This code was an optimization for 32-bit NUMA systems.
It has probably been the cause of a number of subtle bugs over
the years, although the conditions to excite them would have
been hard to trigger. Essentially, we remap part of the kernel
linear mapping area, and then sometimes part of that area gets
freed back in to the bootmem allocator. If those pages get
used by kernel data structures (say mem_map[] or a dentry),
there's no big deal. But, if anyone ever tried to use the
linear mapping for these pages _and_ cared about their physical
address, bad things happen.
For instance, say you passed __GFP_ZERO to the page allocator
and then happened to get handed one of these pages, it zero the
remapped page, but it would make a pte to the _old_ page.
There are probably a hundred other ways that it could screw
with things.
We don't need to hang on to performance optimizations for
these old boxes any more. All my 32-bit NUMA systems are long
dead and buried, and I probably had access to more than most
people.
This code is causing real things to break today:
https://lkml.org/lkml/2013/1/9/376
I looked in to actually fixing this, but it requires surgery
to way too much brittle code, as well as stuff like
per_cpu_ptr_to_phys().
[ hpa: Cc: this for -stable, since it is a memory corruption issue.
However, an alternative is to simply mark NUMA as depends BROKEN
rather than EXPERIMENTAL in the X86_32 subclause... ]
Link: http://lkml.kernel.org/r/20130131005616.1C79F411@kernel.stglabs.ibm.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org>
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