RE: use of shrinker in virtio balloon free page hinting

[Wang, Wei W]

On Wednesday, July 17, 2019 7:21 PM, 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. 

Yes, we wanted to follow the old oom behavior, which allows the oom notifier
to deflate pages only when this feature bit has been negotiated.


> 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

Do you mean virtio_balloon_shrinker_count()? It just reports to
do_shrink_slab the amount of freeable memory that balloon has.
(vb->num_pages and vb->num_free_page_blocks are all included )

> - if free pages are being reported, pages freed
>   by shrinker will just get re-allocated again

fill_balloon will re-try the allocation after sleeping 200ms once allocation fails.

 
> I was unable to make this part of code behave in any reasonable way - was
> shrinker usage tested? What's a good way to test that?

Please see the example that I tested before : https://lkml.org/lkml/2018/8/6/29
(just the first one: *1. V3 patches)

What problem did you see?

I just tried the latest code, and find ballooning reports a #GP (seems caused by
418a3ab1e). 
I'll take a look at the details in the office tomorrow.

Best,
Wei