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Michael S. Tsirkin

2020-Feb-06 07:40 UTC

[PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

On Wed, Feb 05, 2020 at 05:34:02PM +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 slab cache that should be
> shrunk when under memory pressure. The shrinker does not have a concept of
> priorities, so this behavior cannot be configured.
> 
> 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.
> 
> Especially, a drop_slab() will result in the whole balloon getting
> deflated - undesired. While handling it via the OOM handler might not be
> perfect, it keeps existing behavior. If we want a different behavior, then
> we need a new feature bit and document it properly (although, there should
> be a clear use case and the intended effects should be well described).
> 
> 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.
I agree. And to make this case even stronger:

The oom_pages module parameter was known to be broken: whatever its
value, we return at most VIRTIO_BALLOON_ARRAY_PFNS_MAX.  So module
parameter values > 256 never worked, and it seems highly unlikely that
freeing 1Mbyte on OOM is too aggressive.
There was a patch
 virtio-balloon: deflate up to oom_pages on OOM
by Wei Wang to try to fix it:
https://lore.kernel.org/r/1508500466-21165-3-git-send-email-wei.w.wang at
intel.com
but this was dropped.
> Also, pay attention that leak_balloon() returns the number of 4k pages -
> convert it properly in virtio_balloon_oom_notify().
Oh. So it was returning a wrong value originally (before 71994620bb25).
However what really matters for notifiers is whether the value is 0 -
whether we made progress. So it's cosmetic.
> Note1: using the OOM handler is frowned upon, but it really is what we
>        need for this feature.
Quite. However, I went back researching why we dropped the OOM notifier,
and found this:

https://lore.kernel.org/r/1508500466-21165-2-git-send-email-wei.w.wang at
intel.com

To quote from there:

The balloon_lock was used to synchronize the access demand to elements
of struct virtio_balloon and its queue operations (please see commit
e22504296d). This prevents the concurrent run of the leak_balloon and
fill_balloon functions, thereby resulting in a deadlock issue on OOM:

fill_balloon: take balloon_lock and wait for OOM to get some memory;
oom_notify: release some inflated memory via leak_balloon();
leak_balloon: wait for balloon_lock to be released by fill_balloon.




> Note2: without VIRTIO_BALLOON_F_MUST_TELL_HOST (iow, always with QEMU) we
>        could actually skip sending deflation requests to our hypervisor,
>        making the OOM path *very* simple. Besically freeing pages and
>        updating the balloon.
Well not exactly. !VIRTIO_BALLOON_F_MUST_TELL_HOST does not actually
mean "never tell host". It means "host will not discard pages in
the
balloon, you can defer host notification until after use".

This was the original implementation:

+       if (vb->tell_host_first) {
+               tell_host(vb, vb->deflate_vq);
+               release_pages_by_pfn(vb->pfns, vb->num_pfns);
+       } else {
+               release_pages_by_pfn(vb->pfns, vb->num_pfns);
+               tell_host(vb, vb->deflate_vq);
+       }
+}

I don't know whether completely skipping host notifications
when !VIRTIO_BALLOON_F_MUST_TELL_HOST will break any hosts.
>	 If the communication with the host ever
>        becomes a problem on this call path.
> 
> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
> 
> Reported-by: Tyler Sanderson <tysand at google.com>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: Wei Wang <wei.w.wang at intel.com>
> Cc: Alexander Duyck <alexander.h.duyck at linux.intel.com>
> Cc: David Rientjes <rientjes at google.com>
> Cc: Nadav Amit <namit at vmware.com>
> Cc: Michal Hocko <mhocko at kernel.org>
> Signed-off-by: David Hildenbrand <david at redhat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 107 +++++++++++++-------------------
>  1 file changed, 44 insertions(+), 63 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index 7e5d84caeb94..e7b18f556c5e 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>
> @@ -27,7 +28,9 @@
>   */
>  #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)
> @@ -112,8 +115,11 @@ 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;
>  };
>  
>  static struct virtio_device_id id_table[] = {
> @@ -786,50 +792,13 @@ 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,
> @@ -837,26 +806,22 @@ 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 count;
> +	return vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
>  }
>  
> -static void virtio_balloon_unregister_shrinker(struct virtio_balloon *vb)
> +static int virtio_balloon_oom_notify(struct notifier_block *nb,
> +				     unsigned long dummy, void *parm)
>  {
> -	unregister_shrinker(&vb->shrinker);
> -}
> +	struct virtio_balloon *vb = container_of(nb,
> +						 struct virtio_balloon, oom_nb);
> +	unsigned long *freed = parm;
>  
> -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.seeks = DEFAULT_SEEKS;
> +	*freed += leak_balloon(vb, VIRTIO_BALLOON_OOM_NR_PAGES) /
> +		  VIRTIO_BALLOON_PAGES_PER_PAGE;
> +	update_balloon_size(vb);
>  
> -	return register_shrinker(&vb->shrinker);
> +	return NOTIFY_OK;
>  }
>  
>  static int virtballoon_probe(struct virtio_device *vdev)
> @@ -933,22 +898,35 @@ 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);
> +
> +		/*
> +		 * We're allowed to reuse any free pages, even if they are
> +		 * still to be processed by the host.
> +		 */
> +		vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
> +		vb->shrinker.count_objects = virtio_balloon_shrinker_count;
> +		vb->shrinker.seeks = DEFAULT_SEEKS;
> +		err = register_shrinker(&vb->shrinker);
>  		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;
> +	}
> +
>  	virtio_device_ready(vdev);
>  
>  	if (towards_target(vb))
>  		virtballoon_changed(vdev);
>  	return 0;
>  
> +out_unregister_shrinker:
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +		unregister_shrinker(&vb->shrinker);
>  out_del_balloon_wq:
>  	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>  		destroy_workqueue(vb->balloon_wq);
> @@ -987,8 +965,11 @@ static void virtballoon_remove(struct virtio_device
*vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> -	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> -		virtio_balloon_unregister_shrinker(vb);
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		unregister_oom_notifier(&vb->oom_nb);
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +		unregister_shrinker(&vb->shrinker);
> +
>  	spin_lock_irq(&vb->stop_update_lock);
>  	vb->stop_update = true;
>  	spin_unlock_irq(&vb->stop_update_lock);
> -- 
> 2.24.1

David Hildenbrand

2020-Feb-06 08:42 UTC

head link

[PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

On 06.02.20 08:40, Michael S. Tsirkin wrote:> On Wed, Feb 05, 2020 at 05:34:02PM +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 slab cache that should be
>> shrunk when under memory pressure. The shrinker does not have a concept
of
>> priorities, so this behavior cannot be configured.
>>
>> 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.
>>
>> Especially, a drop_slab() will result in the whole balloon getting
>> deflated - undesired. While handling it via the OOM handler might not
be
>> perfect, it keeps existing behavior. If we want a different behavior,
then
>> we need a new feature bit and document it properly (although, there
should
>> be a clear use case and the intended effects should be well described).
>>
>> 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.
> 
> I agree. And to make this case even stronger:
> 
> The oom_pages module parameter was known to be broken: whatever its
> value, we return at most VIRTIO_BALLOON_ARRAY_PFNS_MAX.  So module
> parameter values > 256 never worked, and it seems highly unlikely that
> freeing 1Mbyte on OOM is too aggressive.
> There was a patch
>  virtio-balloon: deflate up to oom_pages on OOM
> by Wei Wang to try to fix it:
> https://lore.kernel.org/r/1508500466-21165-3-git-send-email-wei.w.wang at
intel.com
> but this was dropped.
Makes sense. 1MB is usually good enough.
> 
>> Also, pay attention that leak_balloon() returns the number of 4k pages
-
>> convert it properly in virtio_balloon_oom_notify().
> 
> Oh. So it was returning a wrong value originally (before 71994620bb25).
> However what really matters for notifiers is whether the value is 0 -
> whether we made progress. So it's cosmetic.
Yes, that's also my understanding.
> 
>> Note1: using the OOM handler is frowned upon, but it really is what we
>>        need for this feature.
> 
> Quite. However, I went back researching why we dropped the OOM notifier,
> and found this:
> 
> https://lore.kernel.org/r/1508500466-21165-2-git-send-email-wei.w.wang at
intel.com
> 
> To quote from there:
> 
> The balloon_lock was used to synchronize the access demand to elements
> of struct virtio_balloon and its queue operations (please see commit
> e22504296d). This prevents the concurrent run of the leak_balloon and
> fill_balloon functions, thereby resulting in a deadlock issue on OOM:
> 
> fill_balloon: take balloon_lock and wait for OOM to get some memory;
> oom_notify: release some inflated memory via leak_balloon();
> leak_balloon: wait for balloon_lock to be released by fill_balloon.
fill_balloon does the allocation *before* taking the lock. tell_host()
should not allocate memory AFAIR. So how could this ever happen?

Anyhow, we could simply work around this by doing a trylock in
fill_balloon() and retrying in the caller. That should be easy. But I
want to understand first, how something like that would even be possible.
>> Note2: without VIRTIO_BALLOON_F_MUST_TELL_HOST (iow, always with QEMU)
we
>>        could actually skip sending deflation requests to our
hypervisor,
>>        making the OOM path *very* simple. Besically freeing pages and
>>        updating the balloon.
> 
> Well not exactly. !VIRTIO_BALLOON_F_MUST_TELL_HOST does not actually
> mean "never tell host". It means "host will not discard
pages in the
> balloon, you can defer host notification until after use".
> 
> This was the original implementation:
> 
> +       if (vb->tell_host_first) {
> +               tell_host(vb, vb->deflate_vq);
> +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> +       } else {
> +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> +               tell_host(vb, vb->deflate_vq);
> +       }
> +}
> 
> I don't know whether completely skipping host notifications
> when !VIRTIO_BALLOON_F_MUST_TELL_HOST will break any hosts.
We discussed this already somewhere else, but here is again what I found.

commit bf50e69f63d21091e525185c3ae761412be0ba72
Author: Dave Hansen <dave at linux.vnet.ibm.com>
Date:   Thu Apr 7 10:43:25 2011 -0700

    virtio balloon: kill tell-host-first logic

    The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
    feature bit.  Whenever the bit is set, the guest kernel must
    always tell the host before we free pages back to the allocator.
    Without this feature, we might free a page (and have another
    user touch it) while the hypervisor is unprepared for it.

    But, if the bit is _not_ set, we are under no obligation to
    reverse the order; we're under no obligation to do _anything_.
    As of now, qemu-kvm defines the bit, but doesn't set it.

MUST_TELL_HOST really means "no need to deflate, just reuse a page".
We
should finally document this somewhere.

-- 
Thanks,

David / dhildenb

Michael S. Tsirkin

2020-Feb-06 08:57 UTC

head link

[PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

On Thu, Feb 06, 2020 at 09:42:34AM +0100, David Hildenbrand
wrote:> On 06.02.20 08:40, Michael S. Tsirkin wrote:
> > On Wed, Feb 05, 2020 at 05:34:02PM +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 slab cache that should be
> >> shrunk when under memory pressure. The shrinker does not have a
concept of
> >> priorities, so this behavior cannot be configured.
> >>
> >> 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.
> >>
> >> Especially, a drop_slab() will result in the whole balloon getting
> >> deflated - undesired. While handling it via the OOM handler might
not be
> >> perfect, it keeps existing behavior. If we want a different
behavior, then
> >> we need a new feature bit and document it properly (although,
there should
> >> be a clear use case and the intended effects should be well
described).
> >>
> >> 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.
> > 
> > I agree. And to make this case even stronger:
> > 
> > The oom_pages module parameter was known to be broken: whatever its
> > value, we return at most VIRTIO_BALLOON_ARRAY_PFNS_MAX.  So module
> > parameter values > 256 never worked, and it seems highly unlikely
that
> > freeing 1Mbyte on OOM is too aggressive.
> > There was a patch
> >  virtio-balloon: deflate up to oom_pages on OOM
> > by Wei Wang to try to fix it:
> > https://lore.kernel.org/r/1508500466-21165-3-git-send-email-wei.w.wang
at intel.com
> > but this was dropped.
> 
> Makes sense. 1MB is usually good enough.
> 
> > 
> >> Also, pay attention that leak_balloon() returns the number of 4k
pages -
> >> convert it properly in virtio_balloon_oom_notify().
> > 
> > Oh. So it was returning a wrong value originally (before
71994620bb25).
> > However what really matters for notifiers is whether the value is 0 -
> > whether we made progress. So it's cosmetic.
> 
> Yes, that's also my understanding.
> 
> > 
> >> Note1: using the OOM handler is frowned upon, but it really is
what we
> >>        need for this feature.
> > 
> > Quite. However, I went back researching why we dropped the OOM
notifier,
> > and found this:
> > 
> > https://lore.kernel.org/r/1508500466-21165-2-git-send-email-wei.w.wang
at intel.com
> > 
> > To quote from there:
> > 
> > The balloon_lock was used to synchronize the access demand to elements
> > of struct virtio_balloon and its queue operations (please see commit
> > e22504296d). This prevents the concurrent run of the leak_balloon and
> > fill_balloon functions, thereby resulting in a deadlock issue on OOM:
> > 
> > fill_balloon: take balloon_lock and wait for OOM to get some memory;
> > oom_notify: release some inflated memory via leak_balloon();
> > leak_balloon: wait for balloon_lock to be released by fill_balloon.
> 
> fill_balloon does the allocation *before* taking the lock. tell_host()
> should not allocate memory AFAIR. So how could this ever happen?
> 
> Anyhow, we could simply work around this by doing a trylock in
> fill_balloon() and retrying in the caller. That should be easy. But I
> want to understand first, how something like that would even be possible.
Hmm it looks like you are right.  Sorry!

> >> Note2: without VIRTIO_BALLOON_F_MUST_TELL_HOST (iow, always with
QEMU) we
> >>        could actually skip sending deflation requests to our
hypervisor,
> >>        making the OOM path *very* simple. Besically freeing pages
and
> >>        updating the balloon.
> > 
> > Well not exactly. !VIRTIO_BALLOON_F_MUST_TELL_HOST does not actually
> > mean "never tell host". It means "host will not discard
pages in the
> > balloon, you can defer host notification until after use".
> > 
> > This was the original implementation:
> > 
> > +       if (vb->tell_host_first) {
> > +               tell_host(vb, vb->deflate_vq);
> > +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +       } else {
> > +               release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +               tell_host(vb, vb->deflate_vq);
> > +       }
> > +}
> > 
> > I don't know whether completely skipping host notifications
> > when !VIRTIO_BALLOON_F_MUST_TELL_HOST will break any hosts.
> 
> We discussed this already somewhere else, but here is again what I found.
> 
> commit bf50e69f63d21091e525185c3ae761412be0ba72
> Author: Dave Hansen <dave at linux.vnet.ibm.com>
> Date:   Thu Apr 7 10:43:25 2011 -0700
> 
>     virtio balloon: kill tell-host-first logic
> 
>     The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
>     feature bit.  Whenever the bit is set, the guest kernel must
>     always tell the host before we free pages back to the allocator.
>     Without this feature, we might free a page (and have another
>     user touch it) while the hypervisor is unprepared for it.
> 
>     But, if the bit is _not_ set, we are under no obligation to
>     reverse the order; we're under no obligation to do _anything_.
>     As of now, qemu-kvm defines the bit, but doesn't set it.
Well this is not what the spec says in the end.
To continue that commit message:

    This patch makes the "tell host first" logic the only case.  This
    should make everybody happy, and reduce the amount of untested or
    untestable code in the kernel.

you can try proposing the change to the virtio TC, see what do others
think.

> MUST_TELL_HOST really means "no need to deflate, just reuse a
page". We
> should finally document this somewhere.
I'm not sure it's not too late to change what that flag means.  If not
sending deflate messages at all is a useful optimization, it seems
safer to add a feature flag for that.
> -- 
> Thanks,
> 
> David / dhildenb

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