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

[Joonsoo Kim]

2015-12-21 17:00 GMT+09:00 Zhu Guihua <zhugh.fnst@xxxxxxxxxxxxxx>:
>
> 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 have tried it but there is no message when I type hot-remove command.

Thanks.
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