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

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Date:   Wed, 21 Sep 2016 09:08:47 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...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

On Wed, Sep 21, 2016 at 12:39:51PM +0530, Aneesh Kumar K.V wrote:
>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().

Post boot, the marking mechanism is not necessary. You can create a 
movable node by putting all of the node's memory into ZONE_MOVABLE 
during the 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 ?

You are looking at the addition step of hotplug. You're correct there, 
the memory is added to the default zone, not ZONE_MOVABLE. The 
transition to ZONE_MOVABLE takes place during the onlining step. In 
online_pages():

	zone = move_pfn_range(zone_shift, pfn, pfn + nr_pages);

The reason we need CONFIG_MOVABLE_NODE is right before that:

	if ((zone_idx(zone) > ZONE_NORMAL ||
	    online_type == MMOP_ONLINE_MOVABLE) &&
	    !can_online_high_movable(zone))
		return -EINVAL;

where can_online_high_movable() is defined like this:

	#ifdef CONFIG_MOVABLE_NODE
	/*
	 * When CONFIG_MOVABLE_NODE, we permit onlining of a node which doesn't have
	 * normal memory.
	 */
	static bool can_online_high_movable(struct zone *zone)
	{
		return true;
	}
	#else /* CONFIG_MOVABLE_NODE */
	/* ensure every online node has NORMAL memory */
	static bool can_online_high_movable(struct zone *zone)
	{
		return node_state(zone_to_nid(zone), N_NORMAL_MEMORY);
	}
	#endif /* CONFIG_MOVABLE_NODE */

To be more clear, I can change the commit log to say "Onlining all of a 
node's memory into ZONE_MOVABLE requires CONFIG_MOVABLE_NODE".

-- 
Reza Arbab