A small change to the tests for "Bad page state", to avoid one class of the
page_remove_rmap BUG reports, giving more information while letting the
system continue: check page_mapcount (_mapcount != -1) rather than
page_mapped (_mapcount >= 0).

And how does _mapcount go bad?  In the case under study, it looks sure now
that an overheating(?) Pentium III sometimes gets confused by a pair of
instructions in the no-buddy-bitmap __free_pages_bulk, and clears the
PG_private bit from the _mapcount field while buddying around - changing
PG_private value changes the bit cleared from _mapcount.  Bad page state
mapcount:-4096 would have tracked this down much sooner, and will be
recognizable if other cpus show the same aberrant reaction to 2.6.11.

The page_remove_rmap BUG does need to be replaced by more permissive and
informative handling, but I'm not yet ready to to finalize such a patch.

Please admit Colin Harrison to the Order of the Iridescent Penguin, for his
tireless testing.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>