From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f69.google.com (mail-oi0-f69.google.com [209.85.218.69]) by kanga.kvack.org (Postfix) with ESMTP id 90DA06B0038 for ; Tue, 16 Aug 2016 02:09:28 -0400 (EDT) Received: by mail-oi0-f69.google.com with SMTP id 4so188269009oih.2 for ; Mon, 15 Aug 2016 23:09:28 -0700 (PDT) Received: from lgeamrelo12.lge.com (LGEAMRELO12.lge.com. [156.147.23.52]) by mx.google.com with ESMTP id w204si20029774ita.47.2016.08.15.23.09.27 for ; Mon, 15 Aug 2016 23:09:28 -0700 (PDT) Date: Tue, 16 Aug 2016 15:15:18 +0900 From: Joonsoo Kim Subject: Re: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper Message-ID: <20160816061518.GE17448@js1304-P5Q-DELUXE> References: <20160810091226.6709-1-vbabka@suse.cz> <20160810091226.6709-9-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160810091226.6709-9-vbabka@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Andrew Morton , Michal Hocko , Mel Gorman , David Rientjes , Rik van Riel , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Wed, Aug 10, 2016 at 11:12:23AM +0200, Vlastimil Babka wrote: > Compaction uses a watermark gap of (2UL << order) pages at various places and > it's not immediately obvious why. Abstract it through a compact_gap() wrapper > to create a single place with a thorough explanation. > > Signed-off-by: Vlastimil Babka > Acked-by: Michal Hocko > --- > include/linux/compaction.h | 16 ++++++++++++++++ > mm/compaction.c | 7 +++---- > mm/vmscan.c | 6 +++--- > 3 files changed, 22 insertions(+), 7 deletions(-) > > diff --git a/include/linux/compaction.h b/include/linux/compaction.h > index a1fba9994728..e7f0d34a90fe 100644 > --- a/include/linux/compaction.h > +++ b/include/linux/compaction.h > @@ -58,6 +58,22 @@ enum compact_result { > > struct alloc_context; /* in mm/internal.h */ > > +/* > + * Number of free order-0 pages that should be available above given watermark > + * to make sure compaction has reasonable chance of not running out of free > + * pages that it needs to isolate as migration target during its work. > + */ > +static inline unsigned long compact_gap(unsigned int order) > +{ > + /* > + * Although all the isolations for migration are temporary, compaction > + * may have up to 1 << order pages on its list and then try to split > + * an (order - 1) free page. At that point, a gap of 1 << order might > + * not be enough, so it's safer to require twice that amount. > + */ > + return 2UL << order; > +} I agree with this wrapper function but there is a question. Could you elaborate more on this code comment? Freescanner could keep COMPACT_CLUSTER_MAX freepages on the list. It's not associated with requested order at least for now. Why compact_gap is 2UL << order in this case? 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