Re: WTH is going on with memory hotplug sysf interface (was: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks)

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 13 Mar 2017 11:43:02 +0100
Michal Hocko <mhocko@kernel.org> wrote:

> On Mon 13-03-17 11:31:10, Igor Mammedov wrote:
> > On Fri, 10 Mar 2017 14:58:07 +0100  
> [...]
> > > [    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.  
> > it could be if hotpluggable memory were present at boot time in E802 table
> > (if I remember right when running on hyperv there is movable zone at boot time),
> > 
> > but in qemu hotpluggable memory isn't put into E820,
> > so zone is allocated later when memory is enumerated
> > by ACPI subsystem and onlined.
> > It causes less issues wrt movable zone and works for
> > different versions of linux/windows as well.
> > 
> > That's where in kernel auto-onlining could be also useful,
> > since user would be able to start-up with with small
> > non removable memory plus several removable DIMMs
> > and have all the memory onlined/available by the time
> > initrd is loaded. (missing piece here is onling
> > removable memory as movable by default).  
> 
> Why we should even care to online that memory that early rather than
> making it available via e820?

It's not forbidden by spec and has less complications
when it comes to removable memory. Declaring it in E820
would add following limitations/drawbacks:
 - firmware should be able to exclude removable memory
   from its usage (currently SeaBIOS nor EFI have to
   know/care about it) => less qemu-guest ABI to maintain.
 - OS should be taught to avoid/move (early) nonmovable
   allocations from removable address ranges.
   There were patches targeting that in recent kernels,
   but it won't work with older kernels that don't have it.
   So limiting a range of OSes that could run on QEMU
   and do memory removal.

E820 less approach works reasonably well with wide range
of guest OSes and less complex that if removable memory
were present it E820. Hence I don't have a compelling
reason to introduce removable memory in E820 as it
only adds to hot(un)plug issues.

I have an off-tree QEMU hack that puts hotremovable
memory added with "-device pc-dimm" on CLI into E820
to experiment with. It could be useful to play
with zone layouts at boot time, so if you are
interested I can rebase it on top of current master
and post it here to play with.