Re: [PATCH v1 08/12] powerpc/pseries: CMM: Implement balloon compaction

[David Hildenbrand]

On 12.11.19 11:46, Michael Ellerman wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
> > On 31.10.19 15:29, David Hildenbrand wrote:
> > > We can now get rid of the cmm_lock and completely rely on the balloon
> > > compaction internals, which now also manage the page list and the lock.
> ...
> > > +
> > > +static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
> > > +			   struct page *newpage, struct page *page,
> > > +			   enum migrate_mode mode)
> > > +{
> > > +	unsigned long flags;
> > > +
> > > +	/*
> > > +	 * loan/"inflate" the newpage first.
> > > +	 *
> > > +	 * We might race against the cmm_thread who might discover after our
> > > +	 * loan request that another page is to be unloaned. However, once
> > > +	 * the cmm_thread runs again later, this error will automatically
> > > +	 * be corrected.
> > > +	 */
> > > +	if (plpar_page_set_loaned(newpage)) {
> > > +		/* Unlikely, but possible. Tell the caller not to retry now. */
> > > +		pr_err_ratelimited("%s: Cannot set page to loaned.", __func__);
> > > +		return -EBUSY;
> > > +	}
> > > +
> > > +	/* balloon page list reference */
> > > +	get_page(newpage);
> > > +
> > > +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> > > +	balloon_page_insert(b_dev_info, newpage);
> > > +	balloon_page_delete(page);
> >
> > I think I am missing a b_dev_info->isolated_pages-- here.
>
> I don't know this code at all, but looking at other balloon drivers they
> do seem to do that in roughly the same spot.
>
> I'll add it, how can we test that it's correct?

It's certainly correct. We increment when we isolate 
(balloon_page_isolate()) and decrement when we un-isolate.

Un-isolate happens when we putback a isolated page 
(balloon_page_putback() - migration aborted) or when we successfully 
migrate it (via balloon_page_migrate()).

The issue is that we cannot decrement in balloon_page_migrate(), as we 
have to hold the b_dev_info->pages_lock. That's why we have to do it in 
the registered callback under lock.

Please note that b_dev_info->isolated_pages is only needed for a sanity 
check in balloon_page_dequeue(). That's why I didn't notice during 
testing. I wonder if we should at some point rip out that sanity check ...

Thanks and cheers!

> cheers


--

Thanks,

David / dhildenb