On 08/29/2017 05:34 PM, Peter Zijlstra wrote:

On Tue, Aug 29, 2017 at 09:59:30AM +0200, Laurent Dufour wrote:

quoted

On 27/08/2017 02:18, Kirill A. Shutemov wrote:

quoted

quoted

+
+	if (unlikely(!vma->anon_vma))
+		goto unlock;

It deserves a comment.

You're right I'll add it in the next version.
For the record, the root cause is that __anon_vma_prepare() requires the
mmap_sem to be held because vm_next and vm_prev must be safe.

But should that test not be:

	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
		goto unlock;

This makes more sense. We are backing off from speculative path
because struct anon_vma has not been created for this anonymous
vma and we cannot do that without holding mmap_sem. This should
have nothing to do with vma->vm_ops availability.