Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754534AbbFSN6c (ORCPT ); Fri, 19 Jun 2015 09:58:32 -0400 Received: from cantor2.suse.de ([195.135.220.15]:52713 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752879AbbFSN6Y (ORCPT ); Fri, 19 Jun 2015 09:58:24 -0400 Date: Fri, 19 Jun 2015 14:58:20 +0100 From: Mel Gorman To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , David Rientjes Subject: Re: [PATCH 4/6] mm, compaction: always skip compound pages by order in migrate scanner Message-ID: <20150619135820.GC11809@suse.de> References: <1433928754-966-1-git-send-email-vbabka@suse.cz> <1433928754-966-5-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1433928754-966-5-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 Content-Length: 1681 Lines: 34 On Wed, Jun 10, 2015 at 11:32:32AM +0200, Vlastimil Babka wrote: > The compaction migrate scanner tries to skip compound pages by their order, to > reduce number of iterations for pages it cannot isolate. The check is only done > if PageLRU() is true, which means it applies to THP pages, but not e.g. > hugetlbfs pages or any other non-LRU compound pages, which we have to iterate > by base pages. > > This limitation comes from the assumption that it's only safe to read > compound_order() when we have the zone's lru_lock and THP cannot be split under > us. But the only danger (after filtering out order values that are not below > MAX_ORDER, to prevent overflows) is that we skip too much or too little after > reading a bogus compound_order() due to a rare race. This is the same reasoning > as patch 99c0fd5e51c4 ("mm, compaction: skip buddy pages by their order in the > migrate scanner") introduced for unsafely reading PageBuddy() order. > > After this patch, all pages are tested for PageCompound() and we skip them by > compound_order(). The test is done after the test for balloon_page_movable() > as we don't want to assume if balloon pages (or other pages with own isolation > and migration implementation if a generic API gets implemented) are compound > or not. > > When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs > pages, the vmstat compact_migrate_scanned count decreased by 15%. > > Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/