linux-kernel - Re: [PATCH v2 3/3] mm: enable CONFIG_MOVABLE

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Date:   Wed, 21 Sep 2016 12:39:51 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Balbir Singh <bsingharora@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, devicetree@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 3/3] mm: enable CONFIG_MOVABLE_NODE on powerpc

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

> On Mon, Sep 19, 2016 at 11:59:35AM +0530, Aneesh Kumar K.V wrote:
>>Movable node also does.
>>	memblock_set_bottom_up(true);
>>What is the impact of that. Do we need changes equivalent to that ? Also
>>where are we marking the nodes which can be hotplugged, ie where do we
>>do memblock_mark_hotplug() ?
>
> These are related to the mechanism x86 uses to create movable nodes at 
> boot. The SRAT is parsed to mark any hotplug memory. That marking is 
> used later when initializing ZONE_MOVABLE for each node. [1]
>
> The bottom-up allocation is due to a timing issue [2]. There is a window 
> where kernel memory may be allocated before the SRAT is parsed. Any 
> bottom-up allocations done during that time will likely be in the same 
> (nonmovable) node as the kernel image.
>
> On power, I don't think we have a heuristic equivalent to that SRAT 
> memory hotplug info. So, we'll be limited to dynamically adding movable 
> nodes after boot.
>
> 1. http://events.linuxfoundation.org/sites/events/files/lcjp13_chen.pdf
> 2. commit 79442ed189ac ("mm/memblock.c: introduce bottom-up allocation 
> mode")
>

What I was checking was how will one mark a node movable in ppc64 ? I
don't see ppc64 code doing the equivalent of memblock_mark_hotplug().

So when you say "Onlining memory into ZONE_MOVABLE requires
CONFIG_MOVABLE_NODE" where is that restriction ?. IIUC,
should_add_memory_movable() will only return ZONE_MOVABLE only if it is
non empty and MOVABLE_NODE will create a ZONE_MOVABLE zone by default
only if it finds a memblock marked hotpluggable. So wondering if we
are not calling memblock_mark_hotplug() how is it working. Or am I
missing something ?

-aneesh