Re: [PATCH v1 0/6] mm: online/offline_pages called w.o. mem_hotplug_lock

[Balbir Singh]

On Wed, Sep 19, 2018 at 09:35:07AM +0200, David Hildenbrand wrote:
> Am 19.09.18 um 03:22 schrieb Balbir Singh:
> > On Tue, Sep 18, 2018 at 01:48:16PM +0200, David Hildenbrand wrote:
> >> 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.
> >>
> > 
> > That seems reasonable, but also implies that device_online() would hold
> > back add/remove memory, could you please also document what mode
> > read/write the locks need to be held? For example can the device_hotplug_lock
> > be held in read mode while add/remove memory via (mem_hotplug_lock) is held
> > in write mode?
> 
> device_hotplug_lock is an ordinary mutex. So no option there.
> 
> Only mem_hotplug_lock is a per CPU RW mutex. And as of now it only
> exists to not require get_online_mems()/put_online_mems() to take the
> device_hotplug_lock. Which is perfectly valid, because these users only
> care about memory (not any other devices) not suddenly vanish. And that
> RW lock makes things fast.
> 
> Any modifications (online/offline/add/remove) require the
> mem_hotplug_lock in write.
> 
> I can add some more details to documentation in patch #6.
> 
> "... we should always hold the mem_hotplug_lock (via
> mem_hotplug_begin/mem_hotplug_done) in write mode to serialize memory
> hotplug" ..."
> 
> "In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in
> read mode allows for a quite efficient get_online_mems/put_online_mems
> implementation, so code accessing memory can protect from that memory
> vanishing."
> 
> Would that work for you?

Yes, Thanks

Balbir Singh.