From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756083Ab3KZKXO (ORCPT ); Tue, 26 Nov 2013 05:23:14 -0500 Received: from cantor2.suse.de ([195.135.220.15]:39353 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754652Ab3KZKXJ (ORCPT ); Tue, 26 Nov 2013 05:23:09 -0500 Date: Tue, 26 Nov 2013 10:23:06 +0000 From: Mel Gorman To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Rik van Riel Subject: Re: [PATCH 2/5] mm: compaction: reset cached scanner pfn's before reading them Message-ID: <20131126102306.GF5285@suse.de> References: <1385389570-11393-1-git-send-email-vbabka@suse.cz> <1385389570-11393-3-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1385389570-11393-3-git-send-email-vbabka@suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 25, 2013 at 03:26:07PM +0100, 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: Mel Gorman -- Mel Gorman SUSE Labs