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Date:   Thu, 4 Jun 2020 18:24:57 -0400
From:   Daniel Jordan <daniel.m.jordan@oracle.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Daniel Jordan <daniel.m.jordan@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Sistare <steven.sistare@oracle.com>
Subject: Re: [PATCH] x86/mm: use max memory block size with unaligned memory
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On Thu, Jun 04, 2020 at 08:55:19PM +0200, David Hildenbrand 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.
> 
> On bare metal: none with that big machines AFAIKS. :)

Sounds about right :)

> For VMs/partitions it gives you much more flexibility ("cloud", kata
> containers, memory overcommit, ...).
> 
> Assume you have a VM with some initial memory size (e.g., 32GB). By
> hotplugging up to 256 DIMMs you cab grow in small steps (e.g., 128MB, up
> to 64GB, 256MB, up to 96GB, ...). And if you online all the memory
> blocks MOVABLE, you can shrink in these small steps.

Yeah, sorry for not being clear, I meant why does powerpc hotplug at "only" 16M.

> Regarding PowerPC, AFAIK it also gives the OS more flexibility to find
> memory blocks that can be offlined and unplugged, especially without the
> MOVABLE zone. Finding some scattered 16MB blocks that can be offlined is
> easier than finding one bigger (e.g., 2GB) memory block that can be
> offlined. And the history of powerpc dlpar dates back to pre-MOVABLE
> days (there is a paper from 2003).

Makes sense, thanks!

> I do think your change mostly affects bare metal where you do not care
> about hotplugging small memory blocks. Maybe an even better check would be
> 
> if (!in_vm() {
> 	bz = MAX_BLOCK_SIZE;
> 	goto none;
> }
> 
> because I doubt we have bare metal machines > 64 where we want to
> hot(un)plug DIMMs < 2G.

Yeah, agreed, not these days.

> But maybe there is a use case I am not aware of
> ... and I don't know an easy way to check whether we are running inside
> a VM or not (like kvm_para_available() ... ).

What about this?  Works on bare metal and kvm, so presumably all the other HVs
too.

 if (x86_hyper_type == X86_HYPER_NATIVE) {
	bz = MAX_BLOCK_SIZE;
	goto done;
 }