From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f173.google.com (mail-ig0-f173.google.com [209.85.213.173]) by kanga.kvack.org (Postfix) with ESMTP id 32DB76B0159 for ; Wed, 11 Jun 2014 20:21:41 -0400 (EDT) Received: by mail-ig0-f173.google.com with SMTP id r2so5132845igi.0 for ; Wed, 11 Jun 2014 17:21:40 -0700 (PDT) Received: from mail-ig0-x230.google.com (mail-ig0-x230.google.com [2607:f8b0:4001:c05::230]) by mx.google.com with ESMTPS id ch18si46664806icb.76.2014.06.11.17.21.40 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 11 Jun 2014 17:21:40 -0700 (PDT) Received: by mail-ig0-f176.google.com with SMTP id a13so7072772igq.9 for ; Wed, 11 Jun 2014 17:21:40 -0700 (PDT) Date: Wed, 11 Jun 2014 17:21:37 -0700 (PDT) From: David Rientjes Subject: Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner In-Reply-To: <5398492E.3070406@suse.cz> Message-ID: References: <1401898310-14525-1-git-send-email-vbabka@suse.cz> <1401898310-14525-4-git-send-email-vbabka@suse.cz> <5390374E.5080708@suse.cz> <53916BB0.3070001@suse.cz> <53959C11.2000305@suse.cz> <5396B31B.6080706@suse.cz> <5398492E.3070406@suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Greg Thelen , Minchan Kim , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel On Wed, 11 Jun 2014, Vlastimil Babka wrote: > > I hate to belabor this point, but I think gcc does treat it differently. > > If you look at the assembly comparing your patch to if you do > > > > unsigned long freepage_order = ACCESS_ONCE(page_private(page)); > > > > instead, then if you enable annotation you'll see that gcc treats the > > store as page_x->D.y.private in your patch vs. MEM[(volatile long unsigned > > int *)page_x + 48B] with the above. > > Hm sure you compiled a version that used page_order_unsafe() and not > page_order()? Because I do see: > > MEM[(volatile long unsigned int *)valid_page_114 + 48B]; > > That's gcc 4.8.1, but our gcc guy said he tried 4.5+ and all was like this. > And that it would be a gcc bug if not. > He also did a test where page_order was called twice in one function and > page_order_unsafe twice in another function. page_order() was reduced to a > single access in the assembly, page_order_unsafe were two accesses. > Ok, and I won't continue to push the point. I think the lockless suitable_migration_target() call that looks at page_order() is fine in the free scanner since we use it as a racy check, but it might benefit from either a comment describing the behavior or a sanity check for page_order(page) <= MAX_ORDER as you've done before. -- 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/ . Don't email: email@kvack.org