[PATCH 2/3] Revert "mm/rmap.c: reuse mergeable anon_vma as parent when fork"

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

This reverts commit 4e4a9eb921332b9d1edd99f76998f99f36b195f7
In dup_mmap(), anon_vma_fork() is called for attaching anon_vma and
parameter 'tmp'  (i.e., the new vma of child) has same ->vm_next and
->vm_prev as its parent vma. That causes the anon_vma used by parent been
mistakenly shared by child (In anon_vma_clone(), the code added by that
commit will do this reuse work).

Besides this issue, the design of reusing anon_vma from vma which has
gone through fork should be avoided ([1]). So, this patch reverts that
commit and maintains the consistent logic of reusing anon_vma for
fork/split/merge vma.

[1] commit d0e9fe1758f2 ("Simplify and comment on anon_vma re-use for
    anon_vma_prepare()") explains the test of "list_is_singular()".

Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork")
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/rmap.c | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b3e3819..861435b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -269,19 +269,6 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 {
 	struct anon_vma_chain *avc, *pavc;
 	struct anon_vma *root = NULL;
-	struct vm_area_struct *prev = dst->vm_prev, *pprev = src->vm_prev;
-
-	/*
-	 * If parent share anon_vma with its vm_prev, keep this sharing in in
-	 * child.
-	 *
-	 * 1. Parent has vm_prev, which implies we have vm_prev.
-	 * 2. Parent and its vm_prev have the same anon_vma.
-	 */
-	if (!dst->anon_vma && src->anon_vma &&
-	    pprev && pprev->anon_vma == src->anon_vma)
-		dst->anon_vma = prev->anon_vma;
-
 
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;
-- 
1.8.3.1