Messages in this thread | | | Date | Mon, 21 Dec 2015 16:00:57 +0800 | From | Zhu Guihua <> | Subject | Re: [RFC] theoretical race between memory hotplug and pfn iterator |
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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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