[PATCH mmotm] memcg: fix mem_cgroup_update_mapped_file_stat oops

[Hugh Dickins]

CONFIG_SPARSEMEM=y CONFIG_CGROUP_MEM_RES_CTLR=y cgroup_disable=memory
bootup is oopsing in mem_cgroup_update_mapped_file_stat().  !SPARSEMEM
is fine because its lookup_page_cgroup() contains an explicit check for
NULL node_page_cgroup, but the SPARSEMEM version was missing a check for
NULL section->page_cgroup.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---
Should go in as a fix to
memcg-add-file-based-rss-accounting.patch
but it's curious that's the first thing to suffer from this divergence.

Perhaps this is the wrong fix, and there should be an explicit
mem_cgroup_disable() check somewhere else; but it would then seem
dangerous that SPARSEMEM and !SPARSEMEM diverge in this way,
and there are lots of lookup_page_cgroup NULL tests around.

 mm/page_cgroup.c |    2 ++
 1 file changed, 2 insertions(+)

--- 2.6.30-rc3-mm1/mm/page_cgroup.c	2009-04-29 21:01:06.000000000 +0100
+++ mmotm/mm/page_cgroup.c	2009-04-29 21:12:04.000000000 +0100
@@ -99,6 +99,8 @@ struct page_cgroup *lookup_page_cgroup(s
 	unsigned long pfn = page_to_pfn(page);
 	struct mem_section *section = __pfn_to_section(pfn);
 
+	if (!section->page_cgroup)
+		return NULL;
 	return section->page_cgroup + pfn;
 }
 
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