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authorLi Xinhai <lixinhai.lxh@gmail.com>2020-04-06 20:03:36 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-04-07 10:43:37 -0700
commit23ab76bf90a66d45e557f238f8806d147fc3be5d (patch)
treebc8d9a1e16aeca3a36c185147386b0aded0b592e /mm/rmap.c
parent93949bb21b52aae924fd26f9775c8655dd9e885e (diff)
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()". Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork") Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> 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> Link: http://lkml.kernel.org/r/1581150928-3214-3-git-send-email-lixinhai.lxh@gmail.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/rmap.c')
-rw-r--r--mm/rmap.c13
1 files changed, 0 insertions, 13 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index 2df75a119c83..68fe0472c803 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -275,19 +275,6 @@ 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;