From: Manfred Spraul <manfred@colorfullife.com>

The real memory allocation is usually larger than the actual object size:
either due to L1 cache line padding, or due to page padding with
CONFIG_DEBUG_PAGEALLOC.  Right now objects are placed to the beginning of
the real allocation, but to trigger bugs it's better to move objects to the
end of the real allocation: that way accesses behind the end of the
allocation have a larger chance of hitting the (unmapped) next page.  The
attached patch moves the objects to align them with the end of the real
allocation.

Actually it contains 4 seperate changes:

- Do not page-pad allocations that are <= SMP_CACHE_LINE_SIZE.  This
  crashes.  Right now the limit is hardcoded to 128 bytes, but sooner or
  later an arch will appear with 256 byte cache lines.

- cleanup: redzone bytes are not accessed with inline helper functions,
  instead of magic offsets scattered throughout slab.c

- main change: move objects to the end of the allocation - trivial after
  the cleanup.

- Print old redzone value if a redzone mismatch happens: This makes it
  simpler to figure out what happened [single bit error, wrong redzone
  code, overwritten]