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    Subject[PATCH 3.2 104/152] mm: fix corner case in anon_vma endless growing prevention
    3.2.67-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Konstantin Khlebnikov <koct9i@gmail.com>

    commit b800c91a0517071156e772d4fb329ad33590da62 upstream.

    Fix for BUG_ON(anon_vma->degree) splashes in unlink_anon_vmas() ("kernel
    BUG at mm/rmap.c:399!") caused by commit 7a3ef208e662 ("mm: prevent
    endless growth of anon_vma hierarchy")

    Anon_vma_clone() is usually called for a copy of source vma in
    destination argument. If source vma has anon_vma it should be already
    in dst->anon_vma. NULL in dst->anon_vma is used as a sign that it's
    called from anon_vma_fork(). In this case anon_vma_clone() finds
    anon_vma for reusing.

    Vma_adjust() calls it differently and this breaks anon_vma reusing
    logic: anon_vma_clone() links vma to old anon_vma and updates degree
    counters but vma_adjust() overrides vma->anon_vma right after that. As
    a result final unlink_anon_vmas() decrements degree for wrong anon_vma.

    This patch assigns ->anon_vma before calling anon_vma_clone().

    Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
    Reported-and-tested-by: Chris Clayton <chris2553@googlemail.com>
    Reported-and-tested-by: Oded Gabbay <oded.gabbay@amd.com>
    Reported-and-tested-by: Chih-Wei Huang <cwhuang@android-x86.org>
    Acked-by: Rik van Riel <riel@redhat.com>
    Acked-by: Vlastimil Babka <vbabka@suse.cz>
    Cc: Daniel Forrest <dan.forrest@ssec.wisc.edu>
    Cc: Michal Hocko <mhocko@suse.cz>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.2: vma_adjust() didn't use a variable to propagate
    the error code from anon_vma_clone(); change that at the same time]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -537,9 +537,14 @@ again: remove_next = 1 + (end > next->
    * shrinking vma had, to cover any anon pages imported.
    */
    if (exporter && exporter->anon_vma && !importer->anon_vma) {
    - if (anon_vma_clone(importer, exporter))
    - return -ENOMEM;
    + int error;
    +
    importer->anon_vma = exporter->anon_vma;
    + error = anon_vma_clone(importer, exporter);
    + if (error) {
    + importer->anon_vma = NULL;
    + return error;
    + }
    }
    }



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