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

[Reza Arbab <arbab@xxxxxxxxxxxxxxxxxx>]

On Fri, Mar 10, 2017 at 04:53:33PM +0100, Michal Hocko wrote:
> 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.
It's not always zone Normal. See zone_for_memory(). This leads to a 
workaround for having to do online_movable in descending block order. 
Instead of this:
1. probe block 34, probe block 33, probe block 32, ...
2. online_movable 34, online_movable 33, online_movable 32, ...

you can online_movable the first block before adding the rest:

1. probe block 32, online_movable 32
2. probe block 33, probe block 34, ...
        - zone_for_memory() will cause these to start Movable
3. online 33, online 34, ...
        - they're already in Movable, so online_movable is equivalentr

I agree with your general sentiment that this stuff is very 
nonintuitive.
--
Reza Arbab


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