[patch 003/166] Revert "mm/rmap.c: reuse mergeable anon_vma as parent when fork"

Andrew Morton
2020-04-07T03:03:39Z

From: Li Xinhai <lixinhai.lxh@gmail.com>
Subject: Revert "mm/rmap.c: reuse mergeable anon_vma as parent when fork"

This reverts commit 4e4a9eb921332b9d1 ("mm/rmap.c: reuse mergeable
anon_vma as parent when fork").

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.

Reusing anon_vma within the process is fine. But if a vma has gone
through fork(), then that vma's anon_vma should not be shared with its
neighbor vma. As explained in [1], when vma gone through fork(), the
check for list_is_singular(vma->anon_vma_chain) will be false, and
don't share anon_vma.

With current issue, one example can clarify more. Parent process do
below two steps:

1. p_vma_1 is created and p_anon_vma_1 is prepared;

2. p_vma_2 is created and share p_anon_vma_1; (this is allowed,
becaues p_vma_1 didn't gothrough fork()); parent process do fork():

3. c_vma_1 is dup from p_vma_1, and has its own c_anon_vma_1
prepared; at this point, c_vma_1->anon_vma_chain has two items, one
for p_anon_vma_1 and one for c_anon_vma_1;

4. c_vma_2 is dup from p_vma_2, it is not allowed to share
c_anon_vma_1, because

c_vma_1->anon_vma_chain has two items.
[1] commit d0e9fe1758f2 ("Simplify and comment on anon_vma re-use for
anon_vma_prepare()") explains the test of "list_is_singular()".

Link: http://lkml.kernel.org/r/1581150928-3214-3-git-send-email-lixinhai.lxh@gmail.com
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>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/rmap.c | 13 -------------
1 file changed, 13 deletions(-)

--- a/mm/rmap.c~revert-mm-rmapc-reuse-mergeable-anon_vma-as-parent-when-fork
+++ a/mm/rmap.c
@@ -275,19 +275,6 @@ int anon_vma_clone(struct vm_area_struct
{
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;
_