Re: use of shrinker in virtio balloon free page hinting

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 17, 2019 at 04:10:47PM +0200, David Hildenbrand wrote:
> 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".

Right but it's possible we exhaust all memory, then
someone else asks for a single page and that invokes
the shrinker.

> 
> 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

Sorry I was unclear.  The question was about
VIRTIO_BALLOON_F_FREE_PAGE_HINT specifically.

-- 
MST