Re: [QUESTION] balloon page isolation needs LRU lock?

[Joonsoo Kim]

On Fri, Dec 06, 2013 at 10:21:43AM -0200, Rafael Aquini wrote:
> On Fri, Dec 06, 2013 at 05:53:31PM +0900, Joonsoo Kim wrote:
> > Hello, Rafael.
> > 
> > I looked at some compaction code and found that some oddity about
> > balloon compaction. In isolate_migratepages_range(), if we meet
> > !PageLRU(), we check whether this page is for balloon compaction.
> > In this case, code needs locked. Is the lock really needed? I can't find
> > any relationship between balloon compaction and LRU lock.
> > 
> > Second question is that in above case if we don't hold a lock, we
> > skip this page. I guess that if we meet balloon page repeatedly, there
> > is no change to run isolation. Am I missing?
> > 
> > Please let me know what I am missing.
> > 
> > Thanks in advance.
> 
> Howdy Joonsoo, thanks for your question.
> 
> The major reason I left the 'locked' case in place when isolating balloon pages
> was to keep consistency with the other isolation cases. Among all page types we
> isolate for compaction balloon pages are an exception as, you noticed, they're
> not on LRU lists. So, we (specially) fake balloon pages as LRU to isolate/compact them, 
> withouth having to sort to drastic surgeries into kernel code to implement
> exception cases for isolating/compacting balloon pages.
> 
> As others pages we isolate for compaction are isolated while holding the
> zone->lru_lock, I left the same condition placed for balloon pages as a
> safeguard for consistency. If we hit a balloon page while scanning page blocks
> and we do not have the lru lock held, then the balloon page will be treated 
> by the scanning mechanism just as what it is: a !PageLRU() case, and life will
> go on as described by the algorithm.
> 
> OTOH, there's no direct relationship between the balloon page and the LRU lock,
> other than this consistency one I aforementioned. I've never seen any major
> trouble on letting the lock requirement in place during my tests on workloads
> that mix balloon pages and compaction. However, if you're seeing any trouble and
> that lru lock requirement is acting as an overkill or playing a bad role on your
> tests, you can get rid of it easily, IMHO.

Hello, Rafael.

Thanks for nice explanation.
Now I totally understand what it means and why it does.

Thanks!

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