Re: use of shrinker in virtio balloon free page hinting

[David Hildenbrand <david@redhat.com>]

On 17.07.19 13:20, Michael S. Tsirkin wrote:
> Wei, others,
> 
> ATM virtio_balloon_shrinker_scan will only get registered
> when deflate on oom feature bit is set.
> 
> Not sure whether that's intentional.  Assuming it is:
> 
> virtio_balloon_shrinker_scan will try to locate and free
> pages that are processed by host.
> The above seems broken in several ways:
> - count ignores the free page list completely
> - if free pages are being reported, pages freed
>   by shrinker will just get re-allocated again

Trying to answer your questions (not sure if I fully understood what you
mean)

virtio_balloon_shrinker_scan() will not be called due to inflation
requests (balloon_page_alloc()). It will be called whenever the system
is OOM, e.g., when starting a new application.

I assume you were expecting the shrinker getting called due to
balloon_page_alloc(). however, that is not the case as we pass
"__GFP_NORETRY".


To test, something like:

1. Start a VM with

-device virtio-balloon-pci,deflate-on-oom=true

2. Inflate the balloon, e.g.,

QMP: balloon 1024
QMP: info balloon
-> 1024

See how "MemTotal" in /proc/meminfo in the guest won't change

3. Run a workload that exhausts memory in the guest (OOM).

See how the balloon was automatically deflated

QMP: info balloon
-> Something bigger than 1024


Not sure if it is broken, last time I played with it, it worked, but
that was ~1-2 years ago.

-- 

Thanks,

David / dhildenb