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

[Joonsoo Kim]

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?

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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