On Wed, Nov 17, 2021 at 07:06:34PM +0900, David Stevens wrote:
> From: David Stevens <stevensd@xxxxxxxxxxxx>
>
> Add a module parameters to virtio_balloon to allow specifying whether or
> not the driver should call adjust_managed_page_count. If the parameter
> is set, it overrides the default behavior inferred from the deflate on
> OOM flag. This allows the balloon to operate without changing the amount
> of memory visible to userspace via /proc/meminfo or sysinfo, even on a
> system that cannot set the default on OOM flag.
>
> The motivation for this patch is to allow userspace to more accurately
> take advantage of virtio_balloon's cooperative memory control on a
> system without the ability to use deflate on OOM. As it stands,
> userspace has no way to know how much memory may be available on such a
> system, which makes tasks such as sizing caches impossible.
>
> When deflate on OOM is not enabled, the current behavior of the
> virtio_balloon more or less resembles hotplugging individual pages, at
> least from an accounting perspective. This is basically hardcoding the
> requirement that totalram_pages must be available to the guest
> immediately, regardless of what the host does. While that is a valid
> policy, on Linux (which supports memory overcommit) with virtio_balloon
> (which is designed to facilitate overcommit in the host), it is not the
> only possible policy.
>
> The param added by this patch allows the guest to operate under the
> assumption that pages in the virtio_balloon will generally be made
> available when needed. This assumption may not always hold, but when it
> is violated, the guest will just fall back to the normal mechanisms for
> dealing with overcommitted memory.
>
> Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> ---
>
> Another option to achieve similar behavior would be to add a new feature
> flag that would be used in conjunction with DEFLATE_ON_OOM to determine
> whether or not adjust_managed_page_count should be called. However, I
> feel that this sort of policy decision is a little outside the scope of
> what the virtio_balloon API deals with.
>
> ---
> drivers/virtio/virtio_balloon.c | 23 ++++++++++++++++++-----
> 1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index c22ff0117b46..17dac286899c 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -133,6 +133,21 @@ static const struct virtio_device_id id_table[] = {
> { 0 },
> };
>
> +static char *adjust_pages = "";
> +module_param(adjust_pages, charp, 0);
> +MODULE_PARM_DESC(adjust_pages, "Whether or not pages in the balloon should be removed from the managed page count");
> +
> +static bool should_adjust_pages(struct virtio_balloon *vb)
> +{
> + if (!strcmp(adjust_pages, "always"))
> + return true;
> + else if (!strcmp(adjust_pages, "never"))
> + return false;
> +
> + return !virtio_has_feature(vb->vdev,
> + VIRTIO_BALLOON_F_DEFLATE_ON_OOM);
> +}
> +
> static u32 page_to_balloon_pfn(struct page *page)
> {
> unsigned long pfn = page_to_pfn(page);
> @@ -243,8 +258,7 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>
> set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> - if (!virtio_has_feature(vb->vdev,
> - VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> + if (should_adjust_pages(vb))
> adjust_managed_page_count(page, -1);
> vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
> }
> @@ -264,8 +278,7 @@ static void release_pages_balloon(struct virtio_balloon *vb,
> struct page *page, *next;
>
> list_for_each_entry_safe(page, next, pages, lru) {
> - if (!virtio_has_feature(vb->vdev,
> - VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> + if (should_adjust_pages(vb))
> adjust_managed_page_count(page, 1);
> list_del(&page->lru);
> put_page(page); /* balloon reference */
> @@ -775,7 +788,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
> * managed page count when inflating, we have to fixup the count of
> * both involved zones.
> */
> - if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM) &&
> + if (should_adjust_pages(vb) &&
> page_zone(page) != page_zone(newpage)) {
> adjust_managed_page_count(page, 1);
> adjust_managed_page_count(newpage, -1);
A problem here is that the host also cares: IIUC
with VIRTIO_BALLOON_F_STATS_VQ we are sending the info
about page counts to host, changing what the stats
mean.
So I suspect we need a device feature for this at least
to control this aspect.
> --
> 2.34.0.rc2.393.gf8c9666880-goog
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