thp: split_huge_page anon_vma ordering dependency

This documents how split_huge_page is safe vs new vma inserctions into the
anon_vma that may have already released the anon_vma->lock but not
established pmds yet when split_huge_page starts.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -177,6 +177,10 @@ static void anon_vma_chain_link(struct vm_area_struct *vma,
 	list_add(&avc->same_vma, &vma->anon_vma_chain);
 
 	anon_vma_lock(anon_vma);
+	/*
+	 * It's critical to add new vmas to the tail of the anon_vma,
+	 * see comment in huge_memory.c:__split_huge_page().
+	 */
 	list_add_tail(&avc->same_anon_vma, &anon_vma->head);
 	anon_vma_unlock(anon_vma);
 }