Re: [RFC PATCH 00/12] mm: mirrored memory support for page buddy allocations

From: Luck, Tony
Date: Thu Jun 18 2015 - 16:33:49 EST

On Thu, Jun 18, 2015 at 11:55:42AM +0200, Vlastimil Babka wrote:
> >>>If there are many mirror regions in one node, then it will be many holes in the
> >>>normal zone, is this fine?
> >>
> >>Yeah, it doesn't matter how many holes there are.
> >
> >So mirror zone and normal zone will span each other, right?
> >
> >e.g. node 1: 4G-8G(normal), 8-12G(mirror), 12-16G(normal), 16-24G(mirror), 24-28G(normal) ...
> >normal: start=4G, size=28-4=24G,
> >mirror: start=8G, size=24-8=16G,
> Yes, that works. It's somewhat unfortunate wrt performance that the hardware
> does it like this though.

With current Xeon h/w you can have one mirrored range per memory
controller ... and there are two memory controllers on a cpu socket,
so two mirrored ranges per node. So a map might look like:

SKT0: MC0: 0-2G Mirrored (but we may want to ignore mirror here to keep it for ZONE_DMA)
SKT0: MC0: 2G-4G No memory ... I/O mapping area
SKT0: MC0: 4G-34G Not mirrored
SKT0: MC1: 34G-40G Mirrored
SKT0: MC1: 40G-66G Not mirrored

SKT1: MC0: 66G-70G Mirror
SKT1: MC0: 70G-98G Not Mirrored
SKT1: MC1: 98G-102G Mirror
SKT1: MC1: 102G-130G Not Mirrored

... and so on.

> >I think zone is defined according to the special address range, like 16M(DMA), 4G(DMA32),
> Traditionally yes. But then there is ZONE_MOVABLE, this year's LSF/MM we
> discussed (and didn't outright deny) ZONE_CMA...
> I'm not saying others will favour the new zone approach though, it's just my
> opinion that it might be a better option than a new migratetype.

If we are going to have lots of zones ... then perhaps we will
need a fast way to look at a "struct page" and decide which zone
it belongs to. Complicated math on the address deosn't sound ideal.
If the complex zone model is just for 64-bit, are there enough bits
available in page->flags (3 bits for 8 options ... which we are close
to filling now ... 4 bits for future breathing room).

> >and is it appropriate to add a new mirror zone with a volatile physical address?
> By "volatile" you mean what, that the example above would change
> dynamically? That would be rather challenging...

If we hot-add another cpu together with on die memory controllers connected
to more memory ... then some of the new memory might be mirrored. Current
h/w doesn't allow mirrored areas to grow/shrink (though if there are a lot
of errors we may break a mirror so a whole range could lose the mirror attribute).

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