Re: [PATCH 1 of 6] hotplug-memory: refactor online_pages to separate zone growth from page onlining

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Thu, 2008-04-03 at 18:32 -0700, Jeremy Fitzhardinge wrote:
> Dave Hansen wrote:
> > On Thu, 2008-04-03 at 17:05 -0700, Jeremy Fitzhardinge wrote:
> >> +int prepare_online_pages(unsigned long pfn, unsigned long nr_pages)
> >> +{
> >> +       int ret = notify_going_online(pfn, nr_pages);
> >> +       if (ret)
> >> +               return ret;
> >> +
> >> +       grow_zone_span(pfn, pfn+nr_pages);
> >>         return 0;
> >> +}
> >>     
> >
> > OK, after seeing this used in the Xen driver, I'm even less a fan of the
> > name.  Mostly because it doesn't actually prepare *pages* to be onlined.
> > It prepares the zones/pgdats and notifies that pages might soon be
> > online.  Can you think of any better names?
> >
> > grow_and_notify...??
> 
> Does it even need to be a separately visible function?  Could it just be 
> part of add_memory()?  Is there any reason delay doing it until 
> online_pages()?

Yeah, the add_memory() itself is supposed to be easily able to be rolled
back.  Say the machine didn't actually need the memory, it could just
give the memory back.  No danger of fragmentation or anything.

But, at the same time, we don't want to have this memory sitting there
being unused, but still being accounted for in the VM as real memory.
It's not in use, and making it look in use (by growing the zone spans)
could upset the VM watermarks.

> > It's also a bit funky because you're calling the online notifiers, but
> > you're not actually onlining the pages, yet.  Does that have any
> > repercussions?
> 
> No.  It will always call the GOING_ONLINE notifier, but it will only 
> call the ONLINE notifier if it actually bulk-onlines all the pages.  In 
> my page-by-page case, it will never end up calling the ONLINE notifier.  
> I could call it repeatedly for each page, but I'm not sure how useful 
> that is (the lack of any users of the ONLINE notifier makes it hard to 
> judge).

Ahhh.  You're completely right.  I was confused by ONLINE and
GOING_ONLINE.  Thanks for the reminder!

-- Dave