From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 3F3746B00EF for ; Wed, 8 Jun 2011 05:33:15 -0400 (EDT) Date: Wed, 8 Jun 2011 11:33:10 +0200 From: Michal Hocko Subject: Re: [PATCH 1/4] mm: compaction: Ensure that the compaction free scanner does not move to the next zone Message-ID: <20110608093310.GC6742@tiehlicka.suse.cz> References: <1307459225-4481-1-git-send-email-mgorman@suse.de> <1307459225-4481-2-git-send-email-mgorman@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1307459225-4481-2-git-send-email-mgorman@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Andrew Morton , Andrea Arcangeli , Minchan Kim , Thomas Sattler , Ury Stankevich , Andi Kleen , linux-mm , linux-kernel On Tue 07-06-11 16:07:02, Mel Gorman wrote: > Compaction works with two scanners, a migration and a free > scanner. When the scanners crossover, migration within the zone is > complete. The location of the scanner is recorded on each cycle to > avoid excesive scanning. > > When a zone is small and mostly reserved, it's very easy for the > migration scanner to be close to the end of the zone. Then the following > situation can occurs > > o migration scanner isolates some pages near the end of the zone > o free scanner starts at the end of the zone but finds that the > migration scanner is already there > o free scanner gets reinitialised for the next cycle as > cc->migrate_pfn + pageblock_nr_pages > moving the free scanner into the next zone > o migration scanner moves into the next zone > > When this happens, NR_ISOLATED accounting goes haywire because some > of the accounting happens against the wrong zone. One zones counter > remains positive while the other goes negative even though the overall > global count is accurate. This was reported on X86-32 with !SMP because > !SMP allows the negative counters to be visible. The fact that it is > difficult to reproduce on X86-64 is probably just a co-incidence as > the bug should theoritically be possible there. > > Signed-off-by: Mel Gorman > Reviewed-by: Minchan Kim Reviewed-by: Michal Hocko -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org