Re: [PATCH 1/2] mm: add private lock to serialize memory hotplug operations

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Thursday, March 09, 2017 11:15:47 PM Rafael J. Wysocki wrote:
> On Thursday, March 09, 2017 10:10:31 AM Dan Williams wrote:
> > On Thu, Mar 9, 2017 at 5:39 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > > On Thursday, March 09, 2017 02:06:15 PM Heiko Carstens wrote:
> > >> Commit bfc8c90139eb ("mem-hotplug: implement get/put_online_mems")
> > >> introduced new functions get/put_online_mems() and
> > >> mem_hotplug_begin/end() in order to allow similar semantics for memory
> > >> hotplug like for cpu hotplug.
> > >>
> > >> The corresponding functions for cpu hotplug are get/put_online_cpus()
> > >> and cpu_hotplug_begin/done() for cpu hotplug.
> > >>
> > >> The commit however missed to introduce functions that would serialize
> > >> memory hotplug operations like they are done for cpu hotplug with
> > >> cpu_maps_update_begin/done().
> > >>
> > >> This basically leaves mem_hotplug.active_writer unprotected and allows
> > >> concurrent writers to modify it, which may lead to problems as
> > >> outlined by commit f931ab479dd2 ("mm: fix devm_memremap_pages crash,
> > >> use mem_hotplug_{begin, done}").
> > >>
> > >> That commit was extended again with commit b5d24fda9c3d ("mm,
> > >> devm_memremap_pages: hold device_hotplug lock over mem_hotplug_{begin,
> > >> done}") which serializes memory hotplug operations for some call
> > >> sites by using the device_hotplug lock.
> > >>
> > >> In addition with commit 3fc21924100b ("mm: validate device_hotplug is
> > >> held for memory hotplug") a sanity check was added to
> > >> mem_hotplug_begin() to verify that the device_hotplug lock is held.
> > >
> > > Admittedly, I haven't looked at all of the code paths involved in detail yet,
> > > but there's one concern regarding lock/unlock_device_hotplug().
> > >
> > > The actual main purpose of it is to ensure safe removal of devices in cases
> > > when they cannot be removed separately, like when a whole CPU package
> > > (including possibly an entire NUMA node with memory and all) is removed.
> > >
> > > One of the code paths doing that is acpi_scan_hot_remove() which first
> > > tries to offline devices slated for removal and then finally removes them.
> > >
> > > The reason why this needs to be done in two stages is because the offlining
> > > can fail, in which case we will fail the entire operation, while the final
> > > removal step is, well, final (meaning that the devices are gone after it no
> > > matter what).
> > >
> > > This is done under device_hotplug_lock, so that the devices that were taken
> > > offline in stage 1 cannot be brought back online before stage 2 is carried
> > > out entirely, which surely would be bad if it happened.
> > >
> > > Now, I'm not sure if removing lock/unlock_device_hotplug() from the code in
> > > question actually affects this mechanism, but this in case it does, it is one
> > > thing to double check before going ahead with this patch.
> > >
> > 
> > I *think* we're ok in this case because unplugging the CPU package
> > that contains a persistent memory device will trigger
> > devm_memremap_pages() to call arch_remove_memory(). Removing a pmem
> > device can't fail. It may be held off while pages are pinned for DMA
> > memory, but it will eventually complete.
> 
> What about the offlining, though?  Is it guaranteed that no memory from those
> ranges will go back online after the acpi_scan_try_to_offline() call in
> acpi_scan_hot_remove()?

My point is that after the acpi_evaluate_ej0() in acpi_scan_hot_remove() the
hardware is physically gone, so if anything is still doing DMA to that memory at
that point, then the user is going to be unhappy.

Thanks,
Rafael

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