Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

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

On Thu, Aug 23, 2012 at 12:21:29PM -0300, Rafael Aquini wrote:
> On Thu, Aug 23, 2012 at 04:53:29PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Aug 23, 2012 at 10:06:07AM -0300, Rafael Aquini wrote:
> > > On Thu, Aug 23, 2012 at 03:34:32PM +0300, Michael S. Tsirkin wrote:
> > > > > So, nothing has changed here.
> > > > 
> > > > Yes, your patch does change things:
> > > > leak_balloon now might return without freeing any pages.
> > > > In that case we will not be making any progress, and just
> > > > spin, pinning CPU.
> > > 
> > > That's a transitory condition, that migh happen if leak_balloon() takes place
> > > when compaction, or migration are under their way and it might only affects the
> > > module unload case.
> > 
> > Regular operation seems even more broken: host might ask
> > you to leak memory but because it is under compaction
> > you might leak nothing. No?
> >
> 
> And that is exactely what it wants to do. If there is (temporarily) nothing to leak,
> then not leaking is the only sane thing to do.

It's an internal issue between balloon and mm. User does not care.

> Having balloon pages being migrated
> does not break the leak at all, despite it can last a little longer.
> 

Not "longer" - apparently forever unless user resend the leak command.
It's wrong - it should
1. not tell host if nothing was done
2. after migration finished leak and tell host

> > > 
> > > I already told you that we do not do that by any mean introduced by this patch.
> > > You're just being stubborn here. If those bits are broken, they were already
> > > broken before I did come up with this proposal.
> > 
> > Sorry you don't address the points I am making.  Maybe there are no
> > bugs. But it looks like there are.  And assuming I am just seeing things
> > this just means patch needs more comments, in commit log and in
> > code to explain the design so that it stops looking like that.
> >
> 
> Yes, I belive you're biased here.
> 
> 
> > Basically it was very simple: we assumed page->lru was never
> > touched for an allocated page, so it's safe to use it for
> > internal book-keeping by the driver.
> >
> > Now, this is not the case anymore, you add some logic in mm/ that might
> > or might not touch page->lru depending on things like reference count.
> > And you are asking why things break even though you change very little
> > in balloon itself?  Because the interface between balloon and mm is now
> > big, fragile and largely undocumented.
> > 
> 
> The driver don't use page->lru as its bookeeping at all, it uses
> vb->num_pages instead. 

$ grep lru drivers/virtio/virtio_balloon.c
                list_add(&page->lru, &vb->pages);
                page = list_first_entry(&vb->pages, struct page, lru);
                list_del(&page->lru);


> 
> > Another strangeness I just noticed: if we ever do an extra get_page in
> > balloon, compaction logic in mm will break, yes?  But one expects to be
> > able to do get_page after alloc_page without ill effects
> > as long as one does put_page before free.
> >
> 
> You can do it (bump up the balloon page refcount), and it will only prevent
> balloon pages from being isolated and migrated, thus reducing the effectiveness of
> defragmenting memory when balloon pages are present, just like it happens today.
> 
> It really doesn't seems the case of virtio_balloon driver, or any other driver,
> which allocates pages directly from buddy to keep raising the page refcount,
> though.
>  

E.g. network devices routinely play with pages they get from buddy,
this is used for sharing memory between skbs.

> > Just a thought: maybe it is cleaner to move all balloon page tracking
> > into mm/?  Implement alloc_balloon/free_balloon with methods to fill and
> > leak pages, and callbacks to invoke when done.  This should be good for
> > other hypervisors too. If you like this idea, I can even try to help out
> > by refactoring current code in this way, so that you can build on it.
> > But this is just a thought, not a must.
> >
> 
> That seems to be a good thought to be on a future enhancements wish-list, for sure.
> We can start thinking of it, and I surely would be more than happy on be doing
> it along with you. But I don't think not having it right away is a dealbreaker
> for this proposal, as is.

I grant busywait on module unloading isn't a huge deal breaker.

Poking in mm internals is not a dealbreaker?
Not leaking as much as
you are asked to isn't?

> I'm not against your thoughts, and I'm really glad that you're providing such
> good dicussion over this subject, but, now I'll wait for Rusty thoughts on 
> this one question.
> 
> Cheers!

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