Re: [PATCH v2] virtio_balloon: Fix endless deflation and inflation on arm64

[Gavin Shan <gshan@redhat.com>]

Hi David and Michael,

On 8/31/23 11:10, 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 aligning the target up to the guest page size, 64KB in this
> specific case. With this applied, no flooding QMP events are 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>
> Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
> ---
> v2: Align @num_pages up to the guest page size in towards_target()
>      directly as David suggested.
> ---
>   drivers/virtio/virtio_balloon.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 

If the patch looks good, could you please merge this to Linux 6.6.rc4 since
it's something needed by our downstream. I hope it can land upstream as early
as possible, thanks a lot.

> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 5b15936a5214..2d5d252ef419 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -395,7 +395,11 @@ static inline s64 towards_target(struct virtio_balloon *vb)
>   	virtio_cread_le(vb->vdev, struct virtio_balloon_config, num_pages,
>   			&num_pages);
>   
> -	target = num_pages;
> +	/*
> +	 * Aligned up to guest page size to avoid inflating and deflating
> +	 * balloon endlessly.
> +	 */
> +	target = ALIGN(num_pages, VIRTIO_BALLOON_PAGES_PER_PAGE);
>   	return target - vb->num_pages;
>   }
>   

Thanks,
Gavin