Re: WTH is going on with memory hotplug sysf interface

[YASUAKI ISHIMATSU]

On 03/13/2017 05:19 AM, Michal Hocko wrote:
> On Fri 10-03-17 12:39:27, Yasuaki Ishimatsu wrote:
> > On 03/10/2017 08:58 AM, Michal Hocko wrote:
> [...]
> > > 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.
>
> Could you be more specific please? How could this make management any
> easier when udev is basically racing with the physical hotplug and the
> result is basically undefined?

When changing zone from NORMAL(N) to MOVALBE(M), we must resize both zones,
zone->zone_start_pfn and zone->spanned_pages. Currently there is the
restriction.

So we just simply change:
  zone(N)->spanned_pages -= nr_pages
  zone(M)->zone_start_pfn -= nr_pages

But if every memory can change zone with no restriction, we must recalculate
these zones spanned_pages and zone_start_pfn follows:

  memory section #
   1 2 3 4 5 6 7
  |N|M|N|N|N|M|M|
     |
  |N|N|N|N|N|M|M|
 * change memory section #2 from MOVABLE to NORMAL.
   then we must find next movable memory section (#6) and resize these zones.

I think when implementing movable memory, there is no requirement of this.
So kernel has the current restriction.

Thanks,
Yasuaki Ishimatsu