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

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Date:   Thu, 4 Jun 2020 18:27:52 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH] x86/mm: use max memory block size with unaligned memory
 end

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.