Re: [PATCH v2] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

[Michael S. Tsirkin]

On Tue, Mar 10, 2020 at 11:39:03AM +0100, David Hildenbrand wrote:
> Commit 71994620bb25 ("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 other slab cache that should be
> shrunk when under memory pressure. The shrinker does not have a concept of
> priorities yet, so this behavior cannot be configured. Eventually once
> that is in place, we might want to switch back after doing proper
> testing.
> 
> 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.
> 
> 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 71994620bb25 ("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().
> 
> Testing done by Tyler for future reference:
>   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.
> 
> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
> 
> Reported-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> Tested-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---


A couple of nits in this version.

> v1 -> v2:
> - Rebase on top of linux-next (free page reporting)
> - Clarified some parts in the patch description and added testing
>   instructions/results
> - Added Fixes: and Tested-by:
> 
> As this patch is based on free page reporting, MST suggested to take this
> via Andrew's tree.
> 
> ---
>  drivers/virtio/virtio_balloon.c | 103 +++++++++++++++-----------------
>  1 file changed, 47 insertions(+), 56 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 8511d258dbb4..05c14b541623 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -14,6 +14,7 @@
>  #include <linux/slab.h>
>  #include <linux/module.h>
>  #include <linux/balloon_compaction.h>
> +#include <linux/oom.h>
>  #include <linux/wait.h>
>  #include <linux/mm.h>
>  #include <linux/mount.h>
> @@ -28,7 +29,9 @@
>   */
>  #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
>  #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> -#define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> +/* Maximum number of (4k) pages to deflate on OOM notifications. */
> +#define VIRTIO_BALLOON_OOM_NR_PAGES 256
> +#define VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY 80
>  
>  #define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \
>  					     __GFP_NOMEMALLOC)
> @@ -114,9 +117,12 @@ struct virtio_balloon {
>  	/* Memory statistics */
>  	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
>  
> -	/* To register a shrinker to shrink memory upon memory pressure */
> +	/* Shrinker to return free pages - VIRTIO_BALLOON_F_FREE_PAGE_HINT */
>  	struct shrinker shrinker;
>  
> +	/* OOM notifier to deflate on OOM - VIRTIO_BALLOON_F_DEFLATE_ON_OOM */
> +	struct notifier_block oom_nb;
> +
>  	/* Free page reporting device */
>  	struct virtqueue *reporting_vq;
>  	struct page_reporting_dev_info pr_dev_info;
> @@ -830,50 +836,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb,
>  	return blocks_freed * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
>  }
>  
> -static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> -                                          unsigned long pages_to_free)
> -{
> -	return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) /
> -		VIRTIO_BALLOON_PAGES_PER_PAGE;
> -}
> -
> -static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
> -					  unsigned long pages_to_free)
> -{
> -	unsigned long pages_freed = 0;
> -
> -	/*
> -	 * One invocation of leak_balloon can deflate at most
> -	 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
> -	 * multiple times to deflate pages till reaching pages_to_free.
> -	 */
> -	while (vb->num_pages && pages_freed < pages_to_free)
> -		pages_freed += leak_balloon_pages(vb,
> -						  pages_to_free - pages_freed);
> -
> -	update_balloon_size(vb);
> -
> -	return pages_freed;
> -}
> -
>  static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
>  						  struct shrink_control *sc)
>  {
> -	unsigned long pages_to_free, pages_freed = 0;
>  	struct virtio_balloon *vb = container_of(shrinker,
>  					struct virtio_balloon, shrinker);
>  
> -	pages_to_free = sc->nr_to_scan;
> -
> -	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> -		pages_freed = shrink_free_pages(vb, pages_to_free);
> -
> -	if (pages_freed >= pages_to_free)
> -		return pages_freed;
> -
> -	pages_freed += shrink_balloon_pages(vb, pages_to_free - pages_freed);
> -
> -	return pages_freed;
> +	return shrink_free_pages(vb, sc->nr_to_scan);
>  }
>  
>  static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
> @@ -881,12 +850,22 @@ static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
>  {
>  	struct virtio_balloon *vb = container_of(shrinker,
>  					struct virtio_balloon, shrinker);
> -	unsigned long count;
>  
> -	count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> -	count += vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
> +	return vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
> +}
>  
> -	return count;
> +static int virtio_balloon_oom_notify(struct notifier_block *nb,
> +				     unsigned long dummy, void *parm)
> +{
> +	struct virtio_balloon *vb = container_of(nb,
> +						 struct virtio_balloon, oom_nb);
> +	unsigned long *freed = parm;
> +
> +	*freed += leak_balloon(vb, VIRTIO_BALLOON_OOM_NR_PAGES) /
> +		  VIRTIO_BALLOON_PAGES_PER_PAGE;
> +	update_balloon_size(vb);
> +
> +	return NOTIFY_OK;
>  }
>  
>  static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb)
> @@ -971,7 +950,22 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  						  VIRTIO_BALLOON_CMD_ID_STOP);
>  		spin_lock_init(&vb->free_page_list_lock);
>  		INIT_LIST_HEAD(&vb->free_page_list);
> +		/*
> +		 * We're allowed to reuse any free pages, even if they are
> +		 * still to be processed by the host.
> +		 */
> +		err = virtio_balloon_register_shrinker(vb);
> +		if (err)
> +			goto out_del_balloon_wq;
>  	}
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
> +		vb->oom_nb.notifier_call = virtio_balloon_oom_notify;
> +		vb->oom_nb.priority = VIRTIO_BALLOON_OOM_NOTIFY_PRIORITY;
> +		err = register_oom_notifier(&vb->oom_nb);
> +		if (err < 0)
> +			goto out_unregister_shrinker;
> +	}
> +


Let's decide whether we want an empty line after }, or not, and stick to
it. I prefer an empty line but no biggie as long as we are consistent.

>  	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) {
>  		/* Start with poison val of 0 representing general init */
>  		__u32 poison_val = 0;
> @@ -986,15 +980,6 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		virtio_cwrite(vb->vdev, struct virtio_balloon_config,
>  			      poison_val, &poison_val);
>  	}
> -	/*
> -	 * We continue to use VIRTIO_BALLOON_F_DEFLATE_ON_OOM to decide if a
> -	 * shrinker needs to be registered to relieve memory pressure.
> -	 */
> -	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM)) {
> -		err = virtio_balloon_register_shrinker(vb);
> -		if (err)
> -			goto out_del_balloon_wq;
> -	}
>  
>  	vb->pr_dev_info.report = virtballoon_free_page_report;
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> @@ -1003,12 +988,12 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		capacity = virtqueue_get_vring_size(vb->reporting_vq);
>  		if (capacity < PAGE_REPORTING_CAPACITY) {
>  			err = -ENOSPC;
> -			goto out_unregister_shrinker;
> +			goto out_unregister_oom;
>  		}
>  
>  		err = page_reporting_register(&vb->pr_dev_info);
>  		if (err)
> -			goto out_unregister_shrinker;
> +			goto out_unregister_oom;
>  	}
>  
>  	virtio_device_ready(vdev);
> @@ -1017,8 +1002,11 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		virtballoon_changed(vdev);
>  	return 0;
>  
> +out_unregister_oom:
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		unregister_oom_notifier(&vb->oom_nb);
>  out_unregister_shrinker:
> -	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>  		virtio_balloon_unregister_shrinker(vb);


What's with vdev versus vb->vdev here?
I suggest we keep using vb->vdev to make the patch minimal if we can.
Same elsewhere.

>  out_del_balloon_wq:
>  	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> @@ -1061,7 +1049,10 @@ static void virtballoon_remove(struct virtio_device *vdev)
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
>  		page_reporting_unregister(&vb->pr_dev_info);
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		unregister_oom_notifier(&vb->oom_nb);
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>  		virtio_balloon_unregister_shrinker(vb);
> +

>  	spin_lock_irq(&vb->stop_update_lock);
>  	vb->stop_update = true;
>  	spin_unlock_irq(&vb->stop_update_lock);
> -- 
> 2.24.1