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SubjectRe: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM
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.
> 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.
>
> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
>
> Reported-by: Tyler Sanderson <tysand@google.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Wei Wang <wei.w.wang@intel.com>
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Nadav Amit <namit@vmware.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>


I guess we should add a Fixes tag to the patch it's reverting,
this way it's backported and hypervisors will be able to rely on OOM
behaviour.

> ---
> drivers/virtio/virtio_balloon.c | 107 +++++++++++++-------------------
> 1 file changed, 44 insertions(+), 63 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 7e5d84caeb94..e7b18f556c5e 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -14,6 +14,7 @@
> #include <linux/slab.h>
> #include <linux/module.h>
> #include <linux/balloon_compaction.h>
> +#include <linux/oom.h>
> #include <linux/wait.h>
> #include <linux/mm.h>
> #include <linux/mount.h>
> @@ -27,7 +28,9 @@
> */
> #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
> #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> -#define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> +/* Maximum number of (4k) pages to deflate on OOM notifications. */
> +#define VIRTIO_BALLOON_OOM_NR_PAGES 256
> +#define VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY 80
>
> #define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \
> __GFP_NOMEMALLOC)
> @@ -112,8 +115,11 @@ struct virtio_balloon {
> /* Memory statistics */
> struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
>
> - /* To register a shrinker to shrink memory upon memory pressure */
> + /* Shrinker to return free pages - VIRTIO_BALLOON_F_FREE_PAGE_HINT */
> struct shrinker shrinker;
> +
> + /* OOM notifier to deflate on OOM - VIRTIO_BALLOON_F_DEFLATE_ON_OOM */
> + struct notifier_block oom_nb;
> };
>
> static struct virtio_device_id id_table[] = {
> @@ -786,50 +792,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb,
> return blocks_freed * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
> }
>
> -static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> - unsigned long pages_to_free)
> -{
> - return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) /
> - VIRTIO_BALLOON_PAGES_PER_PAGE;
> -}
> -
> -static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
> - unsigned long pages_to_free)
> -{
> - unsigned long pages_freed = 0;
> -
> - /*
> - * One invocation of leak_balloon can deflate at most
> - * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
> - * multiple times to deflate pages till reaching pages_to_free.
> - */
> - while (vb->num_pages && pages_freed < pages_to_free)
> - pages_freed += leak_balloon_pages(vb,
> - pages_to_free - pages_freed);
> -
> - update_balloon_size(vb);
> -
> - return pages_freed;
> -}
> -
> static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
> struct shrink_control *sc)
> {
> - unsigned long pages_to_free, pages_freed = 0;
> struct virtio_balloon *vb = container_of(shrinker,
> struct virtio_balloon, shrinker);
>
> - pages_to_free = sc->nr_to_scan;
> -
> - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> - pages_freed = shrink_free_pages(vb, pages_to_free);
> -
> - if (pages_freed >= pages_to_free)
> - return pages_freed;
> -
> - pages_freed += shrink_balloon_pages(vb, pages_to_free - pages_freed);
> -
> - return pages_freed;
> + return shrink_free_pages(vb, sc->nr_to_scan);
> }
>
> static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
> @@ -837,26 +806,22 @@ static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
> {
> struct virtio_balloon *vb = container_of(shrinker,
> struct virtio_balloon, shrinker);
> - unsigned long count;
> -
> - count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> - count += vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
>
> - return count;
> + return vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
> }
>
> -static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb)
> +static int virtio_balloon_oom_notify(struct notifier_block *nb,
> + unsigned long dummy, void *parm)
> {
> - unregister_shrinker(&vb->shrinker);
> -}
> + struct virtio_balloon *vb = container_of(nb,
> + struct virtio_balloon, oom_nb);
> + unsigned long *freed = parm;
>
> -static int virtio_balloon_register_shrinker(struct virtio_balloon *vb)
> -{
> - vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
> - vb->shrinker.count_objects = virtio_balloon_shrinker_count;
> - vb->shrinker.seeks = DEFAULT_SEEKS;
> + *freed += leak_balloon(vb, VIRTIO_BALLOON_OOM_NR_PAGES) /
> + VIRTIO_BALLOON_PAGES_PER_PAGE;
> + update_balloon_size(vb);
>
> - return register_shrinker(&vb->shrinker);
> + return NOTIFY_OK;
> }
>
> static int virtballoon_probe(struct virtio_device *vdev)
> @@ -933,22 +898,35 @@ static int virtballoon_probe(struct virtio_device *vdev)
> virtio_cwrite(vb->vdev, struct virtio_balloon_config,
> poison_val, &poison_val);
> }
> - }
> - /*
> - * We continue to use VIRTIO_BALLOON_F_DEFLATE_ON_OOM to decide if a
> - * shrinker needs to be registered to relieve memory pressure.
> - */
> - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
> - err = virtio_balloon_register_shrinker(vb);
> +
> + /*
> + * We're allowed to reuse any free pages, even if they are
> + * still to be processed by the host.
> + */
> + vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
> + vb->shrinker.count_objects = virtio_balloon_shrinker_count;
> + vb->shrinker.seeks = DEFAULT_SEEKS;
> + err = register_shrinker(&vb->shrinker);
> if (err)
> goto out_del_balloon_wq;
> }
> + if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
> + vb->oom_nb.notifier_call = virtio_balloon_oom_notify;
> + vb->oom_nb.priority = VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY;
> + err = register_oom_notifier(&vb->oom_nb);
> + if (err < 0)
> + goto out_unregister_shrinker;
> + }
> +
> virtio_device_ready(vdev);
>
> if (towards_target(vb))
> virtballoon_changed(vdev);
> return 0;
>
> +out_unregister_shrinker:
> + if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> + unregister_shrinker(&vb->shrinker);
> out_del_balloon_wq:
> if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> destroy_workqueue(vb->balloon_wq);
> @@ -987,8 +965,11 @@ static void virtballoon_remove(struct virtio_device *vdev)
> {
> struct virtio_balloon *vb = vdev->priv;
>
> - if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> - virtio_balloon_unregister_shrinker(vb);
> + if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> + unregister_oom_notifier(&vb->oom_nb);
> + if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> + unregister_shrinker(&vb->shrinker);
> +
> spin_lock_irq(&vb->stop_update_lock);
> vb->stop_update = true;
> spin_unlock_irq(&vb->stop_update_lock);
> --
> 2.24.1

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