From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5D68EC49ED6 for ; Wed, 11 Sep 2019 12:54:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 35808206A1 for ; Wed, 11 Sep 2019 12:54:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568206495; bh=WJ75pINOyrjYQ6BUKvIsPRzwG7TTn0l0O+3DEN1RmDc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=O2eMH3T5eTdSMlcF6r8qOVU+Xzu9iamIx+yj0+qmSkXazodySTopngJH1OrRqahdk zNkshbCcg+MelCMMM4XTTzswymDx5/Urimiw8JMaHrahXUCoh0Mp86tKfECGwuY+rl fokNfDgijziMl9nc5ZfpOYV/WITck3eXC5Sad/Ys= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728006AbfIKMyy (ORCPT ); Wed, 11 Sep 2019 08:54:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:59640 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727806AbfIKMyy (ORCPT ); Wed, 11 Sep 2019 08:54:54 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 1D587B635; Wed, 11 Sep 2019 12:54:51 +0000 (UTC) Date: Wed, 11 Sep 2019 14:54:13 +0200 From: Michal Hocko To: David Hildenbrand Cc: "Michael S. Tsirkin" , Alexander Duyck , Alexander Duyck , virtio-dev@lists.oasis-open.org, kvm list , Catalin Marinas , Dave Hansen , LKML , Matthew Wilcox , linux-mm , Andrew Morton , will@kernel.org, linux-arm-kernel@lists.infradead.org, Oscar Salvador , Yang Zhang , Pankaj Gupta , Konrad Rzeszutek Wilk , Nitesh Narayan Lal , Rik van Riel , lcapitulino@redhat.com, "Wang, Wei W" , Andrea Arcangeli , ying.huang@intel.com, Paolo Bonzini , Dan Williams , Fengguang Wu , "Kirill A. Shutemov" Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \ Message-ID: <20190911125413.GY4023@dhcp22.suse.cz> References: <20190910144713.GF2063@dhcp22.suse.cz> <20190910175213.GD4023@dhcp22.suse.cz> <1d7de9f9f4074f67c567dbb4cc1497503d739e30.camel@linux.intel.com> <20190911113619.GP4023@dhcp22.suse.cz> <20190911080804-mutt-send-email-mst@kernel.org> <20190911121941.GU4023@dhcp22.suse.cz> <20190911122526.GV4023@dhcp22.suse.cz> <4748a572-57b3-31da-0dde-30138e550c3a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4748a572-57b3-31da-0dde-30138e550c3a@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 11-09-19 14:42:41, David Hildenbrand wrote: > On 11.09.19 14:25, Michal Hocko wrote: > > On Wed 11-09-19 14:19:41, Michal Hocko wrote: > >> On Wed 11-09-19 08:08:38, Michael S. Tsirkin wrote: > >>> On Wed, Sep 11, 2019 at 01:36:19PM +0200, Michal Hocko wrote: > >>>> On Tue 10-09-19 14:23:40, Alexander Duyck wrote: > >>>> [...] > >>>>> We don't put any limitations on the allocator other then that it needs to > >>>>> clean up the metadata on allocation, and that it cannot allocate a page > >>>>> that is in the process of being reported since we pulled it from the > >>>>> free_list. If the page is a "Reported" page then it decrements the > >>>>> reported_pages count for the free_area and makes sure the page doesn't > >>>>> exist in the "Boundary" array pointer value, if it does it moves the > >>>>> "Boundary" since it is pulling the page. > >>>> > >>>> This is still a non-trivial limitation on the page allocation from an > >>>> external code IMHO. I cannot give any explicit reason why an ordering on > >>>> the free list might matter (well except for page shuffling which uses it > >>>> to make physical memory pattern allocation more random) but the > >>>> architecture seems hacky and dubious to be honest. It shoulds like the > >>>> whole interface has been developed around a very particular and single > >>>> purpose optimization. > >>>> > >>>> I remember that there was an attempt to report free memory that provided > >>>> a callback mechanism [1], which was much less intrusive to the internals > >>>> of the allocator yet it should provide a similar functionality. Did you > >>>> see that approach? How does this compares to it? Or am I completely off > >>>> when comparing them? > >>>> > >>>> [1] mostly likely not the latest version of the patchset > >>>> http://lkml.kernel.org/r/1502940416-42944-5-git-send-email-wei.w.wang@intel.com > >>> > >>> Linus nacked that one. He thinks invoking callbacks with lots of > >>> internal mm locks is too fragile. > >> > >> I would be really curious how much he would be happy about injecting > >> other restrictions on the allocator like this patch proposes. This is > >> more intrusive as it has a higher maintenance cost longterm IMHO. > > > > Btw. I do agree that callbacks with internal mm locks are not great > > either. We do have a model for that in mmu_notifiers and it is something > > I do consider PITA, on the other hand it is mostly sleepable part of the > > interface which makes it the real pain. The above callback mechanism was > > explicitly documented with restrictions and that the context is > > essentially atomic with no access to particular struct pages and no > > expensive operations possible. So in the end I've considered it > > acceptably painful. Not that I want to override Linus' nack but if > > virtualization usecases really require some form of reporting and no > > other way to do that push people to invent even more interesting > > approaches then we should simply give them/you something reasonable > > and least intrusive to our internals. > > > > The issue with "[PATCH v14 4/5] mm: support reporting free page blocks" > is that it cannot really handle the use case we have here if I am not > wrong. While a page is getting processed by the hypervisor (e.g. > MADV_DONTNEED), it must not get reused. What prevents to use the callback to get a list of pfn ranges to work on and then use something like start_isolate_page_range on the collected pfn ranges to make sure nobody steals pages from under your feet, do your thing and drop the isolated state afterwards. I am saying somethig like because you wouldn't really want a generic has_unmovable_pages but rather if (!page_ref_count(page)) { if (PageBuddy(page)) iter += (1 << page_order(page)) - 1; continue; } subset of it. -- Michal Hocko SUSE Labs