From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id F34A46B00F3 for ; Mon, 13 Sep 2010 05:14:21 -0400 (EDT) Date: Mon, 13 Sep 2010 10:14:05 +0100 From: Mel Gorman Subject: Re: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page() Message-ID: <20100913091405.GB23508@csn.ul.ie> References: <20100909131211.C93C.A69D9226@jp.fujitsu.com> <20100909092203.GL29263@csn.ul.ie> <20100909182649.C94F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100909182649.C94F.A69D9226@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: KAMEZAWA Hiroyuki , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Linux Kernel List , Rik van Riel , Johannes Weiner , Minchan Kim , Wu Fengguang , Andrea Arcangeli , Dave Chinner , Chris Mason , Christoph Hellwig , Andrew Morton List-ID: On Fri, Sep 10, 2010 at 07:25:43PM +0900, KOSAKI Motohiro wrote: > > On Thu, Sep 09, 2010 at 01:13:22PM +0900, KOSAKI Motohiro wrote: > > > > On Thu, 9 Sep 2010 12:04:48 +0900 > > > > KAMEZAWA Hiroyuki wrote: > > > > > > > > > On Mon, 6 Sep 2010 11:47:28 +0100 > > > > > Mel Gorman wrote: > > > > > > > > > > > From: KOSAKI Motohiro > > > > > > > > > > > > With synchrounous lumpy reclaim, there is no reason to give up to reclaim > > > > > > pages even if page is locked. This patch uses lock_page() instead of > > > > > > trylock_page() in this case. > > > > > > > > > > > > Signed-off-by: KOSAKI Motohiro > > > > > > Signed-off-by: Mel Gorman > > > > > > > > > > Reviewed-by: KAMEZAWA Hiroyuki > > > > > > > > > Ah......but can't this change cause dead lock ?? > > > > > > Yes, this patch is purely crappy. please drop. I guess I was poisoned > > > by poisonous mushroom of Mario Bros. > > > > > > > Lets be clear on what the exact dead lock conditions are. The ones I had > > thought about when I felt this patch was ok were; > > > > o We are not holding the LRU lock (or any lock, we just called cond_resched()) > > o We do not have another page locked because we cannot lock multiple pages > > o Kswapd will never be in LUMPY_MODE_SYNC so it is not getting blocked > > o lock_page() itself is not allocating anything that we could recurse on > > True, all. > > > > > One potential dead lock would be if the direct reclaimer held a page > > lock and ended up here but is that situation even allowed? > > example, > > __do_fault() > { > (snip) > if (unlikely(!(ret & VM_FAULT_LOCKED))) > lock_page(vmf.page); > else > VM_BUG_ON(!PageLocked(vmf.page)); > > /* > * Should we do an early C-O-W break? > */ > page = vmf.page; > if (flags & FAULT_FLAG_WRITE) { > if (!(vma->vm_flags & VM_SHARED)) { > anon = 1; > if (unlikely(anon_vma_prepare(vma))) { > ret = VM_FAULT_OOM; > goto out; > } > page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, > vma, address); > Correct, this is a problem. I already had dropped the patch but thanks for pointing out a deadlock because I was missing this case. Nothing stops the page being faulted being sent to shrink_page_list() when alloc_page_vma() is called. The deadlock might be hard to hit, but it's there. > > Afaik, detailed rule is, > > o kswapd can call lock_page() because they never take page lock outside vmscan lock_page_nosync as you point out in your next mail. While it can call it, kswapd shouldn't because normally it avoids stalls but it would not deadlock as a result of calling it. > o if try_lock() is successed, we can call lock_page_nosync() against its page after unlock. > because the task have gurantee of no lock taken. > o otherwise, direct reclaimer can't call lock_page(). the task may have a lock already. > I think the safer bet is simply to say "direct reclaimers should not call lock_page() because the fault path could be holding a lock on that page already". Thanks. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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