From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752435AbaEGVVN (ORCPT ); Wed, 7 May 2014 17:21:13 -0400 Received: from mail-pa0-f49.google.com ([209.85.220.49]:35340 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751111AbaEGVVM (ORCPT ); Wed, 7 May 2014 17:21:12 -0400 Date: Wed, 7 May 2014 14:21:10 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Andrew Morton cc: Mel Gorman , Rik van Riel , Vlastimil Babka , Joonsoo Kim , Greg Thelen , Hugh Dickins , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch v3 2/6] mm, compaction: return failed migration target pages back to freelist In-Reply-To: <20140507141534.d4def933b3a9999e7826df5c@linux-foundation.org> Message-ID: References: <20140507141534.d4def933b3a9999e7826df5c@linux-foundation.org> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 7 May 2014, Andrew Morton wrote: > > Memory compaction works by having a "freeing scanner" scan from one end of a > > zone which isolates pages as migration targets while another "migrating scanner" > > scans from the other end of the same zone which isolates pages for migration. > > > > When page migration fails for an isolated page, the target page is returned to > > the system rather than the freelist built by the freeing scanner. This may > > require the freeing scanner to continue scanning memory after suitable migration > > targets have already been returned to the system needlessly. > > > > This patch returns destination pages to the freeing scanner freelist when page > > migration fails. This prevents unnecessary work done by the freeing scanner but > > also encourages memory to be as compacted as possible at the end of the zone. > > > > Reported-by: Greg Thelen > > What did Greg actually report? IOW, what if any observable problem is > being fixed here? > Greg reported by code inspection that he found isolated free pages were returned back to the VM rather than the compaction freelist. This will cause holes behind the free scanner and cause it to reallocate additional memory if necessary later. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f43.google.com (mail-pa0-f43.google.com [209.85.220.43]) by kanga.kvack.org (Postfix) with ESMTP id 647236B005A for ; Wed, 7 May 2014 17:21:13 -0400 (EDT) Received: by mail-pa0-f43.google.com with SMTP id hz1so1674349pad.16 for ; Wed, 07 May 2014 14:21:13 -0700 (PDT) Received: from mail-pa0-x22d.google.com (mail-pa0-x22d.google.com [2607:f8b0:400e:c03::22d]) by mx.google.com with ESMTPS id vw5si14424255pab.251.2014.05.07.14.21.12 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 07 May 2014 14:21:12 -0700 (PDT) Received: by mail-pa0-f45.google.com with SMTP id ey11so1720710pad.4 for ; Wed, 07 May 2014 14:21:12 -0700 (PDT) Date: Wed, 7 May 2014 14:21:10 -0700 (PDT) From: David Rientjes Subject: Re: [patch v3 2/6] mm, compaction: return failed migration target pages back to freelist In-Reply-To: <20140507141534.d4def933b3a9999e7826df5c@linux-foundation.org> Message-ID: References: <20140507141534.d4def933b3a9999e7826df5c@linux-foundation.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Mel Gorman , Rik van Riel , Vlastimil Babka , Joonsoo Kim , Greg Thelen , Hugh Dickins , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Wed, 7 May 2014, Andrew Morton wrote: > > Memory compaction works by having a "freeing scanner" scan from one end of a > > zone which isolates pages as migration targets while another "migrating scanner" > > scans from the other end of the same zone which isolates pages for migration. > > > > When page migration fails for an isolated page, the target page is returned to > > the system rather than the freelist built by the freeing scanner. This may > > require the freeing scanner to continue scanning memory after suitable migration > > targets have already been returned to the system needlessly. > > > > This patch returns destination pages to the freeing scanner freelist when page > > migration fails. This prevents unnecessary work done by the freeing scanner but > > also encourages memory to be as compacted as possible at the end of the zone. > > > > Reported-by: Greg Thelen > > What did Greg actually report? IOW, what if any observable problem is > being fixed here? > Greg reported by code inspection that he found isolated free pages were returned back to the VM rather than the compaction freelist. This will cause holes behind the free scanner and cause it to reallocate additional memory if necessary later. -- 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