Re: [PATCH v3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

[David Rientjes]

On Tue, 10 Mar 2020, 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 other slab cache that should be
> shrunk when under memory pressure. The shrinker does not have a concept of
> priorities yet, so this behavior cannot be configured. Eventually once
> that is in place, we might want to switch back after doing proper
> testing.
> 
> 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.
> 
> 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().
> 
> Testing done by Tyler for future reference:
>   Test setup: VM with 16 CPU, 64GB RAM. Running Debian 10. We have a 42
>   GB file full of random bytes that we continually cat to /dev/null.
>   This fills the page cache as the file is read. Meanwhile we trigger
>   the balloon to inflate, with a target size of 53 GB. This setup causes
>   the balloon inflation to pressure the page cache as the page cache is
>   also trying to grow. Afterwards we shrink the balloon back to zero (so
>   total deflate = total inflate).
> 
>   Without patch (kernel 4.19.0-5):
>   Inflation never reaches the target until we stop the "cat file >
>   /dev/null" process. Total inflation time was 542 seconds. The longest
>   period that made no net forward progress was 315 seconds (see attached
>   graph).
>   Result of "grep balloon /proc/vmstat" after the test:
>   balloon_inflate 154828377
>   balloon_deflate 154828377
> 
>   With patch (kernel 5.6.0-rc4+):
>   Total inflation duration was 63 seconds. No deflate-queue activity
>   occurs when pressuring the page-cache.
>   Result of "grep balloon /proc/vmstat" after the test:
>   balloon_inflate 12968539
>   balloon_deflate 12968539
> 
>   Conclusion: This patch fixes the issue. In the test it reduced
>   inflate/deflate activity by 12x, and reduced inflation time by 8.6x.
>   But more importantly, if we hadn't killed the "grep balloon
>   /proc/vmstat" process then, without the patch, the inflation process
>   would never reach the target.
> 
> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
> 
> Reported-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> Tested-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

Should this have:

Cc: stable@xxxxxxxxxxxxxxx # 4.19+

?