From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751407AbdEBLsK (ORCPT ); Tue, 2 May 2017 07:48:10 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:33899 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750884AbdEBLsI (ORCPT ); Tue, 2 May 2017 07:48:08 -0400 Date: Tue, 2 May 2017 14:37:46 +0300 From: "Kirill A. Shutemov" To: Jerome Glisse Cc: Dan Williams , Ingo Molnar , "linux-kernel@vger.kernel.org" , Linux MM , Ingo Molnar , Andrew Morton , Logan Gunthorpe , Kirill Shutemov Subject: Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference Message-ID: <20170502113746.5ybuix3lnvlk7kxt@node.shutemov.name> References: <1579714997.4315035.1493402406629.JavaMail.zimbra@redhat.com> <1295710462.4327805.1493406971970.JavaMail.zimbra@redhat.com> <20170428193305.GA3912@redhat.com> <20170429101726.cdczojcjjupb7myy@node.shutemov.name> <20170430231421.GA15163@redhat.com> <20170501102359.abopw7hpd4eb6x2w@node.shutemov.name> <20170501135545.GA16772@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170501135545.GA16772@redhat.com> User-Agent: NeoMutt/20170306 (1.8.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 01, 2017 at 09:55:48AM -0400, Jerome Glisse wrote: > On Mon, May 01, 2017 at 01:23:59PM +0300, Kirill A. Shutemov wrote: > > On Sun, Apr 30, 2017 at 07:14:24PM -0400, Jerome Glisse wrote: > > > On Sat, Apr 29, 2017 at 01:17:26PM +0300, Kirill A. Shutemov wrote: > > > > On Fri, Apr 28, 2017 at 03:33:07PM -0400, Jerome Glisse wrote: > > > > > On Fri, Apr 28, 2017 at 12:22:24PM -0700, Dan Williams wrote: > > > > > > 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? > > > > > > > > > > 100% sure on that :) I need to callback into driver for 2->1 transition > > > > > no way around that. If we change ZONE_DEVICE to not have an elevated > > > > > reference count that you need to make a lot more change to mm so that > > > > > ZONE_DEVICE is never use as fallback for memory allocation. Also need > > > > > to make change to be sure that ZONE_DEVICE page never endup in one of > > > > > the path that try to put them back on lru. There is a lot of place that > > > > > would need to be updated and it would be highly intrusive and add a > > > > > lot of special cases to other hot code path. > > > > > > > > Could you explain more on where the requirement comes from or point me to > > > > where I can read about this. > > > > > > > > > > HMM ZONE_DEVICE pages are use like other pages (anonymous or file back page) > > > in _any_ vma. So i need to know when a page is freed ie either as result of > > > unmap, exit or migration or anything that would free the memory. For zone > > > device a page is free once its refcount reach 1 so i need to catch refcount > > > transition from 2->1 > > > > What if we would rework zone device to have pages with refcount 0 at > > start? > > That is a _lot_ of work from top of my head because it would need changes > to a lot of places and likely more hot code path that simply adding some- > thing to put_page() note that i only need something in put_page() i do not > need anything in the get page path. Is adding a conditional branch for > HMM pages in put_page() that much of a problem ? Well, it gets inlined everywhere. Removing zone_device code from get_page() and put_page() saved non-trivial ~140k in vmlinux for allyesconfig. Re-introducing part this bloat would be unfortunate. > > > This is the only way i can inform the device that the page is now free. See > > > > > > https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-v21&id=52da8fe1a088b87b5321319add79e43b8372ed7d > > > > > > There is _no_ way around that. > > > > I'm still not convinced that it's impossible. > > > > Could you describe lifecycle for pages in case of HMM? > > Process malloc something, end it over to some function in the program > that use the GPU that function call GPU API (OpenCL, CUDA, ...) that > trigger a migration to device memory. > > So in the kernel you get a migration like any existing migration, > original page is unmap, if refcount is all ok (no pin) then a device > page is allocated and thing are migrated to device memory. > > What happen after is unknown. Either userspace/kernel driver decide > to migrate back to system memory, either there is an munmap, either > there is a CPU page fault, ... So from that point on the device page > as the exact same life as a regular page. > > Above i describe the migrate case, but you can also have new memory > allocation that directly allocate device memory. For instance if the > GPU do a page fault on an address that isn't back by anything then > we can directly allocate a device page. No migration involve in that > case. > > HMM pages are like any other pages in most respect. Exception are: > - no GUP Hm. How do you exclude GUP? And why is it required? -- Kirill A. Shutemov