[PATCH] fix rmap walk during fork

[Andrea Arcangeli <aarcange@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>

The below bug in fork lead to the rmap walk finding the parent huge-pmd twice
instead of just one, because the anon_vma_chain objects of the child vma still
point to the vma->vm_mm of the parent. The below patch fixes it by making the
rmap walk accurate during fork. It's not a big deal normally but it
worth being accurate considering the cost is the same.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/kernel/fork.c b/kernel/fork.c
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -360,10 +360,10 @@ static int dup_mmap(struct mm_struct *mm
 		if (IS_ERR(pol))
 			goto fail_nomem_policy;
 		vma_set_policy(tmp, pol);
+		tmp->vm_mm = mm;
 		if (anon_vma_fork(tmp, mpnt))
 			goto fail_nomem_anon_vma_fork;
 		tmp->vm_flags &= ~VM_LOCKED;
-		tmp->vm_mm = mm;
 		tmp->vm_next = tmp->vm_prev = NULL;
 		file = tmp->vm_file;
 		if (file) {

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