From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f199.google.com (mail-qt0-f199.google.com [209.85.216.199]) by kanga.kvack.org (Postfix) with ESMTP id 302076B1E45 for ; Tue, 21 Aug 2018 06:44:33 -0400 (EDT) Received: by mail-qt0-f199.google.com with SMTP id o18-v6so15861954qtm.11 for ; Tue, 21 Aug 2018 03:44:33 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id x11-v6si996761qtf.296.2018.08.21.03.44.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 21 Aug 2018 03:44:32 -0700 (PDT) From: David Hildenbrand Subject: [PATCH RFCv2 0/6] mm: online/offline_pages called w.o. mem_hotplug_lock Date: Tue, 21 Aug 2018 12:44:12 +0200 Message-Id: <20180821104418.12710-1-david@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-acpi@vger.kernel.org, xen-devel@lists.xenproject.org, devel@linuxdriverproject.org, David Hildenbrand , Andrew Morton , Balbir Singh , Benjamin Herrenschmidt , Boris Ostrovsky , Dan Williams , Greg Kroah-Hartman , Haiyang Zhang , Heiko Carstens , John Allen , Jonathan Corbet , Joonsoo Kim , Juergen Gross , Kate Stewart , "K. Y. Srinivasan" , Len Brown , Martin Schwidefsky , Mathieu Malaterre , Michael Ellerman , Michael Neuling , Michal Hocko , Nathan Fontenot , Oscar Salvador , Paul Mackerras , Pavel Tatashin , Philippe Ombredanne , "Rafael J. Wysocki" , Rashmica Gupta , Stephen Hemminger , Thomas Gleixner , Vlastimil Babka , YASUAKI ISHIMATSU This is the same approach as in the first RFC, but this time without exporting device_hotplug_lock (requested by Greg) and with some more details and documentation regarding locking. Tested only on x86 so far. -------------------------------------------------------------------------- Reading through the code and studying how mem_hotplug_lock is to be used, I noticed that there are two places where we can end up calling device_online()/device_offline() - online_pages()/offline_pages() without the mem_hotplug_lock. And there are other places where we call device_online()/device_offline() without the device_hotplug_lock. While e.g. echo "online" > /sys/devices/system/memory/memory9/state is fine, e.g. echo 1 > /sys/devices/system/memory/memory9/online Will not take the mem_hotplug_lock. However the device_lock() and device_hotplug_lock. E.g. via memory_probe_store(), we can end up calling add_memory()->online_pages() without the device_hotplug_lock. So we can have concurrent callers in online_pages(). We e.g. touch in online_pages() basically unprotected zone->present_pages then. Looks like there is a longer history to that (see Patch #2 for details), and fixing it to work the way it was intended is not really possible. We would e.g. have to take the mem_hotplug_lock in device/base/core.c, which sounds wrong. Summary: We had a lock inversion on mem_hotplug_lock and device_lock(). More details can be found in patch 3 and patch 6. I propose the general rules (documentation added in patch 6): 1. add_memory/add_memory_resource() must only be called with device_hotplug_lock. 2. remove_memory() must only be called with device_hotplug_lock. This is already documented and holds for all callers. 3. device_online()/device_offline() must only be called with device_hotplug_lock. This is already documented and true for now in core code. Other callers (related to memory hotplug) have to be fixed up. 4. mem_hotplug_lock is taken inside of add_memory/remove_memory/ online_pages/offline_pages. To me, this looks way cleaner than what we have right now (and easier to verify). And looking at the documentation of remove_memory, using lock_device_hotplug also for add_memory() feels natural. RFC -> RFCv2: - Don't export device_hotplug_lock, provide proper remove_memory/add_memory wrappers. - Split up the patches a bit. - Try to improve powernv memtrace locking - Add some documentation for locking that matches my knowledge David Hildenbrand (6): mm/memory_hotplug: make remove_memory() take the device_hotplug_lock mm/memory_hotplug: make add_memory() take the device_hotplug_lock mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock powerpc/powernv: hold device_hotplug_lock when calling device_online() powerpc/powernv: hold device_hotplug_lock in memtrace_offline_pages() memory-hotplug.txt: Add some details about locking internals Documentation/memory-hotplug.txt | 39 +++++++++++- arch/powerpc/platforms/powernv/memtrace.c | 14 +++-- .../platforms/pseries/hotplug-memory.c | 8 +-- drivers/acpi/acpi_memhotplug.c | 4 +- drivers/base/memory.c | 22 +++---- drivers/xen/balloon.c | 3 + include/linux/memory_hotplug.h | 4 +- mm/memory_hotplug.c | 59 +++++++++++++++---- 8 files changed, 115 insertions(+), 38 deletions(-) -- 2.17.1