Re: WTH is going on with memory hotplug sysf interface

[Yasuaki Ishimatsu <yasu.isimatu@gmail.com>]

On 03/10/2017 08:58 AM, Michal Hocko wrote:
> Let's CC people touching this logic. A short summary is that onlining
> memory via udev is currently unusable for online_movable because blocks
> are added from lower addresses while movable blocks are allowed from
> last blocks. More below.
>
> On Thu 09-03-17 13:54:00, Michal Hocko wrote:
>> On Tue 07-03-17 13:40:04, Igor Mammedov wrote:
>>> On Mon, 6 Mar 2017 15:54:17 +0100
>>> Michal Hocko <mhocko@kernel.org> wrote:
>>>
>>>> On Fri 03-03-17 18:34:22, Igor Mammedov wrote:
>> [...]
>>>>> in current mainline kernel it triggers following code path:
>>>>>
>>>>> online_pages()
>>>>>   ...
>>>>>        if (online_type == MMOP_ONLINE_KERNEL) {
>>>>>                 if (!zone_can_shift(pfn, nr_pages, ZONE_NORMAL, &zone_shift))
>>>>>                         return -EINVAL;
>>>>
>>>> Are you sure? I would expect MMOP_ONLINE_MOVABLE here
>>> pretty much, reproducer is above so try and see for yourself
>>
>> I will play with this...
>
> OK so I did with -m 2G,slots=4,maxmem=4G -numa node,mem=1G -numa node,mem=1G which generated
> [...]
> [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00000000-0x0009ffff]
> [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00100000-0x3fffffff]
> [    0.000000] ACPI: SRAT: Node 1 PXM 1 [mem 0x40000000-0x7fffffff]
> [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x27fffffff] hotplug
> [    0.000000] NUMA: Node 0 [mem 0x00000000-0x0009ffff] + [mem 0x00100000-0x3fffffff] -> [mem 0x00000000-0x3fffffff]
> [    0.000000] NODE_DATA(0) allocated [mem 0x3fffc000-0x3fffffff]
> [    0.000000] NODE_DATA(1) allocated [mem 0x7ffdc000-0x7ffdffff]
> [    0.000000] Zone ranges:
> [    0.000000]   DMA      [mem 0x0000000000001000-0x0000000000ffffff]
> [    0.000000]   DMA32    [mem 0x0000000001000000-0x000000007ffdffff]
> [    0.000000]   Normal   empty
> [    0.000000] Movable zone start for each node
> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000000001000-0x000000000009efff]
> [    0.000000]   node   0: [mem 0x0000000000100000-0x000000003fffffff]
> [    0.000000]   node   1: [mem 0x0000000040000000-0x000000007ffdffff]
>
> so there is neither any normal zone nor movable one at the boot time.
> Then I hotplugged 1G slot
> (qemu) object_add memory-backend-ram,id=mem1,size=1G
> (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
>
> unfortunatelly the memory didn't show up automatically and I got
> [  116.375781] acpi PNP0C80:00: Enumeration failure
>
> so I had to probe it manually (prbably the BIOS my qemu uses doesn't
> support auto probing - I haven't really dug further). Anyway the SRAT
> table printed during the boot told that we should start at 0x100000000
>
> # echo 0x100000000 > /sys/devices/system/memory/probe
> # grep . /sys/devices/system/memory/memory32/valid_zones
> Normal Movable
>
> which looks reasonably right? Both Normal and Movable zones are allowed
>
> # echo $((0x100000000+(128<<20))) > /sys/devices/system/memory/probe
> # grep . /sys/devices/system/memory/memory3?/valid_zones
> /sys/devices/system/memory/memory32/valid_zones:Normal
> /sys/devices/system/memory/memory33/valid_zones:Normal Movable
>
> Huh, so our valid_zones have changed under our feet...
>
> # echo $((0x100000000+2*(128<<20))) > /sys/devices/system/memory/probe
> # grep . /sys/devices/system/memory/memory3?/valid_zones
> /sys/devices/system/memory/memory32/valid_zones:Normal
> /sys/devices/system/memory/memory33/valid_zones:Normal
> /sys/devices/system/memory/memory34/valid_zones:Normal Movable
>
> and again. So only the last memblock is considered movable. Let's try to
> online them now.
>
> # echo online_movable > /sys/devices/system/memory/memory34/state
> # grep . /sys/devices/system/memory/memory3?/valid_zones
> /sys/devices/system/memory/memory32/valid_zones:Normal
> /sys/devices/system/memory/memory33/valid_zones:Normal Movable
> /sys/devices/system/memory/memory34/valid_zones:Movable Normal
>

I think there is no strong reason which kernel has the restriction.
By setting the restrictions, it seems to have made management of
these zone structs simple.

Thanks,
Yasuaki Ishimatsu

> This would explain why onlining from the last block actually works but
> to me this sounds like a completely crappy behavior. All we need to
> guarantee AFAICS is that Normal and Movable zones do not overlap. I
> believe there is even no real requirement about ordering of the physical
> memory in Normal vs. Movable zones as long as they do not overlap. But
> let's keep it simple for the start and always enforce the current status
> quo that Normal zone is physically preceeding Movable zone.
> Can somebody explain why we cannot have a simple rule for Normal vs.
> Movable which would be:
> 	- block [pfn, pfn+block_size] can be Normal if
> 	  !zone_populated(MOVABLE) || pfn+block_size < ZONE_MOVABLE->zone_start_pfn
> 	- block [pfn, pfn+block_size] can be Movable if
> 	  !zone_populated(NORMAL) || ZONE_NORMAL->zone_end_pfn < pfn
>
> I haven't fully grokked all the restrictions on the movable zone size
> based on the kernel parameters (find_zone_movable_pfns_for_nodes) but
> this shouldn't really make the situation really much more complicated I
> believe because those parameters should be mostly about static
> initialization rather than hotplug but I might be easily missing
> something.
>