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

[Rafael J. Wysocki]

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@xxxxxxxxxxxxx> 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()?

Thanks,
Rafael