From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753598Ab3KZNQV (ORCPT ); Tue, 26 Nov 2013 08:16:21 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46012 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751540Ab3KZNQT (ORCPT ); Tue, 26 Nov 2013 08:16:19 -0500 Message-ID: <52949F1C.2060607@redhat.com> Date: Tue, 26 Nov 2013 08:16:12 -0500 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Vlastimil Babka CC: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman Subject: Re: [PATCH 2/5] mm: compaction: reset cached scanner pfn's before reading them References: <1385389570-11393-1-git-send-email-vbabka@suse.cz> <1385389570-11393-3-git-send-email-vbabka@suse.cz> In-Reply-To: <1385389570-11393-3-git-send-email-vbabka@suse.cz> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/25/2013 09:26 AM, Vlastimil Babka wrote: > Compaction caches pfn's for its migrate and free scanners to avoid scanning > the whole zone each time. In compact_zone(), the cached values are read to > set up initial values for the scanners. There are several situations when > these cached pfn's are reset to the first and last pfn of the zone, > respectively. One of these situations is when a compaction has been deferred > for a zone and is now being restarted during a direct compaction, which is also > done in compact_zone(). > > However, compact_zone() currently reads the cached pfn's *before* resetting > them. This means the reset doesn't affect the compaction that performs it, and > with good chance also subsequent compactions, as update_pageblock_skip() is > likely to be called and update the cached pfn's to those being processed. > Another chance for a successful reset is when a direct compaction detects that > migration and free scanners meet (which has its own problems addressed by > another patch) and sets update_pageblock_skip flag which kswapd uses to do the > reset because it goes to sleep. > > This is clearly a bug that results in non-deterministic behavior, so this patch > moves the cached pfn reset to be performed *before* the values are read. > > Cc: Mel Gorman > Cc: Rik van Riel > Signed-off-by: Vlastimil Babka Acked-by: Rik van Riel -- All rights reversed