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    Subject[PATCH v12 21/31] mm: Introduce find_vma_rcu()
    Date
    This allows to search for a VMA structure without holding the mmap_sem.

    The search is repeated while the mm seqlock is changing and until we found
    a valid VMA.

    While under the RCU protection, a reference is taken on the VMA, so the
    caller must call put_vma() once it not more need the VMA structure.

    At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
    reference is taken to the VMA by calling get_vma().

    When removing a VMA from the MM RB tree, the VMA is not release immediately
    but at the end of the RCU grace period through vm_rcu_put(). This ensures
    that the VMA remains allocated until the end the RCU grace period.

    Since the vm_file pointer, if valid, is released in put_vma(), there is no
    guarantee that the file pointer will be valid on the returned VMA.

    Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
    ---
    include/linux/mm_types.h | 1 +
    mm/internal.h | 5 ++-
    mm/mmap.c | 76 ++++++++++++++++++++++++++++++++++++++--
    3 files changed, 78 insertions(+), 4 deletions(-)

    diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
    index 6a6159e11a3f..9af6694cb95d 100644
    --- a/include/linux/mm_types.h
    +++ b/include/linux/mm_types.h
    @@ -287,6 +287,7 @@ struct vm_area_struct {

    #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
    atomic_t vm_ref_count;
    + struct rcu_head vm_rcu;
    #endif
    struct rb_node vm_rb;

    diff --git a/mm/internal.h b/mm/internal.h
    index 302382bed406..1e368e4afe3c 100644
    --- a/mm/internal.h
    +++ b/mm/internal.h
    @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma)
    __free_vma(vma);
    }

    -#else
    +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm,
    + unsigned long addr);
    +
    +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */

    static inline void get_vma(struct vm_area_struct *vma)
    {
    diff --git a/mm/mmap.c b/mm/mmap.c
    index c106440dcae7..34bf261dc2c8 100644
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
    {
    write_sequnlock(&mm->mm_seq);
    }
    +
    +static void __vm_rcu_put(struct rcu_head *head)
    +{
    + struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
    + vm_rcu);
    + put_vma(vma);
    +}
    +static void vm_rcu_put(struct vm_area_struct *vma)
    +{
    + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
    + call_rcu(&vma->vm_rcu, __vm_rcu_put);
    +}
    #else
    static inline void mm_write_seqlock(struct mm_struct *mm)
    {
    @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)

    void __free_vma(struct vm_area_struct *vma)
    {
    + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
    + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
    mpol_put(vma_policy(vma));
    vm_area_free(vma);
    }
    @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma)
    /*
    * Close a vm structure and free it, returning the next.
    */
    -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
    +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma)
    {
    struct vm_area_struct *next = vma->vm_next;

    might_sleep();
    + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) &&
    + !RB_EMPTY_NODE(&vma->vm_rb)) {
    + /*
    + * If the VMA is still linked in the RB tree, we must release
    + * that reference by calling put_vma().
    + * This should only happen when called from exit_mmap().
    + * We forcely clear the node to satisfy the chec in
    + * __free_vma(). This is safe since the RB tree is not walked
    + * anymore.
    + */
    + RB_CLEAR_NODE(&vma->vm_rb);
    + put_vma(vma);
    + }
    if (vma->vm_ops && vma->vm_ops->close)
    vma->vm_ops->close(vma);
    if (vma->vm_file)
    @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
    return next;
    }

    +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
    +{
    + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
    + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
    + return __remove_vma(vma);
    +}
    +
    static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
    struct list_head *uf);
    SYSCALL_DEFINE1(brk, unsigned long, brk)
    @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma,

    /* All rb_subtree_gap values must be consistent prior to insertion */
    validate_mm_rb(root, NULL);
    -
    + get_vma(vma);
    rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
    }

    @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
    mm_write_seqlock(mm);
    rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
    mm_write_sequnlock(mm); /* wmb */
    +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
    + /*
    + * Ensure the removal is complete before clearing the node.
    + * Matched by vma_has_changed()/handle_speculative_fault().
    + */
    + RB_CLEAR_NODE(&vma->vm_rb);
    + vm_rcu_put(vma);
    +#endif
    }

    static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
    @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)

    EXPORT_SYMBOL(find_vma);

    +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
    +/*
    + * Like find_vma() but under the protection of RCU and the mm sequence counter.
    + * The vma returned has to be relaesed by the caller through the call to
    + * put_vma()
    + */
    +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr)
    +{
    + struct vm_area_struct *vma = NULL;
    + unsigned int seq;
    +
    + do {
    + if (vma)
    + put_vma(vma);
    +
    + seq = read_seqbegin(&mm->mm_seq);
    +
    + rcu_read_lock();
    + vma = find_vma(mm, addr);
    + if (vma)
    + get_vma(vma);
    + rcu_read_unlock();
    + } while (read_seqretry(&mm->mm_seq, seq));
    +
    + return vma;
    +}
    +#endif
    +
    /*
    * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
    */
    @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm)
    while (vma) {
    if (vma->vm_flags & VM_ACCOUNT)
    nr_accounted += vma_pages(vma);
    - vma = remove_vma(vma);
    + vma = __remove_vma(vma);
    }
    vm_unacct_memory(nr_accounted);
    }
    --
    2.21.0
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