Re: [PATCH] virtio_balloon: Fix endless deflation and inflation on arm64

[David Hildenbrand <david@redhat.com>]

On 29.08.23 03:54, Gavin Shan wrote:
> The deflation request to the target, which isn't unaligned to the
> guest page size causes endless deflation and inflation actions. For
> example, we receive the flooding QMP events for the changes on memory
> balloon's size after a deflation request to the unaligned target is
> sent for the ARM64 guest, where we have 64KB base page size.
> 
>    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64      \
>    -accel kvm -machine virt,gic-version=host -cpu host          \
>    -smp maxcpus=8,cpus=8,sockets=2,clusters=2,cores=2,threads=1 \
>    -m 1024M,slots=16,maxmem=64G                                 \
>    -object memory-backend-ram,id=mem0,size=512M                 \
>    -object memory-backend-ram,id=mem1,size=512M                 \
>    -numa node,nodeid=0,memdev=mem0,cpus=0-3                     \
>    -numa node,nodeid=1,memdev=mem1,cpus=4-7                     \
>      :                                                          \
>    -device virtio-balloon-pci,id=balloon0,bus=pcie.10
> 
>    { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
>    {"return": {}}
>    {"timestamp": {"seconds": 1693272173, "microseconds": 88667},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272174, "microseconds": 89704},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272175, "microseconds": 90819},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272176, "microseconds": 91961},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272177, "microseconds": 93040},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
>    {"timestamp": {"seconds": 1693272178, "microseconds": 94117},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
>    {"timestamp": {"seconds": 1693272179, "microseconds": 95337},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272180, "microseconds": 96615},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
>    {"timestamp": {"seconds": 1693272181, "microseconds": 97626},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272182, "microseconds": 98693},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
>    {"timestamp": {"seconds": 1693272183, "microseconds": 99698},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272184, "microseconds": 100727},  \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272185, "microseconds": 90430},   \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    {"timestamp": {"seconds": 1693272186, "microseconds": 102999},  \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
>       :
>    <The similar QMP events repeat>
> 
> Fix it by having the target aligned to the guest page size, 64KB
> in this specific case. With this applied, no flooding QMP event
> is observed and the memory balloon's size can be stablizied to
> 0x3ffe0000 soon after the deflation request is sent.
> 
>    { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
>    {"return": {}}
>    {"timestamp": {"seconds": 1693273328, "microseconds": 793075},  \
>     "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
>    { "execute" : "query-balloon" }
>    {"return": {"actual": 1073610752}}
> 
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>   drivers/virtio/virtio_balloon.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 5b15936a5214..625caac35264 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -386,6 +386,17 @@ static void stats_handle_request(struct virtio_balloon *vb)
>   	virtqueue_kick(vq);
>   }
>   
> +static inline s64 align_pages_up(s64 diff)
> +{
> +	if (diff == 0)
> +		return diff;
> +
> +	if (diff > 0)
> +		return ALIGN(diff, VIRTIO_BALLOON_PAGES_PER_PAGE);
> +
> +	return -ALIGN(-diff, VIRTIO_BALLOON_PAGES_PER_PAGE);
> +}
> +
>   static inline s64 towards_target(struct virtio_balloon *vb)
>   {
>   	s64 target;
> @@ -396,7 +407,7 @@ static inline s64 towards_target(struct virtio_balloon *vb)
>   			&num_pages);
>   
>   	target = num_pages;
> -	return target - vb->num_pages;

We know that vb->num_pages is always multiples of 
VIRTIO_BALLOON_PAGES_PER_PAGE.

Why not simply align target down?

target = ALIGN(num_pages, VIRTIO_BALLOON_PAGES_PER_PAGE);
return target - vb->num_pages;


-- 
Cheers,

David / dhildenb

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