Re: [PATCH 32/94] kernel/fork: Convert dup_mmap to use maple tree

[Suren Baghdasaryan]

On Wed, Apr 28, 2021 at 8:36 AM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
>
> Use the maple tree iterator to duplicate the mm_struct trees.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
>  include/linux/mm.h       |  2 --
>  include/linux/sched/mm.h |  3 +++
>  kernel/fork.c            | 24 +++++++++++++++++++-----
>  mm/mmap.c                |  4 ----
>  4 files changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e89bacfa9145..7f7dff6ad884 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2498,8 +2498,6 @@ extern bool arch_has_descending_max_zone_pfns(void);
>  /* nommu.c */
>  extern atomic_long_t mmap_pages_allocated;
>  extern int nommu_shrink_inode_mappings(struct inode *, size_t, size_t);
> -/* maple_tree */
> -void vma_store(struct mm_struct *mm, struct vm_area_struct *vma);
>
>  /* interval_tree.c */
>  void vma_interval_tree_insert(struct vm_area_struct *node,
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index e24b1fe348e3..76cab3aea6ab 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -8,6 +8,7 @@
>  #include <linux/mm_types.h>
>  #include <linux/gfp.h>
>  #include <linux/sync_core.h>
> +#include <linux/maple_tree.h>
>
>  /*
>   * Routines for handling mm_structs
> @@ -67,11 +68,13 @@ static inline void mmdrop(struct mm_struct *mm)
>   */
>  static inline void mmget(struct mm_struct *mm)
>  {
> +       mt_set_in_rcu(&mm->mm_mt);
>         atomic_inc(&mm->mm_users);
>  }
>
>  static inline bool mmget_not_zero(struct mm_struct *mm)
>  {
> +       mt_set_in_rcu(&mm->mm_mt);

Should you be calling mt_set_in_rcu() if atomic_inc_not_zero() failed?
I don't think mmput() is called after mmget_not_zero() fails and
mt_clear_in_rcu() will not be called.

>         return atomic_inc_not_zero(&mm->mm_users);
>  }
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index c37abaf28eb9..832416ff613e 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -477,7 +477,9 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>         struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
>         struct rb_node **rb_link, *rb_parent;
>         int retval;
> -       unsigned long charge;
> +       unsigned long charge = 0;
> +       MA_STATE(old_mas, &oldmm->mm_mt, 0, 0);
> +       MA_STATE(mas, &mm->mm_mt, 0, 0);
>         LIST_HEAD(uf);
>
>         uprobe_start_dup_mmap();
> @@ -511,7 +513,13 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>                 goto out;
>
>         prev = NULL;
> -       for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
> +
> +       retval = mas_entry_count(&mas, oldmm->map_count);
> +       if (retval)
> +               goto fail_nomem;
> +
> +       rcu_read_lock();
> +       mas_for_each(&old_mas, mpnt, ULONG_MAX) {
>                 struct file *file;
>
>                 if (mpnt->vm_flags & VM_DONTCOPY) {
> @@ -525,7 +533,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>                  */
>                 if (fatal_signal_pending(current)) {
>                         retval = -EINTR;
> -                       goto out;
> +                       goto loop_out;
>                 }
>                 if (mpnt->vm_flags & VM_ACCOUNT) {
>                         unsigned long len = vma_pages(mpnt);
> @@ -594,7 +602,9 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>                 rb_parent = &tmp->vm_rb;
>
>                 /* Link the vma into the MT */
> -               vma_store(mm, tmp);
> +               mas.index = tmp->vm_start;
> +               mas.last = tmp->vm_end - 1;
> +               mas_store(&mas, tmp);
>
>                 mm->map_count++;
>                 if (!(tmp->vm_flags & VM_WIPEONFORK))
> @@ -604,14 +614,17 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>                         tmp->vm_ops->open(tmp);
>
>                 if (retval)
> -                       goto out;
> +                       goto loop_out;
>         }
>         /* a new mm has just been created */
>         retval = arch_dup_mmap(oldmm, mm);
> +loop_out:
>  out:
> +       rcu_read_unlock();
>         mmap_write_unlock(mm);
>         flush_tlb_mm(oldmm);
>         mmap_write_unlock(oldmm);
> +       mas_destroy(&mas);
>         dup_userfaultfd_complete(&uf);
>  fail_uprobe_end:
>         uprobe_end_dup_mmap();
> @@ -1092,6 +1105,7 @@ static inline void __mmput(struct mm_struct *mm)
>  {
>         VM_BUG_ON(atomic_read(&mm->mm_users));
>
> +       mt_clear_in_rcu(&mm->mm_mt);
>         uprobe_clear_state(mm);
>         exit_aio(mm);
>         ksm_exit(mm);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 929c2f9eb3f5..1bd43f4db28e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -780,10 +780,6 @@ static inline void vma_mt_store(struct mm_struct *mm, struct vm_area_struct *vma
>                 GFP_KERNEL);
>  }
>
> -void vma_store(struct mm_struct *mm, struct vm_area_struct *vma) {
> -       vma_mt_store(mm, vma);
> -}
> -
>  static void
>  __vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
>         struct vm_area_struct *prev, struct rb_node **rb_link,
> --
> 2.30.2