On Wed 14-11-18 10:48:09, David Hildenbrand wrote: > On 14.11.18 10:41, Michal Hocko wrote: > > On Wed 14-11-18 10:25:57, David Hildenbrand wrote: > >> On 14.11.18 10:00, Baoquan He wrote: > >>> Hi David, > >>> > >>> On 11/14/18 at 09:18am, David Hildenbrand wrote: > >>>> Code seems to be waiting for the mem_hotplug_lock in read. > >>>> We hold mem_hotplug_lock in write whenever we online/offline/add/remove > >>>> memory. There are two ways to trigger offlining of memory: > >>>> > >>>> 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state" > >>>> > >>>> This always properly took the mem_hotplug_lock. Nothing changed > >>>> > >>>> 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online" > >>>> > >>>> This didn't take the mem_hotplug_lock and I fixed that for this release. > >>>> > >>>> So if you were testing with 1., you should have seen the same error > >>>> before this release (unless there is something else now broken in this > >>>> release). > >>> > >>> Thanks a lot for looking into this. > >>> > >>> I triggered sysrq+t to check threads' state. You can see that we use > >>> firmware to trigger ACPI event to go to acpi_bus_offline(), it truly > >>> didn't take mem_hotplug_lock() and has taken it with your fix in > >>> commit 381eab4a6ee mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock > >>> > >>> [ +0.007062] Workqueue: kacpi_hotplug acpi_hotplug_work_fn > >>> [ +0.005398] Call Trace: > >>> [ +0.002476] ? page_vma_mapped_walk+0x307/0x710 > >>> [ +0.004538] ? page_remove_rmap+0xa2/0x340 > >>> [ +0.004104] ? ptep_clear_flush+0x54/0x60 > >>> [ +0.004027] ? enqueue_entity+0x11c/0x620 > >>> [ +0.005904] ? schedule+0x28/0x80 > >>> [ +0.003336] ? rmap_walk_file+0xf9/0x270 > >>> [ +0.003940] ? try_to_unmap+0x9c/0xf0 > >>> [ +0.003695] ? migrate_pages+0x2b0/0xb90 > >>> [ +0.003959] ? try_offline_node+0x160/0x160 > >>> [ +0.004214] ? __offline_pages+0x6ce/0x8e0 > >>> [ +0.004134] ? memory_subsys_offline+0x40/0x60 > >>> [ +0.004474] ? device_offline+0x81/0xb0 > >>> [ +0.003867] ? acpi_bus_offline+0xdb/0x140 > >>> [ +0.004117] ? acpi_device_hotplug+0x21c/0x460 > >>> [ +0.004458] ? acpi_hotplug_work_fn+0x1a/0x30 > >>> [ +0.004372] ? process_one_work+0x1a1/0x3a0 > >>> [ +0.004195] ? worker_thread+0x30/0x380 > >>> [ +0.003851] ? drain_workqueue+0x120/0x120 > >>> [ +0.004117] ? kthread+0x112/0x130 > >>> [ +0.003411] ? kthread_park+0x80/0x80 > >>> [ +0.005325] ? ret_from_fork+0x35/0x40 > >>> > >> > >> Yes, this is indeed another code path that was fixed (and I didn't > >> actually realize it ;) ). Thanks for the callchain. Before my fix > >> hotplug still would have never succeeded (offline_pages would have > >> silently looped forever) as far as I can tell. > > > > I haven't studied your patch yet so I am not really sure why you have > > added the lock into this path. The memory hotplug locking is certainly > > far from great but I believe we should really rething the scope of the > > lock. There shouldn't be any fundamental reason to use the global lock > > for the full offlining. So rather than moving the lock from one place to > > another we need a range locking I believe. > See the patches for details, the lock was removed on this path by > mistake not by design. OK, so I guess we should plug that hole first I guess. > Replacing the lock by some range lock can now be done. The tricky part > will be get_online_mems(), we'll have to indicate a range somehow. For > online/offline/add/remove, we have the range. I would argue that get_online_mems() needs some rethinking. Callers shouldn't really care that a node went offline. If they care about the specific pfn range of the node to not go away then the range locking should work just fine for them. -- Michal Hocko SUSE Labs
