From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f45.google.com (mail-pb0-f45.google.com [209.85.160.45]) by kanga.kvack.org (Postfix) with ESMTP id 815916B004D for ; Mon, 9 Dec 2013 03:44:34 -0500 (EST) Received: by mail-pb0-f45.google.com with SMTP id rp16so4987801pbb.18 for ; Mon, 09 Dec 2013 00:44:34 -0800 (PST) Received: from LGEMRELSE7Q.lge.com (LGEMRELSE7Q.lge.com. [156.147.1.151]) by mx.google.com with ESMTP id v7si6614542pbi.68.2013.12.09.00.44.31 for ; Mon, 09 Dec 2013 00:44:33 -0800 (PST) Date: Mon, 9 Dec 2013 17:47:20 +0900 From: Joonsoo Kim Subject: Re: [QUESTION] balloon page isolation needs LRU lock? Message-ID: <20131209084720.GB27201@lge.com> References: <20131206085331.GA24706@lge.com> <20131206122142.GC26883@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131206122142.GC26883@localhost.localdomain> Sender: owner-linux-mm@kvack.org List-ID: To: Rafael Aquini Cc: Mel Gorman , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org