[PATCH 1/3] mm: don't prepare anon_vma if vma has VM_WIPEONFORK

[Li Xinhai <lixinhai.lxh@gmail.com>]

In dup_mmap(), anon_vma_prepare() is called for vma has VM_WIPEONFORK,
and parameter 'tmp' (i.e., the new vma of child) has same ->vm_next and
->vm_prev as its parent vma. That allows anon_vma used by parent been
mistakenly shared by child (find_mergeable_anon_vma() will do this reuse
work).

Besides this issue, call anon_vma_prepare() should be avoided because we
don't copy page for this vma. Preparing anon_vma will be handled during
fault.

Fixes: d2cd9ede6e19 ("mm,fork: introduce MADV_WIPEONFORK")
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
---
 kernel/fork.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 0808095..1bbd49a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -552,10 +552,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 		if (retval)
 			goto fail_nomem_anon_vma_fork;
 		if (tmp->vm_flags & VM_WIPEONFORK) {
-			/* VM_WIPEONFORK gets a clean slate in the child. */
+			/*
+			 * VM_WIPEONFORK gets a clean slate in the child.
+			 * Don't prepare anon_vma until fault since we don't
+			 * copy page for current vma.
+			 */
 			tmp->anon_vma = NULL;
-			if (anon_vma_prepare(tmp))
-				goto fail_nomem_anon_vma_fork;
 		} else if (anon_vma_fork(tmp, mpnt))
 			goto fail_nomem_anon_vma_fork;
 		tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
-- 
1.8.3.1