Re: [Xen-devel] [PATCH RFCv2 0/6] mm: online/offline_pages called w.o. mem_hotplug_lock

[David Hildenbrand <david@xxxxxxxxxx>]

On 21.08.2018 12:44, David Hildenbrand wrote:
> 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.
> 

I'll be on vacation for two weeks starting on Saturday. If there are no
comments I'll send this as !RFC once I return.

Thanks!

> --------------------------------------------------------------------------
> 
> 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(-)
> 


-- 

Thanks,

David / dhildenb

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