Re: [PATCH v6 1/3] mm: introduce compaction and migration for virtio ballooned pages

[Rafael Aquini <aquini@redhat.com>]

On Thu, Aug 09, 2012 at 10:00:19AM +0100, Mel Gorman wrote:
> On Wed, Aug 08, 2012 at 07:53:19PM -0300, Rafael Aquini wrote:
> > Memory fragmentation introduced by ballooning might reduce significantly
> > the number of 2MB contiguous memory blocks that can be used within a guest,
> > thus imposing performance penalties associated with the reduced number of
> > transparent huge pages that could be used by the guest workload.
> > 
> > This patch introduces the helper functions as well as the necessary changes
> > to teach compaction and migration bits how to cope with pages which are
> > part of a guest memory balloon, in order to make them movable by memory
> > compaction procedures.
> > 
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> 
> Mostly looks ok but I have one question;
> 
> > <SNIP>
> >
> > +/* putback_lru_page() counterpart for a ballooned page */
> > +bool putback_balloon_page(struct page *page)
> > +{
> > +	if (WARN_ON(!movable_balloon_page(page)))
> > +		return false;
> > +
> > +	if (likely(trylock_page(page))) {
> > +		if (movable_balloon_page(page)) {
> > +			__putback_balloon_page(page);
> > +			put_page(page);
> > +			unlock_page(page);
> > +			return true;
> > +		}
> > +		unlock_page(page);
> > +	}
> 
> You might have answered this already as I skipped over a few revisions
> and if you have, sorry about that and please add a comment :)
> 
> This trylock_page looks risky as it looks like it can fail if another
> process running compaction tries to isolate this page. It locks the page,
> finds it cant and releases the lock but in the meantime this trylock can
> fail. It triggers a WARN_ON so we'll get a bug report but it leaves the
> reference count elevated and this page has now leaked.
>
Good catch!
I had those bits changed to follow the same logics you had suggested for
isolate_balloon_page(), but I ended up completely missing this potential page
leak case you spotted. Thanks a lot!
 
> Why not just lock_page(page)? As you have already isolated this page you
> know that the lock is only going to be held by a parallel compacting
> process checking the reference count and the delay will be short. As a
> bonus you can drop the WARN_ON check in the caller and make this void as
> the WARN_ON check in the caller becomes redundant.
> 
Sure! 
what do you think of:

+/* putback_lru_page() counterpart for a ballooned page */
+void putback_balloon_page(struct page *page)
+{
+   lock_page(page);
+   if (!WARN_ON(!movable_balloon_page(page))) {
+           __putback_balloon_page(page);
+           put_page(page);
+   }
+   unlock_page(page);
+}

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