Re: [PATCH] x86/mm: use max memory block size with unaligned memory end

From: Daniel Jordan
Date: Thu Jun 04 2020 - 18:27:43 EST

On Thu, Jun 04, 2020 at 01:00:55PM -0700, Dave Hansen wrote:
> On 6/4/20 11:12 AM, Daniel Jordan wrote:
> >> E.g., on powerpc that's 16MB so they have *a lot* of memory blocks.
> >> That's why that's not papering over the problem. Increasing the memory
> >> block size isn't always the answer.
> > Ok. If you don't mind, what's the purpose of hotplugging at that granularity?
> > I'm simply curious.
> FWIW, the 128MB on x86 came from the original sparsemem/hotplug
> implementation. It was the size of the smallest DIMM that my server
> system at the time would take. ppc64's huge page size was and is 16MB
> and that's also the granularity with which hypervisors did hot-add way
> back then. I'm not actually sure what they do now.

Interesting, that tells me a lot more than the "matt - 128 is convenient right
now" comment that has always weirdly stuck out at me.

> I actually can't think of anything that's *keeping* it at 128MB on x86
> though. We don't, for instance, require a whole section to be
> pfn_valid().

Hm, something to look into.