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author | Li Xinhai <lixinhai.lxh@gmail.com> | 2020-04-06 20:03:36 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2020-04-07 10:43:37 -0700 |
commit | 23ab76bf90a66d45e557f238f8806d147fc3be5d (patch) | |
tree | bc8d9a1e16aeca3a36c185147386b0aded0b592e | |
parent | 93949bb21b52aae924fd26f9775c8655dd9e885e (diff) | |
download | linux-23ab76bf90a66d45e557f238f8806d147fc3be5d.tar.bz2 |
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>
-rw-r--r-- | mm/rmap.c | 13 |
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; |