virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM
Commit 71994620 ("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 71994620 ("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 Test report by Tyler Sanderson: 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. Attached [1] is a png of a graph showing the problematic behavior without this patch. It shows deflate-queue activity increasing linearly while balloon size stays constant over the course of more than 8 minutes of the test. [1] https://lore.kernel.org/linux-mm/CAJuQAmphPcfew1v_EOgAdSFiprzjiZjmOf3iJDmFX0gD6b9TYQ@mail.gmail.com/2-without_patch.png Full test report and discussion [2]: [2] https://lore.kernel.org/r/CAJuQAmphPcfew1v_EOgAdSFiprzjiZjmOf3iJDmFX0gD6b9TYQ@mail.gmail.comTested-by:Tyler Sanderson <tysand@google.com> Reported-by:
David Hildenbrand <david@redhat.com> Link: https://lore.kernel.org/r/20200205163402.42627-4-david@redhat.comSigned-off-by:
Michael S. Tsirkin <mst@redhat.com>
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