From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
virtualization@lists.linux-foundation.org,
Tyler Sanderson <tysand@google.com>,
Wei Wang <wei.w.wang@intel.com>,
Alexander Duyck <alexander.h.duyck@linux.intel.com>,
David Rientjes <rientjes@google.com>,
Nadav Amit <namit@vmware.com>, Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM
Date: Sun, 16 Feb 2020 04:47:12 -0500 [thread overview]
Message-ID: <20200216044641-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <f31eff75-b328-de41-c2cc-e55471aa27d8@redhat.com>
On Fri, Feb 14, 2020 at 10:51:43AM +0100, David Hildenbrand wrote:
> On 05.02.20 17:34, 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.
> > Also, pay attention that leak_balloon() returns the number of 4k pages -
> > convert it properly in virtio_balloon_oom_notify().
> >
> > Note1: using the OOM handler is frowned upon, but it really is what we
> > need for this feature.
> >
> > 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. If the communication with the host ever
> > becomes a problem on this call path.
> >
>
> @Michael, how to proceed with this?
>
I'd like to see some reports that this helps people.
e.g. a tested-by tag.
> --
> Thanks,
>
> David / dhildenb
next prev parent reply other threads:[~2020-02-16 9:47 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-05 16:33 [PATCH v1 0/3] virtio-balloon: Fixes + switch back to OOM handler David Hildenbrand
2020-02-05 16:34 ` [PATCH v1 1/3] virtio-balloon: Fix memory leak when unloading while hinting is in progress David Hildenbrand
2020-02-06 8:36 ` Michael S. Tsirkin
2020-02-05 16:34 ` [PATCH v1 2/3] virtio_balloon: Fix memory leaks on errors in virtballoon_probe() David Hildenbrand
2020-02-06 8:36 ` Michael S. Tsirkin
2020-02-05 16:34 ` [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM David Hildenbrand
2020-02-05 22:37 ` Tyler Sanderson
2020-02-05 22:52 ` David Hildenbrand
2020-02-05 23:06 ` Tyler Sanderson
2020-02-06 7:40 ` Michael S. Tsirkin
2020-02-06 8:42 ` David Hildenbrand
2020-02-06 8:57 ` Michael S. Tsirkin
2020-02-06 9:05 ` David Hildenbrand
2020-02-06 9:09 ` Michael S. Tsirkin
2020-02-06 8:57 ` Wang, Wei W
2020-02-06 9:11 ` Michael S. Tsirkin
2020-02-06 9:12 ` Michael S. Tsirkin
2020-02-06 9:21 ` David Hildenbrand
2020-02-14 9:51 ` David Hildenbrand
2020-02-14 13:31 ` Wang, Wei W
2020-02-16 9:47 ` Michael S. Tsirkin [this message]
2020-02-21 3:29 ` Tyler Sanderson
2020-03-08 4:47 ` Tyler Sanderson
2020-03-09 9:03 ` David Hildenbrand
2020-03-09 10:14 ` Michael S. Tsirkin
2020-03-09 10:59 ` David Hildenbrand
2020-03-09 10:24 ` Michael S. Tsirkin
2020-02-14 14:06 ` Michal Hocko
2020-02-14 14:18 ` David Hildenbrand
2020-02-14 20:48 ` Tyler Sanderson
2020-02-14 21:17 ` David Hildenbrand
2020-02-16 9:46 ` Michael S. Tsirkin
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