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This reverts commit4e4a9eb921("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] commitd0e9fe1758("Simplify and comment on anon_vma re-use for anon_vma_prepare()") explains the test of "list_is_singular()". Fixes:4e4a9eb921("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>
Linux kernel
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