Re: [PATCH 2/3] mm: ensure safe rb_subtree_gap update when removing VMA

[Rik van Riel <riel@redhat.com>]

On 11/12/2012 06:51 AM, Michel Lespinasse wrote:
> 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: during VMA removal, remove VMA from the rbtree before we
> remove it from the linked list. The implication is the next vma's
> rb_subtree_gap value becomes stale when next->vm_prev is updated,
> and we want to make sure vma_rb_erase() runs before there are any
> such stale rb_subtree_gap values in the rbtree.
>
> (I don't know of a reproduceable test case for this particular issue)
>
> Signed-off-by: Michel Lespinasse <walken@google.com>

Reviewed-by: Rik van Riel <riel@redhat.com>


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