Re: [PATCH v2 4/6] mm: introduce common page state for ballooned memory

[Rafael Aquini]

On Tue, Sep 23, 2014 at 12:06:11AM +0400, Konstantin Khlebnikov wrote:
> On Mon, Sep 22, 2014 at 11:22 PM, Rafael Aquini <aquini@xxxxxxxxxx> wrote:
> > On Mon, Sep 22, 2014 at 10:40:34PM +0400, Konstantin Khlebnikov wrote:
> >> On Sat, Sep 20, 2014 at 10:23 AM, Andrew Morton
> >> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> > On Sat, 20 Sep 2014 09:25:01 +0400 Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> >> >
> >> >> >
> >> >> > So I'm going to send "fix for
> >> >> > mm-balloon_compaction-use-common-page-ballooning-v2" to Linus
> >> >> > separately, but it has no changelog at all.
> >> >>
> >> >> Probably it would be better if you drop everything except actually
> >> >> fixes and stresstest. This is gone too far, now balloon won't compile
> >> >> in the middle of patchset. Just tell me and I'll redo the rest.
> >> >
> >> > I think it's best if I drop everything:
> >> >
> >> > mm-balloon_compaction-ignore-anonymous-pages.patch
> >> > mm-balloon_compaction-keep-ballooned-pages-away-from-normal-migration-path.patch
> >> > mm-balloon_compaction-isolate-balloon-pages-without-lru_lock.patch
> >> > selftests-vm-transhuge-stress-stress-test-for-memory-compaction.patch
> >> > mm-introduce-common-page-state-for-ballooned-memory.patch
> >> > mm-balloon_compaction-use-common-page-ballooning.patch
> >> > mm-balloon_compaction-general-cleanup.patch
> >> > mm-balloon_compaction-use-common-page-ballooning-v2-fix-1.patch
> >> >
> >> > Please go through it and send out a new version?
> >> >
> >> >
> >>
> >> I've found yet another bug in this code. It seems here is a nest.
> >> balloon_page_dequeue can race with  balloon_page_isolate:
> >> balloon_page_isolate can remove page from list between
> >> llist_for_each_entry_safe and trylock_page in balloon_page_dequeue.
> >> balloon_page_dequeue runs under mutex_lock(&vb->balloon_lock);
> >> both of them lock page using trylock_page so race is tight but it is
> >> not impossible.
> > Plausible to happen if stress testing compaction simultaneously with
> > freezing/unloading the balloon driver. As you noted, it's quite tight
> > despite not impossible. Nice catch.
> >
> >
> >> Probably it's really easier to rewrite it than to fix bugs one by one =/
> > I'm not against a rewrite, but I don't think that rewriting the code to get rid
> > of such bugs changes the fact we still have to address them in the actual placed
> > code as we go on finding them. That's why I thought your inital changeset fine,
> > with patches for stable going first and code overhaul for next following them up.
> >
> > For this race you spotted, I think a simple change like the following
> > might be enough (not-tested)
> 
> This locking scheme is too fragile and uncommon.
>

page_lock and refcounting was what I had at my disposal to sort these
races out since I haven't thoutgh on a special page->_mapcount when
designing this feature. It's the way other page races are sorted out.
Not arguing it's not fragile, but it's the way code is layed out since
some time, so we must check the feasibility of a total overhaul for
stable branches.

 
> What about this:
> 
> * special page->_mapcount marks ballooned pages
> * page->private points to balloon (directly, without intermediate mapping)
> * flag PagePrivate means page currently in balloon page list (i.e. not
> isolated, like PageLRU for normal pages)
> * lock_page protects all of them
> 
> balloon_page_dequeue() will delete page from balloon list only if it's
> not isolated, also it always clears page->private and balloon mark.
> put-back rechecks mark after locking the page and releases it as
> normal page if mark is gone.
> 

I have already agreed with you here, since the changes above are mostly from
your original overhaul proposal. It's a much better approach for that
balloon code, no doubts. Thanks for doing it. Only thing we need to take
care here is about its requirement on changing the semantics for those
interfaces might turn the changes unfeasible for old stable branches. If we
can ignore this mentioned fact entirely, I don't see why not going with
your idea all branches across, otherwise I think we should overhaul the 
code for -next, and send pontual fixes for stable.


Cheers,
-- Rafael

> >
> > diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> > index 6e45a50..fd3a497 100644
> > --- a/mm/balloon_compaction.c
> > +++ b/mm/balloon_compaction.c
> > @@ -93,6 +93,16 @@ struct page *balloon_page_dequeue(struct
> > balloon_dev_info *b_dev_info)
> >                  * to be released by the balloon driver.
> >                  */
> >                 if (trylock_page(page)) {
> > +                       /*
> > +                        * Skip dequeue attempt for this page to a later round
> > +                        * if balloon_page_isolate() has sucessfully isolated
> > +                        * it just before we got the page lock here.
> > +                        */
> > +                       if (page_count(page) != 1) {
> > +                               unlock_page(page);
> > +                               continue
> > +                       }
> > +
> >                         spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> >                         /*
> >                          * Raise the page refcount here to prevent any
> >                          * wrong
> >
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