keep using the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT, because
> >> this has no such side effects. Always register the shrinker with
> >> VIRTIO_BALLOON_F_FREE_PAGE_HINT now. We are always allowed to reuse free
> >> pages that are still to be processed by the guest. The hypervisor takes
> >> care of identifying and resolving possible races between processing a
> >> hinting request and the guest reusing a page.
> >>
> >> In contrast to pre commit 71994620bb25 ("virtio_balloon: replace oom
> >> notifier with shrinker"), don't add a moodule parameter to configure the
> >> number of pages to deflate on OOM. Can be re-added if really needed.
> > 
> > I agree. And to make this case even stronger:
> > 
> > The oom_pages module parameter was known to be broken: whatever its
> > value, we return at most VIRTIO_BALLOON_ARRAY_PFNS_MAX.  So module
> > parameter values > 256 never worked, and it seems highly unlikely that
> > freeing 1Mbyte on OOM is too aggressive.
> > There was a patch
> >  virtio-balloon: deflate up to oom_pages on OOM
> > by Wei Wang to try to fix it:
> > https://lore.kernel.org/r/1508500466-21165-3-git-send-email-wei.w.wang@intel.com
> > but this was dropped.
> 
> Makes sense. 1MB is usually good enough.
> 
> > 
> >> Also, pay attention that leak_balloon() returns the number of 4k pages -
> >> convert it properly in virtio_balloon_oom_notify().
> > 
> > Oh. So it was returning a wrong value originally (before 71994620bb25).
> > However what really matters for notifiers is whether the value is 0 -
> > whether we made progress. So it's cosmetic.
> 
> Yes, that's also my understanding.
> 
> > 
> >> Note1: using the OOM handler is frowned upon, but it really is what we
> >>        need for this feature.
> > 
> > Quite. However, I went back researching why we dropped the OOM notifier,
> > and found this:
> > 
> > https://lore.kernel.org/r/1508500466-21165-2-git-send-email-wei.w.wang@intel.com
> > 
> > To quote from there:
> > 
> > The balloon_lock was used to synchronize the access demand to elements
> > of struct virtio_balloon and its queue operations (please see commit
> > e22504296d). This prevents the concurrent run of the leak_balloon and
> > fill_balloon functions, thereby resulting in a deadlock issue on OOM:
> > 
> > fill_balloon: take balloon_lock and wait for OOM to get some memory;
> > oom_notify: release some inflated memory via leak_balloon();
> > leak_balloon: wait for balloon_lock to be released by fill_balloon.
> 
> fill_balloon does the allocation *before* taking the lock. tell_host()
> should not allocate memory AFAIR. So how could this ever happen?
> 
> Anyhow, we could simply work around this by doing a trylock in
> fill_balloon() and retrying in the caller. That should be easy. But I
> want to understand first, how something like that would even be possible.

Hmm it looks like you are right.  Sorry!


> >> Note2: without VIRTIO_BALLOON_F_MUST_TELL_HOST (iow, always with QEMU) we
> >>        could actually skip sending deflation requests to our hypervisor,
> >>        making the OOM path *very* simple. Besically freeing pages and
> >>        updating the balloon.
> > 
> > Well not exactly. !VIRTIO_BALLOON_F_MUST_TELL_HOST does not actually
> > mean "never tell host". It means "host will not discard pages in the
> > balloon, you can defer host notification until after use".
> > 
> > This was the original implementation:
> > 
> > +       if (vb->tell_host_first) {
> > +               tell_host(vb, vb->deflate_vq);
> > +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +       } else {
> > +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +               tell_host(vb, vb->deflate_vq);
> > +       }
> > +}
> > 
> > I don't know whether completely skipping host notifications
> > when !VIRTIO_BALLOON_F_MUST_TELL_HOST will break any hosts.
> 
> We discussed this already somewhere else, but here is again what I found.
> 
> commit bf50e69f63d21091e525185c3ae761412be0ba72
> Author: Dave Hansen <dave@linux.vnet.ibm.com>
> Date:   Thu Apr 7 10:43:25 2011 -0700
> 
>     virtio balloon: kill tell-host-first logic
> 
>     The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
>     feature bit.  Whenever the bit is set, the guest kernel must
>     always tell the host before we free pages back to the allocator.
>     Without this feature, we might free a page (and have another
>     user touch it) while the hypervisor is unprepared for it.
> 
>     But, if the bit is _not_ set, we are under no obligation to
>     reverse the order; we're under no obligation to do _anything_.
>     As of now, qemu-kvm defines the bit, but doesn't set it.

Well this is not what the spec says in the end.
To continue that commit message:

    This patch makes the "tell host first" logic the only case.  This
    should make everybody happy, and reduce the amount of untested or
    untestable code in the kernel.

you can try proposing the change to the virtio TC, see what do others
think.


> MUST_TELL_HOST really means "no need to deflate, just reuse a page". We
> should finally document this somewhere.

I'm not sure it's not too late to change what that flag means.  If not
sending deflate messages at all is a useful optimization, it seems
safer to add a feature flag for that.

> -- 
> Thanks,
> 
> David / dhildenb