I have no idea why you are maintaining the page->index case here, it's
completely broken for page cache pages. Unless someone more familiar with
the page->index BUG_ON() tests objects, the BUG_ON() there should be
removed completely. Neither Christoph nor I were able to work out what the
point of the page->index tests were, and no one else has spoken up about
them, so it's likely safe to treat them as bogus.
SLOB on nommu will oops on any page cache page in the current git kernels
until those BUG_ON()'s are killed. The more distressing thing is the fact
that the blackfin people have been patching SLOB when it's obvious that
they haven't been testing (ie, even booting!) it.