RE: Balloon pressuring page cache
From: Wang, Wei W <hidden>
Date: 2020-02-05 06:49:32
Also in:
linux-mm
Possibly related (same subject, not in this thread)
- 2020-02-06 · Re: Balloon pressuring page cache · David Hildenbrand <hidden>
- 2020-02-05 · Re: Balloon pressuring page cache · Tyler Sanderson <hidden>
- 2020-02-03 · Re: Balloon pressuring page cache · Tyler Sanderson <hidden>
- 2020-02-03 · Re: Balloon pressuring page cache · "Michael S. Tsirkin" <mst@redhat.com>
- 2020-02-02 · Re: Balloon pressuring page cache · David Rientjes via Virtualization <hidden>
On Tuesday, February 4, 2020 10:30 PM, David Hildenbrand wrote:
So, I am trying to understand how the code is intended to work, but I am afraid I am missing something (or to rephrase: I think I found a BUG :) and there is lack of proper documentation about this feature). a) We allocate pages and add them to the list as long as we are told to do so. We send these pages to the host one by one. b) We free all pages once we get a STOP signal. Until then, we keep pages allocated.
Yes. Either host sends to the guest a STOP cmd or when the guest fails to allocate a page (meaning that all the possible free pages are taken already), the reporting ends.
quoted hunk ↗ jump to hunk
c) When called via the shrinker, we want to free pages from the list, even though the hypervisor did not notify us to do so. Issue 1: When we unload the balloon driver in the guest in an unlucky event, we won't free the pages. We are missing something like (if I am not wrong):diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c index b1d2068fa2bd..e2b0925e1e83 100644 --- a/drivers/virtio/virtio_balloon.c +++ b/drivers/virtio/virtio_balloon.c@@ -929,6 +929,10 @@ static void remove_common(struct virtio_balloon*vb) leak_balloon(vb, vb->num_pages); update_balloon_size(vb); + /* There might be free pages that are being reported: release them. */ + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) + return_free_pages_to_mm(vb, ULONG_MAX); + /* Now we reset the device so we can clean up the queues. */ vb->vdev->config->reset(vb->vdev);
Right, thanks!
Issue 2: When called via the shrinker, (but also to fix Issue 1), it could be that we do have VIRTIO_BALLOON_F_MUST_TELL_HOST.
I don't think it is an issue here. MUST_TELL_HOST is for the ballooning pages, where pages are offered to host to _USE_. For free page hint, as the name already suggests, it's just a _HINT_ , so in whatever use case, the host should not take the page to use. So the guest doesn't need to tell host and wait. Back to the implementation of virtio_balloon_shrinker_scan, which I don't see an issue so far: shrink_free_pages just return pages to mm without waiting for the ack from host shrink_balloon_pages goes through leak_balloon which tell_host before release the balloon pages. Best, Wei