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    Subject[PATCH 08/21] mm: unlink_file_vma, remove_vma
    Divide remove_vm_struct into two parts: first anon_vma_unlink plus
    unlink_file_vma, to unlink the vma from the list and tree by which rmap
    or vmtruncate might find it; then remove_vma to close, fput and free.

    The intention here is to do the anon_vma_unlink and unlink_file_vma
    earlier, in free_pgtables before freeing any page tables: so we can be
    sure that any page tables traversed by rmap and vmtruncate are stable
    (and other, ordinary cases are stabilized by holding mmap_sem).

    This will be crucial to traversing pgd,pud,pmd without page_table_lock.
    But testing the split-out patch showed that lifting the page_table_lock
    is symbiotically necessary to make this change - the lock ordering is
    wrong to move those unlinks into free_pgtables while it's under ptlock.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>
    ---

    include/linux/mm.h | 1 +
    mm/mmap.c | 41 +++++++++++++++++++++++++++--------------
    2 files changed, 28 insertions(+), 14 deletions(-)

    --- mm07/include/linux/mm.h 2005-09-24 19:27:33.000000000 +0100
    +++ mm08/include/linux/mm.h 2005-09-24 19:28:01.000000000 +0100
    @@ -840,6 +840,7 @@ extern int split_vma(struct mm_struct *,
    extern int insert_vm_struct(struct mm_struct *, struct vm_area_struct *);
    extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
    struct rb_node **, struct rb_node *);
    +extern void unlink_file_vma(struct vm_area_struct *);
    extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
    unsigned long addr, unsigned long len, pgoff_t pgoff);
    extern void exit_mmap(struct mm_struct *);
    --- mm07/mm/mmap.c 2005-09-24 19:27:47.000000000 +0100
    +++ mm08/mm/mmap.c 2005-09-24 19:28:01.000000000 +0100
    @@ -177,26 +177,44 @@ static void __remove_shared_vm_struct(st
    }

    /*
    - * Remove one vm structure and free it.
    + * Unlink a file-based vm structure from its prio_tree, to hide
    + * vma from rmap and vmtruncate before freeing its page tables.
    */
    -static void remove_vm_struct(struct vm_area_struct *vma)
    +void unlink_file_vma(struct vm_area_struct *vma)
    {
    struct file *file = vma->vm_file;

    - might_sleep();
    if (file) {
    struct address_space *mapping = file->f_mapping;
    spin_lock(&mapping->i_mmap_lock);
    __remove_shared_vm_struct(vma, file, mapping);
    spin_unlock(&mapping->i_mmap_lock);
    }
    +}
    +
    +/*
    + * Close a vm structure and free it, returning the next.
    + */
    +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
    +{
    + struct vm_area_struct *next = vma->vm_next;
    +
    + /*
    + * Hide vma from rmap and vmtruncate before freeing page tables:
    + * to be moved into free_pgtables once page_table_lock is lifted
    + * from it, but until then lock ordering forbids that move.
    + */
    + anon_vma_unlink(vma);
    + unlink_file_vma(vma);
    +
    + might_sleep();
    if (vma->vm_ops && vma->vm_ops->close)
    vma->vm_ops->close(vma);
    - if (file)
    - fput(file);
    - anon_vma_unlink(vma);
    + if (vma->vm_file)
    + fput(vma->vm_file);
    mpol_free(vma_policy(vma));
    kmem_cache_free(vm_area_cachep, vma);
    + return next;
    }

    asmlinkage unsigned long sys_brk(unsigned long brk)
    @@ -1608,15 +1626,13 @@ find_extend_vma(struct mm_struct * mm, u
    static void remove_vma_list(struct mm_struct *mm, struct vm_area_struct *vma)
    {
    do {
    - struct vm_area_struct *next = vma->vm_next;
    long nrpages = vma_pages(vma);

    mm->total_vm -= nrpages;
    if (vma->vm_flags & VM_LOCKED)
    mm->locked_vm -= nrpages;
    vm_stat_account(mm, vma->vm_flags, vma->vm_file, -nrpages);
    - remove_vm_struct(vma);
    - vma = next;
    + vma = remove_vma(vma);
    } while (vma);
    validate_mm(mm);
    }
    @@ -1940,11 +1956,8 @@ void exit_mmap(struct mm_struct *mm)
    * Walk the list again, actually closing and freeing it
    * without holding any MM locks.
    */
    - while (vma) {
    - struct vm_area_struct *next = vma->vm_next;
    - remove_vm_struct(vma);
    - vma = next;
    - }
    + while (vma)
    + vma = remove_vma(vma);

    BUG_ON(mm->nr_ptes > (FIRST_USER_ADDRESS+PMD_SIZE-1)>>PMD_SHIFT);
    }
    -
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