Re: use of shrinker in virtio balloon free page hinting

[Michael S. Tsirkin]

On Thu, Jul 18, 2019 at 01:57:06PM +0800, Wei Wang wrote:
> 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?

I think so, yes. I also wonder whether we should stop reporting
at that point - otherwise we'll just allocate the freed pages again.

> 
> > 
> > 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.

Oh - you are right. Thanks!

> 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.

what if it does not fail?


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

Memory pressure could be triggered by an unrelated allocation
e.g. from another driver.

> 
> 
> Best,
> Wei