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Date:   Thu, 6 Feb 2020 04:04:21 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Wei Wang <wei.w.wang@intel.com>
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, tysand@google.com,
        david@redhat.com, alexander.h.duyck@linux.intel.com,
        rientjes@google.com, mhocko@kernel.org, namit@vmware.com,
        penguin-kernel@i-love.sakura.ne.jp
Subject: Re: [PATCH RFC] virtio_balloon: conservative balloon page shrinking
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On Thu, Feb 06, 2020 at 04:01:47PM +0800, Wei Wang wrote:
> There are cases that users want to shrink balloon pages after the
> pagecache depleted. The conservative_shrinker lets the shrinker
> shrink balloon pages when all the pagecache has been reclaimed.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>

I'd rather avoid module parameters, but otherwise looks 
like a reasonable idea.
Tyler, what do you think?


> ---
>  drivers/virtio/virtio_balloon.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 93f995f6cf36..b4c5bb13a867 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -42,6 +42,10 @@
>  static struct vfsmount *balloon_mnt;
>  #endif
>  
> +static bool conservative_shrinker = true;
> +module_param(conservative_shrinker, bool, 0644);
> +MODULE_PARM_DESC(conservative_shrinker, "conservatively shrink balloon pages");
> +
>  enum virtio_balloon_vq {
>  	VIRTIO_BALLOON_VQ_INFLATE,
>  	VIRTIO_BALLOON_VQ_DEFLATE,
> @@ -796,6 +800,10 @@ static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>  {
>  	unsigned long pages_freed = 0;
>  
> +	/* Balloon pages only gets shrunk when the pagecache depleted */
> +	if (conservative_shrinker && global_node_page_state(NR_FILE_PAGES))
> +		return 0;
> +
>  	/*
>  	 * One invocation of leak_balloon can deflate at most
>  	 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
> @@ -837,7 +845,11 @@ static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
>  					struct virtio_balloon, shrinker);
>  	unsigned long count;
>  
> -	count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> +	if (conservative_shrinker && global_node_page_state(NR_FILE_PAGES))

I'd rather have an API for that in mm/. In particular, do we want other
shrinkers to run, not just pagecache? To pick an example I'm familiar
with, kvm mmu cache for nested virt?

> +		count = 0;
> +	else
> +		count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> +
>  	count += vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
>  
>  	return count;
> -- 
> 2.17.1