From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932268AbeCKRP7 (ORCPT ); Sun, 11 Mar 2018 13:15:59 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:43723 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932167AbeCKRP5 (ORCPT ); Sun, 11 Mar 2018 13:15:57 -0400 X-Google-Smtp-Source: AG47ELt5PMjzM68rhcnJ0aNY+BbMjk2henQDFF/02ajQGn0unO1Ec7zhq+PPsV/wXO7eU4p8aHbrUcYmh1sA+mJWk9w= MIME-Version: 1.0 In-Reply-To: <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> <20180311112725.GC4043@hirez.programming.kicks-ass.net> From: Dan Williams Date: Sun, 11 Mar 2018 10:15:55 -0700 Message-ID: Subject: Re: [PATCH v5 08/11] wait_bit: introduce {wait_on,wake_up}_atomic_one To: Peter Zijlstra Cc: linux-nvdimm , Ingo Molnar , Christoph Hellwig , david , linux-xfs , linux-fsdevel , Jan Kara , Ross Zwisler , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 11, 2018 at 4:27 AM, Peter Zijlstra wrote: > 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 That thread seems to be worried about the object disappearing the moment it's reference count reaches a target. That isn't the case with the memmap / struct page objects for ZONE_DEVICE pages. I understand wait_for_atomic_one() is broken in the general case, but as far as I can see it works fine specifically for ZONE_DEVICE page busy tracking, just not generic object lifetime.