Re: [PATCH v3] mm: make expand_downwards symmetrical to expand_upwards

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On Tue, 2011-04-19 at 12:15 -0500, Christoph Lameter wrote:
> On Tue, 19 Apr 2011, James Bottomley wrote:
> > On Tue, 2011-04-19 at 20:05 +0300, Pekka Enberg wrote:
> > > > It seems to be a random intermittent mm crash because the next reboot
> > > > crashed with the same trace but after the fsck had completed and the
> > > > third came up to the login prompt.
> > >
> > > Looks like a genuine SLUB problem on parisc. Christoph?
> >
> > Looking through the slub code, it seems to be making invalid
> > assumptions.  All of the node stuff is dependent on CONFIG_NUMA.
> > However, we're CONFIG_DISCONTIGMEM (with CONFIG_NUMA not set): on the
> > machines I and Dave Anglin have, our physical memory ranges are 0-1GB
> > and 64-65GB, so I think slub crashes when we get a page from the high
> > memory range ... because it's not expecting a non-zero node number.
> 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.

This simple program triggers the problem instantly because it forces
allocation up into the upper discontigmem range:

#include <stdlib.h>

void main(void)
  const long size = 1024*1024*1024;
  char *a = malloc(size);
  int i;

  for (i=0; i < size; i += 4096)
    a[i] = '\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?



diff --git a/mm/slub.c b/mm/slub.c
index 94d2a33..243bd9c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -235,7 +235,11 @@ int slab_is_available(void)
 static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
 	return s->node[node];
+	return s->node[0];
 /* Verify that a pointer has an address that is valid within a slab page */

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