From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030974AbcCQRdq (ORCPT ); Thu, 17 Mar 2016 13:33:46 -0400 Received: from metis.ext.4.pengutronix.de ([92.198.50.35]:51452 "EHLO metis.ext.4.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030207AbcCQRdo (ORCPT ); Thu, 17 Mar 2016 13:33:44 -0400 Message-ID: <1458236020.18134.17.camel@pengutronix.de> Subject: Re: [PATCH] mm/page_isolation: let caller take the zone lock for test_pages_isolated From: Lucas Stach To: Joonsoo Kim Cc: Linux Memory Management List , LKML , David Rientjes , Vlastimil Babka , kernel@pengutronix.de, patchwork-lst@pengutronix.de Date: Thu, 17 Mar 2016 18:33:40 +0100 In-Reply-To: References: <1458146962-15401-1-git-send-email-l.stach@pengutronix.de> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1+b1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 2001:67c:670:100:fa0f:41ff:fe58:4010 X-SA-Exim-Mail-From: l.stach@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am Freitag, den 18.03.2016, 02:18 +0900 schrieb Joonsoo Kim: > 2016-03-17 1:49 GMT+09:00 Lucas Stach : > > This fixes an annoying race in the CMA code leading to lots of "PFNs busy" > > messages when CMA is used concurrently. This is harmless normally as CMA > > will just retry the allocation at a different place, but it might lead to > > increased fragmentation of the CMA area as well as failing allocations > > when CMA is under memory pressure. > > > > The issue is that test_pages_isolated checks if the range is free by > > checking that all pages in the range are buddy pages. For this to work > > the start pfn needs to be aligned to the higher order buddy page > > including the start pfn if there is any. > > > > This is not a problem for the memory hotplug code, as it always offlines > > whole pageblocks, but CMA may want to isolate a smaller range. So for > > the check to work correctly it down-aligns the start pfn to the higher > > order buddy page. As the zone is not yet locked at that point a > > concurrent page free might coalesce the pages to be checked into an > > even bigger buddy page, causing the check to fail, while all pages are > > in fact buddy pages. > > > > By moving the zone locking to the caller of the test function, it's > > possible to do it before CMA tries to find the proper start page and stop > > any concurrent page coalescing to happen until the check is finished. > > I think that this patch cannot prevent the same race on > isolate_freepages_range(). If buddy merging happens after we > passed test_pages_isolated(), isolate_freepages_range() cannot see > buddy page and will fail. > Your analysis seems correct. I'll fix this patch to hold the zone lock across isolate_freepages_range. Thanks, Lucas