[PATCH] page_cgroup: fix horrid swap accounting regression

[Hugh Dickins]

Why is memcg's swap accounting so broken?  Insane counts, wrong ownership,
unfreeable structures, which later get freed and then accessed after free.

Turns out to be a tiny a little 3.3-rc1 regression in 9fb4b7cc0724
"page_cgroup: add helper function to get swap_cgroup": the helper
function (actually named lookup_swap_cgroup()) returns an address
using void* arithmetic, but the structure in question is a short.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/page_cgroup.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- 3.3-rc6/mm/page_cgroup.c	2012-01-20 08:42:35.320020840 -0800
+++ linux/mm/page_cgroup.c	2012-03-05 19:51:13.535372098 -0800
@@ -379,13 +379,15 @@ static struct swap_cgroup *lookup_swap_c
 	pgoff_t offset = swp_offset(ent);
 	struct swap_cgroup_ctrl *ctrl;
 	struct page *mappage;
+	struct swap_cgroup *sc;
 
 	ctrl = &swap_cgroup_ctrl[swp_type(ent)];
 	if (ctrlp)
 		*ctrlp = ctrl;
 
 	mappage = ctrl->map[offset / SC_PER_PAGE];
-	return page_address(mappage) + offset % SC_PER_PAGE;
+	sc = page_address(mappage);
+	return sc + offset % SC_PER_PAGE;
 }
 
 /**
--
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