linux-kernel - Re: [PATCH v3] mm: make expand_downwards symmetrical to expand

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Date:	Tue, 19 Apr 2011 13:10:57 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	Pekka Enberg <penberg@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	linux-parisc@...r.kernel.org, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards

On Tue, 19 Apr 2011, James Bottomley wrote:

> > Right !NUMA systems only have node 0.
>
> That's rubbish.  Discontigmem uses the nodes field to identify the
> discontiguous region.  page_to_nid() returns this value.  Your code
> wrongly assumes this is zero for non NUMA.

Sorry the kernel has no node awareness if you do not set CONFIG_NUMA

F.e. zone node lookups work the following way

static inline int
zone_to_nid(struct zone *zone)
{
#ifdef CONFIG_NUMA
        return zone->node;
#else
        return 0;
#endif
}

How in the world did you get a zone setup in node 1 with a !NUMA config?

The problem seems to be that the kernel seems to allow a
definition of a page_to_nid() function that returns non zero in the !NUMA
case. And slub relies on page_to_nid returning zero in the !NUMA case.
Because NODES_WIDTH should be 0 in the !NUMA case and therefore
page_to_nid must return 0.

> I can fix the panic by hard coding get_nodes() to return the zero node
> for the non-numa case ... however, presumably it's more than just this
> that's broken in slub?

If you think that is broken then we have brokenness all over the kernel
whenever we determine the node from a page and use that to do a lookup.

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