From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f197.google.com (mail-qk0-f197.google.com [209.85.220.197]) by kanga.kvack.org (Postfix) with ESMTP id B783C6B0007 for ; Tue, 17 Apr 2018 07:50:10 -0400 (EDT) Received: by mail-qk0-f197.google.com with SMTP id z128so12316326qka.8 for ; Tue, 17 Apr 2018 04:50:10 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id x20si18521642qtb.365.2018.04.17.04.50.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 17 Apr 2018 04:50:09 -0700 (PDT) Subject: Re: [PATCH RFC 2/8] mm: introduce PG_offline References: <20180413131632.1413-1-david@redhat.com> <20180413131632.1413-3-david@redhat.com> <20180413134047.GR17484@dhcp22.suse.cz> From: David Hildenbrand Message-ID: <86717b44-7aad-19d9-b55f-7f9eb40bcaa1@redhat.com> Date: Tue, 17 Apr 2018 13:50:01 +0200 MIME-Version: 1.0 In-Reply-To: <20180413134047.GR17484@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, Steven Rostedt , Ingo Molnar , Andrew Morton , Huang Ying , Greg Kroah-Hartman , Pavel Tatashin , Miles Chen , Vlastimil Babka , Mel Gorman , Rik van Riel , James Hogan , "Levin, Alexander (Sasha Levin)" , open list On 13.04.2018 15:40, Michal Hocko wrote: > On Fri 13-04-18 15:16:26, David Hildenbrand wrote: >> online_pages()/offline_pages() theoretically allows us to work on >> sub-section sizes. This is especially relevant in the context of >> virtualization. It e.g. allows us to add/remove memory to Linux in a VM in >> 4MB chunks. > > Well, theoretically possible but this would require a lot of auditing > because the hotplug and per section assumption is quite a spread one. > >> While the whole section is marked as online/offline, we have to know >> the state of each page. E.g. to not read memory that is not online >> during kexec() or to properly mark a section as offline as soon as all >> contained pages are offline. > > But you cannot use a page flag for that, I am afraid. Page flags are > extremely scarce resource. I haven't looked at the rest of the series > but _if_ we have a bit spare which I am not really sure about then you > should prove there are no other ways around this. BTW, looking at the possible layouts of page->flags, I don't think it will be a problem adding this flag. Especially if we compile this flag only if really needed. We could glue this flag to CONFIG_MEMORY_HOTPLUG_SUBSECTION to something like that, that will be set when our new driver is compiled. So this would not affect anybody just wanting to use ordinary DIMM based hotplug (CONFIG_MEMORY_HOTPLUG). But I am open for other suggestions. I don't think PG_reserved is the right thing to use. And storing for each section which parts are online/offline is also something I would like to avoid. -- Thanks, David / dhildenb