Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

[David Hildenbrand]

On 24.10.19 10:42, Michal Hocko wrote:
> On Wed 23-10-19 12:03:51, David Hildenbrand wrote:
> > > Do you see any downsides?
> >
> > The only downside I see is that we get more false negatives on
> > has_unmovable_pages(), eventually resulting in the offlining stage after
> > isolation to loop forever (as some PageOffline() pages are not movable
> > (especially, XEN balloon, HyperV balloon), there won't be progress).
> >
> > I somewhat don't like forcing everybody that uses PageOffline() (especially
> > all users of balloon compaction) to implement memory notifiers just to avoid
> > that. Maybe, we even want to use PageOffline() in the future in the core
> > (e.g., for memory holes instead of PG_reserved or similar).
>
> There is only a handful of those and we need to deal with them anyway.
> If you do not want to enforce them to create their own notifiers then we
> can accomodate the hotplug code. __test_page_isolated_in_pageblock resp.

Yeah, I would prefer offlining code to be able to deal with that without 
notifier changes for all users.

> the call chain up can distinguish temporary and permanent failures
> (EAGAIN vs. EBUSY). The current state when we always return EBUSY and
> keep retrying for ever is not optimal at all, right? A referenced PageOffline

Very right!

> could be an example of EBUSY all other failures where we are effectively
> waiting for pages to get freed finaly would be EAGAIN.

We have to watch out for PageOffline() pages that are actually movable 
(balloon compaction). But that doesn't sound too hard.
> It is a bit late in the process because a large portion of the work has
> been done already but this doesn't sound like something to lose sleep
> over.

Right. I'll look into that to find out if this would work. And see if I 
can reproduce what I described at all (theoretical thoughts) :)

Again, thanks for looking into this Michal!

--

Thanks,

David / dhildenb