Re: [RFC] theoretical race between memory hotplug and pfn iterator

[Zhu Guihua]

On 12/21/2015 03:17 PM, Joonsoo Kim wrote:
> On Mon, Dec 21, 2015 at 03:00:08PM +0800, Zhu Guihua wrote:
> > On 12/21/2015 11:15 AM, Joonsoo Kim wrote:
> > > Hello, memory-hotplug folks.
> > >
> > > I found theoretical problems between memory hotplug and pfn iterator.
> > > For example, pfn iterator works something like below.
> > >
> > > for (pfn = zone_start_pfn; pfn < zone_end_pfn; pfn++) {
> > >          if (!pfn_valid(pfn))
> > >                  continue;
> > >
> > >          page = pfn_to_page(pfn);
> > >          /* Do whatever we want */
> > > }
> > >
> > > Sequence of hotplug is something like below.
> > >
> > > 1) add memmap (after then, pfn_valid will return valid)
> > > 2) memmap_init_zone()
> > >
> > > So, if pfn iterator runs between 1) and 2), it could access
> > > uninitialized page information.
> > >
> > > This problem could be solved by re-ordering initialization steps.
> > >
> > > Hot-remove also has a problem. If memory is hot-removed after
> > > pfn_valid() succeed in pfn iterator, access to page would cause NULL
> > > deference because hot-remove frees corresponding memmap. There is no
> > > guard against free in any pfn iterators.
> > >
> > > This problem can be solved by inserting get_online_mems() in all pfn
> > > iterators but this looks error-prone for future usage. Another idea is
> > > that delaying free corresponding memmap until synchronization point such
> > > as system suspend. It will guarantee that there is no running pfn
> > > iterator. Do any have a better idea?
> > >
> > > Btw, I tried to memory-hotremove with QEMU 2.5.5 but it didn't work. I
> > > followed sequences in doc/memory-hotplug. Do you have any comment on this?
> > I tried memory hot remove with qemu 2.5.5 and RHEL 7, it works well.
> > Maybe you can provide more details, such as guest version, err log.
> I'm testing with qemu 2.5.5 and linux-next-20151209 with reverting
> following two patches.
>
> "mm/memblock.c: use memblock_insert_region() for the empty array"
> "mm-memblock-use-memblock_insert_region-for-the-empty-array-checkpatch-fixes"
>
> When I type "device_del dimm1" in qemu monitor, there is no err log in
> kernel and it looks like command has no effect. I inserted log to
> acpi_memory_device_remove() but there is no message, too. Is there
> another way to check that device_del event is actually transmitted to kernel?

You can use udev to monitor memory device remove event. (udevadm monitor)

> I launch the qemu with following command.
> ./qemu-system-x86_64-recent -enable-kvm -smp 8 -m 4096,slots=16,maxmem=8G ...
>
> Thanks.
>
>
> .



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