Christoph reported a nice splat which illustrated a race in the new
stack based kmap_atomic implementation.
The problem is that we pop our stack slot before we're completely done
resetting its state -- in particular clearing the PTE (sometimes that's
CONFIG_DEBUG_HIGHMEM). If an interrupt happens before we actually clear
the PTE used for the last slot, that interrupt can reuse the slot in a
dirty state, which triggers a BUG in kmap_atomic().
Fix this by introducing kmap_atomic_idx() which reports the current slot
index without actually releasing it and use that to find the PTE and
delay the _pop() until after we're completely done.
Reported-by: Christoph Hellwig<hch@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra<a.p.zijlstra@xxxxxxxxx>