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    SubjectRe: [PATCH 8/8] mm: enable maple tree RCU mode by default.
    On Mon, 27 Mar 2023 14:55:32 -0400 "Liam R. Howlett" <Liam.Howlett@oracle.com> wrote:

    > Use the maple tree in RCU mode for VMA tracking.
    >
    > The maple tree tracks the stack and is able to update the pivot
    > (lower/upper boundary) in-place to allow the page fault handler to write
    > to the tree while holding just the mmap read lock. This is safe as the
    > writes to the stack have a guard VMA which ensures there will always be
    > a NULL in the direction of the growth and thus will only update a pivot.
    >
    > It is possible, but not recommended, to have VMAs that grow up/down
    > without guard VMAs. syzbot has constructed a testcase which sets up a
    > VMA to grow and consume the empty space. Overwriting the entire NULL
    > entry causes the tree to be altered in a way that is not safe for
    > concurrent readers; the readers may see a node being rewritten or one
    > that does not match the maple state they are using.
    >
    > Enabling RCU mode allows the concurrent readers to see a stable node and

    This differs from what had. Intended?

    --- a/mm/mmap.c~mm-enable-maple-tree-rcu-mode-by-default-v8
    +++ a/mm/mmap.c
    @@ -2277,8 +2277,7 @@ do_vmi_align_munmap(struct vma_iterator
    int count = 0;
    int error = -ENOMEM;
    MA_STATE(mas_detach, &mt_detach, 0, 0);
    - mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags &
    - (MT_FLAGS_LOCK_MASK | MT_FLAGS_USE_RCU));
    + mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
    mt_set_external_lock(&mt_detach, &mm->mmap_lock);

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