Let's add Andi

On Fri 10-03-17 16:53:33, Michal Hocko wrote:

On Fri 10-03-17 14:58:07, Michal Hocko wrote:
[...]

quoted

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

OK, so while I was playing with this setup some more I probably got why
this is done this way. All new memblocks are added to the zone Normal
where they are accounted as spanned but not present. When we do
online_movable we just cut from the end of the Normal zone and move it
to Movable zone. This sounds really awkward. What was the reason to go
this way? Why cannot we simply add those pages to the zone at the online
time?

Answering to myself. So the reason seems to be 9d99aaa31f59 ("[PATCH]
x86_64: Support memory hotadd without sparsemem") which is no longer
true because 
config MEMORY_HOTPLUG
        bool "Allow for memory hot-add"
        depends on SPARSEMEM || X86_64_ACPI_NUMA
        depends on ARCH_ENABLE_MEMORY_HOTPLUG
        depends on COMPILE_TEST || !KASAN

so it is either SPARSEMEM or X86_64_ACPI_NUMA that would have to be enabled.
config X86_64_ACPI_NUMA
        def_bool y
        prompt "ACPI NUMA detection"
        depends on X86_64 && NUMA && ACPI && PCI
        select ACPI_NUMA

But I do not see any way how to enable anything but SPARSEMEM for x86_64
choice
        prompt "Memory model"
        depends on SELECT_MEMORY_MODEL
        default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
        default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
        default FLATMEM_MANUAL

ARCH_SPARSEMEM_DEFAULT is 32b only
config ARCH_DISCONTIGMEM_DEFAULT
        def_bool y
        depends on NUMA && X86_32

and ARCH_SPARSEMEM_DEFAULT is enabeld on 64b. So I guess whatever was
the reason to add this code back in 2006 is not true anymore. So I am
really wondering. Do we absolutely need to assign pages which are not
onlined yet to the ZONE_NORMAL unconditionally? Why cannot we put them
out of any zone and wait for memory online operation to put them where
requested?
-- 
Michal Hocko
SUSE Labs

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