Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752896AbaBDDsJ (ORCPT ); Mon, 3 Feb 2014 22:48:09 -0500 Received: from mail-pb0-f46.google.com ([209.85.160.46]:55238 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752394AbaBDDsH (ORCPT ); Mon, 3 Feb 2014 22:48:07 -0500 Date: Mon, 3 Feb 2014 19:47:21 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: David Rientjes cc: Joonsoo Kim , Mel Gorman , Andrew Morton , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch] mm, compaction: avoid isolating pinned pages fix In-Reply-To: Message-ID: References: <20140203095329.GH6732@suse.de> <20140204000237.GA17331@lge.com> <20140204015332.GA14779@lge.com> <20140204021533.GA14924@lge.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) 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 Mon, 3 Feb 2014, David Rientjes wrote: > On Tue, 4 Feb 2014, Joonsoo Kim wrote: > > > > > Okay. It can't fix your situation. Anyway, *normal* anon pages may be mapped > > > > and have positive page_count(), so your code such as > > > > '!page_mapping(page) && page_count(page)' makes compaction skip these *normal* > > > > anon pages and this is incorrect behaviour. > > > > > > > > > > So how does that work with migrate_page_move_mapping() which demands > > > page_count(page) == 1 and the get_page_unless_zero() in > > > __isolate_lru_page()? > > > > Before doing migrate_page_move_mapping(), try_to_unmap() is called so that all > > mapping is unmapped. Then, remained page_count() is 1 which is grabbed by > > __isolate_lru_page(). Am I missing something? > > > > Ah, good point. I wonder if we can get away with > page_count(page) - page_mapcount(page) > 1 to avoid the get_user_pages() > pin? Something like that. But please go back to migrate_page_move_mapping() to factor in what it's additionally considering. Whether you can share code with it, I don't know - it has to do some things under a lock you cannot take at the preliminary stage - you haven't isolated or locked the page yet. There is a separate issue, that a mapping may supply its own non-default mapping->a_ops->migratepage(): can we assume that the page_counting is the same whatever migratepage() is in use? I'm not sure. If you stick to special-casing PageAnon pages, you won't face that issue; but your proposed change would be a lot more satisfying if we can convince ourselves that it's good for !PageAnon too. May need a trawl through the different migratepage() methods that exist in tree. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/