From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ob0-f199.google.com (mail-ob0-f199.google.com [209.85.214.199]) by kanga.kvack.org (Postfix) with ESMTP id 118D6828E1 for ; Tue, 5 Jul 2016 21:37:47 -0400 (EDT) Received: by mail-ob0-f199.google.com with SMTP id fq2so460502273obb.2 for ; Tue, 05 Jul 2016 18:37:47 -0700 (PDT) Received: from lgeamrelo12.lge.com (LGEAMRELO12.lge.com. [156.147.23.52]) by mx.google.com with ESMTP id b75si4150386ioe.115.2016.07.05.18.37.45 for ; Tue, 05 Jul 2016 18:37:46 -0700 (PDT) Date: Wed, 6 Jul 2016 10:41:09 +0900 From: Joonsoo Kim Subject: Re: [patch for-4.7] mm, compaction: prevent VM_BUG_ON when terminating freeing scanner Message-ID: <20160706014109.GC23627@js1304-P5Q-DELUXE> References: <7ecb4f2d-724f-463f-961f-efba1bdb63d2@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: David Rientjes Cc: Vlastimil Babka , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, hughd@google.com, mgorman@techsingularity.net, minchan@kernel.org, stable@vger.kernel.org On Tue, Jul 05, 2016 at 02:01:29PM -0700, David Rientjes wrote: > On Thu, 30 Jun 2016, Vlastimil Babka wrote: > > > > Note: I really dislike the low watermark check in split_free_page() and > > > consider it poor software engineering. The function should split a free > > > page, nothing more. Terminating memory compaction because of a low > > > watermark check when we're simply trying to migrate memory seems like an > > > arbitrary heuristic. There was an objection to removing it in the first > > > proposed patch, but I think we should really consider removing that > > > check so this is simpler. > > > > There's a patch changing it to min watermark (you were CC'd on the series). We > > could argue whether it belongs to split_free_page() or some wrapper of it, but > > I don't think removing it completely should be done. If zone is struggling > > with order-0 pages, a functionality for making higher-order pages shouldn't > > make it even worse. It's also not that arbitrary, even if we succeeded the > > migration and created a high-order page, the higher-order allocation would > > still fail due to watermark checks. Worse, __compact_finished() would keep > > telling the compaction to continue, creating an even longer lag, which is also > > against your recent patches. > > > > I'm suggesting we shouldn't check any zone watermark in split_free_page(): > that function should just split the free page. > > I don't find our current watermark checks to determine if compaction is > worthwhile to be invalid, but I do think that we should avoid checking or > acting on any watermark in isolate_freepages() itself. We could do more > effective checking in __compact_finished() to determine if we should > terminate compaction, but the freeing scanner feels like the wrong place > to do it -- it's also expensive to check while gathering free pages for > memory that we have already successfully isolated as part of the > iteration. > > Do you have any objection to this fix for 4.7? > > Joonson and/or Minchan, does this address the issue that you reported? Unfortunately, I have no test case to trigger it. But, I think that this patch will address it. Anyway, I commented one problem on this patch in other e-mail so please fix it. 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