From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S642390AbdD1TWf (ORCPT ); Fri, 28 Apr 2017 15:22:35 -0400 Received: from mail-it0-f54.google.com ([209.85.214.54]:36223 "EHLO mail-it0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S642374AbdD1TW0 (ORCPT ); Fri, 28 Apr 2017 15:22:26 -0400 MIME-Version: 1.0 In-Reply-To: <1295710462.4327805.1493406971970.JavaMail.zimbra@redhat.com> References: <20170428063913.iz6xjcxblecofjlq@gmail.com> <149339998297.24933.1129582806028305912.stgit@dwillia2-desk3.amr.corp.intel.com> <1743017574.4309811.1493400875692.JavaMail.zimbra@redhat.com> <1579714997.4315035.1493402406629.JavaMail.zimbra@redhat.com> <1295710462.4327805.1493406971970.JavaMail.zimbra@redhat.com> From: Dan Williams Date: Fri, 28 Apr 2017 12:22:24 -0700 Message-ID: Subject: Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference To: Jerome Glisse Cc: Ingo Molnar , "linux-kernel@vger.kernel.org" , Linux MM , Ingo Molnar , Andrew Morton , Logan Gunthorpe , Kirill Shutemov 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 Fri, Apr 28, 2017 at 12:16 PM, Jerome Glisse wrote: >> On Fri, Apr 28, 2017 at 11:00 AM, Jerome Glisse wrote: >> >> On Fri, Apr 28, 2017 at 10:34 AM, Jerome Glisse >> >> wrote: >> >> >> Kirill points out that the calls to {get,put}_dev_pagemap() can be >> >> >> removed from the mm fast path if we take a single get_dev_pagemap() >> >> >> reference to signify that the page is alive and use the final put of >> >> >> the >> >> >> page to drop that reference. >> >> >> >> >> >> This does require some care to make sure that any waits for the >> >> >> percpu_ref to drop to zero occur *after* devm_memremap_page_release(), >> >> >> since it now maintains its own elevated reference. >> >> > >> >> > This is NAK from HMM point of view as i need those call. So if you >> >> > remove >> >> > them now i will need to add them back as part of HMM. >> >> >> >> I thought you only need them at page free time? You can still hook >> >> __put_page(). >> > >> > No, i need a hook when page refcount reach 1, not 0. That being said >> > i don't care about put_dev_pagemap(page->pgmap); so that part of the >> > patch is fine from HMM point of view but i definitly need to hook my- >> > self in the general put_page() function. >> > >> > So i will have to undo part of this patch for HMM (put_page() will >> > need to handle ZONE_DEVICE page differently). >> >> Ok, I'd rather this go in now since it fixes the existing use case, >> and unblocks the get_user_pages_fast() conversion to generic code. >> That also gives Kirill and -mm folks a chance to review what HMM wants >> to do on top of the page_ref infrastructure. The >> {get,put}_zone_device_page interface went in in 4.5 right before >> page_ref went in during 4.6, so it was just an oversight that >> {get,put}_zone_device_page were not removed earlier. >> > > I don't mind this going in, i am hopping people won't ignore HMM patchset > once i repost after 4.12 merge window. Note that there is absolutely no way > around me hooking up inside put_page(). The only other way to do it would > be to modify virtualy all places that call that function to handle ZONE_DEVICE > case. Are you sure about needing to hook the 2 -> 1 transition? Could we change ZONE_DEVICE pages to not have an elevated reference count when they are created so you can keep the HMM references out of the mm hot path?