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 <aqu...@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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