linux-kernel - Re: [PATCH v4 4/5] mm: make processing of movable

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Date:   Wed, 26 Oct 2016 21:52:53 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Stewart Smith <stewart@...ux.vnet.ibm.com>,
        Alistair Popple <apopple@....ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Tang Chen <tangchen@...fujitsu.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        devicetree@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 4/5] mm: make processing of movable_node arch-specific

Reza Arbab <arbab@...ux.vnet.ibm.com> writes:

> On Wed, Oct 26, 2016 at 09:34:18AM +1100, Balbir Singh wrote:
>>I still believe we need your changes, I was wondering if we've tested
>>it against normal memory nodes and checked if any memblock
>>allocations end up there. Michael showed me some memblock
>>allocations on node 1 of a two node machine with movable_node
>
> The movable_node option is x86-only. Both of those nodes contain normal 
> memory, so allocations on both are allowed.
>
>>> Longer; if you use "movable_node", x86 can identify these nodes at 
>>> boot. They call memblock_mark_hotplug() while parsing the SRAT. Then, 
>>> when the zones are initialized, those markings are used to determine 
>>> ZONE_MOVABLE.
>>>
>>> We have no analog of this SRAT information, so our movable nodes can 
>>> only be created post boot, by hotplugging and explicitly onlining 
>>> with online_movable.
>>
>>Is this true for all of system memory as well or only for nodes
>>hotplugged later?
>
> As far as I know, power has nothing like the SRAT that tells us, at 
> boot, which memory is hotpluggable.

On pseries we have the ibm,dynamic-memory device tree property, which
can contain ranges of memory that are not yet "assigned to the
partition" - ie. can be hotplugged later.

So in general that statement is not true.

But I think you're focused on bare-metal, in which case you might be
right. But that doesn't mean we couldn't have a similar property, if
skiboot/hostboot knew what the ranges of memory were going to be.

cheers