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! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>