Virtualization - Nov 2021 - [RFC PATCH] virtio_balloon: add param to skip adjusting pages

On Wed, Nov 17, 2021 at 07:06:34PM +0900, David Stevens
wrote:
> From: David Stevens <stevensd at chromium.org>
> 
> 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 at chromium.org>
> ---
> 
> 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