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

Michal Hocko
2017-03-10T15:53:33Z

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