From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932184AbeCKL2F (ORCPT ); Sun, 11 Mar 2018 07:28:05 -0400 Received: from merlin.infradead.org ([205.233.59.134]:60740 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932120AbeCKL2D (ORCPT ); Sun, 11 Mar 2018 07:28:03 -0400 Date: Sun, 11 Mar 2018 12:27:26 +0100 From: Peter Zijlstra To: Dan Williams Cc: linux-nvdimm@lists.01.org, Ingo Molnar , Christoph Hellwig , david@fromorbit.com, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, jack@suse.cz, ross.zwisler@linux.intel.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 08/11] wait_bit: introduce {wait_on,wake_up}_atomic_one Message-ID: <20180311112725.GC4043@hirez.programming.kicks-ass.net> References: <152066488891.40260.14605734226832760468.stgit@dwillia2-desk3.amr.corp.intel.com> <152066493247.40260.10849841915366086021.stgit@dwillia2-desk3.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <152066493247.40260.10849841915366086021.stgit@dwillia2-desk3.amr.corp.intel.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 09, 2018 at 10:55:32PM -0800, Dan Williams wrote: > Add a generic facility for awaiting an atomic_t to reach a value of 1. > > Page reference counts typically need to reach 0 to be considered a > free / inactive page. However, ZONE_DEVICE pages allocated via > devm_memremap_pages() are never 'onlined', i.e. the put_page() typically > done at init time to assign pages to the page allocator is skipped. > > These pages will have their reference count elevated > 1 by > get_user_pages() when they are under DMA. In order to coordinate DMA to > these pages vs filesytem operations like hole-punch and truncate the > filesystem-dax implementation needs to capture the DMA-idle event i.e. > the 2 to 1 count transition). > > For now, this implementation does not have functional behavior change, > follow-on patches will add waiters for these page-idle events. Argh, no no no.. That whole wait_for_atomic_t thing is a giant trainwreck already and now you're making it worse still. Please have a look here: https://lkml.kernel.org/r/20171101190644.chwhfpoz3ywxx2m7@hirez.programming.kicks-ass.net