mm: debug code to verify rb_subtree_gap updates are safe

mm: debug code to verify rb_subtree_gap updates are safe

Using the trinity fuzzer, Sasha Levin uncovered a case where
rb_subtree_gap wasn't correctly updated.

Digging into this, the root cause was that vma insertions and removals
require both an rbtree insert or erase operation (which may trigger
tree rotations), and an update of the next vma's gap (which does not
change the tree topology, but may require iterating on the node's
ancestors to propagate the update). The rbtree rotations caused the
rb_subtree_gap values to be updated in some of the internal nodes, but
without upstream propagation. Then the subsequent update on the next
vma didn't iterate as high up the tree as it should have, as it
stopped as soon as it hit one of the internal nodes that had been
updated as part of a tree rotation.

The fix is to impose that all rb_subtree_gap values must be up to date
before any rbtree insertion or erase, with the possible exception that
the node being erased doesn't need to have an up to date rb_subtree_gap.

This change: introduce validate_mm_rb() to verify that the rbtree does
not include any stale rb_subtree_gap values before node insertion or
erase, so as to avoid the issue where a subsequent vma_gap_update() would
fail to propagate the rb_subtree_gap updates as high up as necessary.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>