Linux Virtualization - Aug 2018 - [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker

On Fri 27-07-18 17:24:55, Wei Wang wrote:
> The OOM notifier is getting deprecated to use for the reasons mentioned
> here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
> 
> This patch replaces the virtio-balloon oom notifier with a shrinker
> to release balloon pages on memory pressure.
It would be great to document the replacement. This is not a small
change...
> In addition, the bug in the replaced virtballoon_oom_notify that only
> VIRTIO_BALLOON_ARRAY_PFNS_MAX (i.e 256) balloon pages can be freed
> though the user has specified more than that number is fixed in the
> shrinker_scan function.
> 
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> ---
>  drivers/virtio/virtio_balloon.c | 115
+++++++++++++++++++++++-----------------
>  1 file changed, 65 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index 9356a1a..6b2229b 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -27,7 +27,6 @@
>  #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>
> @@ -40,12 +39,12 @@
>   */
>  #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >>
VIRTIO_BALLOON_PFN_SHIFT)
>  #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> -#define OOM_VBALLOON_DEFAULT_PAGES 256
> +#define DEFAULT_BALLOON_PAGES_TO_SHRINK 256
>  #define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
>  
> -static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
> -module_param(oom_pages, int, S_IRUSR | S_IWUSR);
> -MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
> +static unsigned long balloon_pages_to_shrink =
DEFAULT_BALLOON_PAGES_TO_SHRINK;
> +module_param(balloon_pages_to_shrink, ulong, 0600);
> +MODULE_PARM_DESC(balloon_pages_to_shrink, "pages to free on memory
presure");
>  
>  #ifdef CONFIG_BALLOON_COMPACTION
>  static struct vfsmount *balloon_mnt;
> @@ -86,8 +85,8 @@ struct virtio_balloon {
>  	/* Memory statistics */
>  	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
>  
> -	/* To register callback in oom notifier call chain */
> -	struct notifier_block nb;
> +	/* To register a shrinker to shrink memory upon memory pressure */
> +	struct shrinker shrinker;
>  };
>  
>  static struct virtio_device_id id_table[] = {
> @@ -365,38 +364,6 @@ static void update_balloon_size(struct virtio_balloon
*vb)
>  		      &actual);
>  }
>  
> -/*
> - * virtballoon_oom_notify - release pages when system is under severe
> - *			    memory pressure (called from out_of_memory())
> - * @self : notifier block struct
> - * @dummy: not used
> - * @parm : returned - number of freed pages
> - *
> - * The balancing of memory by use of the virtio balloon should not cause
> - * the termination of processes while there are pages in the balloon.
> - * If virtio balloon manages to release some memory, it will make the
> - * system return and retry the allocation that forced the OOM killer
> - * to run.
> - */
> -static int virtballoon_oom_notify(struct notifier_block *self,
> -				  unsigned long dummy, void *parm)
> -{
> -	struct virtio_balloon *vb;
> -	unsigned long *freed;
> -	unsigned num_freed_pages;
> -
> -	vb = container_of(self, struct virtio_balloon, nb);
> -	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> -		return NOTIFY_OK;
> -
> -	freed = parm;
> -	num_freed_pages = leak_balloon(vb, oom_pages);
> -	update_balloon_size(vb);
> -	*freed += num_freed_pages;
> -
> -	return NOTIFY_OK;
> -}
> -
>  static void update_balloon_stats_func(struct work_struct *work)
>  {
>  	struct virtio_balloon *vb;
> @@ -548,6 +515,54 @@ static struct file_system_type balloon_fs = {
>  
>  #endif /* CONFIG_BALLOON_COMPACTION */
>  
> +static unsigned long virtio_balloon_shrinker_scan(struct shrinker
*shrinker,
> +						  struct shrink_control *sc)
> +{
> +	unsigned long pages_to_free = balloon_pages_to_shrink,
> +		      pages_freed = 0;
> +	struct virtio_balloon *vb = container_of(shrinker,
> +					struct virtio_balloon, shrinker);
> +
> +	/*
> +	 * 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
> +	 * balloon_pages_to_shrink pages.
> +	 */
> +	while (vb->num_pages && pages_to_free) {
> +		pages_to_free = balloon_pages_to_shrink - pages_freed;
> +		pages_freed += leak_balloon(vb, pages_to_free);
> +	}
> +	update_balloon_size(vb);
> +
> +	return pages_freed / VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
> +static unsigned long virtio_balloon_shrinker_count(struct shrinker
*shrinker,
> +						   struct shrink_control *sc)
> +{
> +	struct virtio_balloon *vb = container_of(shrinker,
> +					struct virtio_balloon, shrinker);
> +
> +	return min_t(unsigned long, vb->num_pages, balloon_pages_to_shrink) /
> +	       VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
> +static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb)
> +{
> +	unregister_shrinker(&vb->shrinker);
> +}
> +
> +static int virtio_balloon_register_shrinker(struct virtio_balloon *vb)
> +{
> +	vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
> +	vb->shrinker.count_objects = virtio_balloon_shrinker_count;
> +	vb->shrinker.batch = 0;
> +	vb->shrinker.seeks = DEFAULT_SEEKS;
> +
> +	return register_shrinker(&vb->shrinker);
> +}
> +
>  static int virtballoon_probe(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb;
> @@ -580,17 +595,10 @@ static int virtballoon_probe(struct virtio_device
*vdev)
>  	if (err)
>  		goto out_free_vb;
>  
> -	vb->nb.notifier_call = virtballoon_oom_notify;
> -	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
> -	err = register_oom_notifier(&vb->nb);
> -	if (err < 0)
> -		goto out_del_vqs;
> -
>  #ifdef CONFIG_BALLOON_COMPACTION
>  	balloon_mnt = kern_mount(&balloon_fs);
>  	if (IS_ERR(balloon_mnt)) {
>  		err = PTR_ERR(balloon_mnt);
> -		unregister_oom_notifier(&vb->nb);
>  		goto out_del_vqs;
>  	}
>  
> @@ -599,13 +607,20 @@ static int virtballoon_probe(struct virtio_device
*vdev)
>  	if (IS_ERR(vb->vb_dev_info.inode)) {
>  		err = PTR_ERR(vb->vb_dev_info.inode);
>  		kern_unmount(balloon_mnt);
> -		unregister_oom_notifier(&vb->nb);
>  		vb->vb_dev_info.inode = NULL;
>  		goto out_del_vqs;
>  	}
>  	vb->vb_dev_info.inode->i_mapping->a_ops = &balloon_aops;
>  #endif
> -
> +	/*
> +	 * 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_vqs;
> +	}
>  	virtio_device_ready(vdev);
>  
>  	if (towards_target(vb))
> @@ -637,8 +652,8 @@ static void virtballoon_remove(struct virtio_device
*vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> -	unregister_oom_notifier(&vb->nb);
> -
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		virtio_balloon_unregister_shrinker(vb);
>  	spin_lock_irq(&vb->stop_update_lock);
>  	vb->stop_update = true;
>  	spin_unlock_irq(&vb->stop_update_lock);
> -- 
> 2.7.4
-- 
Michal Hocko
SUSE Labs

On 07/30/2018 05:00 PM, Michal Hocko wrote:
> On Fri 27-07-18 17:24:55, Wei Wang wrote:
>> The OOM notifier is getting deprecated to use for the reasons mentioned
>> here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
>>
>> This patch replaces the virtio-balloon oom notifier with a shrinker
>> to release balloon pages on memory pressure.
> It would be great to document the replacement. This is not a small
> change...
OK. I plan to document the following to the commit log:

   The OOM notifier is getting deprecated to use for the reasons:
     - As a callout from the oom context, it is too subtle and easy to
       generate bugs and corner cases which are hard to track;
     - It is called too late (after the reclaiming has been performed).
       Drivers with large amuont of reclaimable memory is expected to be
       released them at an early age of memory pressure;
     - The notifier callback isn't aware of the oom contrains;
     Link: https://lkml.org/lkml/2018/7/12/314

     This patch replaces the virtio-balloon oom notifier with a shrinker
     to release balloon pages on memory pressure. Users can set the 
amount of
     memory pages to release each time a shrinker_scan is called via the
     module parameter balloon_pages_to_shrink, and the default amount is 256
     pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has
     been used to release balloon pages on OOM. We continue to use this
     feature bit for the shrinker, so the shrinker is only registered when
     this feature bit has been negotiated with host.

     In addition, the bug in the replaced virtballoon_oom_notify that only
     VIRTIO_BALLOON_ARRAY_PFNS_MAX (i.e 256) balloon pages can be freed
     though the user has specified more than that number is fixed in the
     shrinker_scan function.


Best,
Wei

On Wed 01-08-18 19:12:25, Wei Wang wrote:
> On 07/30/2018 05:00 PM, Michal Hocko wrote:
> > On Fri 27-07-18 17:24:55, Wei Wang wrote:
> > > The OOM notifier is getting deprecated to use for the reasons
mentioned
> > > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
> > > 
> > > This patch replaces the virtio-balloon oom notifier with a
shrinker
> > > to release balloon pages on memory pressure.
> > It would be great to document the replacement. This is not a small
> > change...
> 
> OK. I plan to document the following to the commit log:
> 
>   The OOM notifier is getting deprecated to use for the reasons:
>     - As a callout from the oom context, it is too subtle and easy to
>       generate bugs and corner cases which are hard to track;
>     - It is called too late (after the reclaiming has been performed).
>       Drivers with large amuont of reclaimable memory is expected to be
>       released them at an early age of memory pressure;
>     - The notifier callback isn't aware of the oom contrains;
>     Link: https://lkml.org/lkml/2018/7/12/314
> 
>     This patch replaces the virtio-balloon oom notifier with a shrinker
>     to release balloon pages on memory pressure. Users can set the amount
of
>     memory pages to release each time a shrinker_scan is called via the
>     module parameter balloon_pages_to_shrink, and the default amount is 256
>     pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has
>     been used to release balloon pages on OOM. We continue to use this
>     feature bit for the shrinker, so the shrinker is only registered when
>     this feature bit has been negotiated with host.
Do you have any numbers for how does this work in practice? Let's say
you have a medium page cache workload which triggers kswapd to do a
light reclaim? Hardcoded shrinking sounds quite dubious to me but I have
no idea how people expect this to work. Shouldn't this be more
adaptive? How precious are those pages anyway?
-- 
Michal Hocko
SUSE Labs