Re: use of shrinker in virtio balloon free page hinting

[Wei Wang]

On 07/18/2019 12:13 PM, Michael S. Tsirkin wrote:
> It makes sense for pages in the balloon (requested by hypervisor).
> However free page hinting can freeze up lots of memory for its own
> internal reasons. It does not make sense to ask hypervisor
> to set flags in order to fix internal guest issues.

Sounds reasonable to me. Probably we could move the flag check to
shrinker_count and shrinker_scan as a reclaiming condition for
ballooning pages only?


> Right. But that does not include the pages in the hint vq,
> which could be a significant amount of memory.

I think it includes, as vb->num_free_page_blocks records the total number
of free page blocks that balloon has taken from mm.

For shrink_free_pages, it calls return_free_pages_to_mm, which pops pages
from vb->free_page_list (this is the list where pages get enlisted after 
they
are put to the hint vq, see get_free_page_and_send).


> > > - 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.
> Even if ballon was never inflated, if shrinker frees some memory while
> we are hinting, hint vq will keep going and allocate it back without
> sleeping.

Still see get_free_page_and_send. -EINTR is returned when page 
allocation fails,
and reporting ends then.

Shrinker is called on system memory pressure. On memory pressure
get_free_page_and_send will fail memory allocation, so it stops 
allocating more.


Best,
Wei