[PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from arch-specific code

[Jann Horn]

On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan <surenb at google.com> wrote:
> Introduce lock_vma_under_rcu function to lookup and lock a VMA during
> page fault handling. When VMA is not found, can't be locked or changes
> after being locked, the function returns NULL. The lookup is performed
> under RCU protection to prevent the found VMA from being destroyed before
> the VMA lock is acquired. VMA lock statistics are updated according to
> the results.
> For now only anonymous VMAs can be searched this way. In other cases the
> function returns NULL.
[...]
> +struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> +                                         unsigned long address)
> +{
> +       MA_STATE(mas, &mm->mm_mt, address, address);
> +       struct vm_area_struct *vma, *validate;
> +
> +       rcu_read_lock();
> +       vma = mas_walk(&mas);
> +retry:
> +       if (!vma)
> +               goto inval;
> +
> +       /* Only anonymous vmas are supported for now */
> +       if (!vma_is_anonymous(vma))
> +               goto inval;
> +
> +       if (!vma_read_trylock(vma))
> +               goto inval;
> +
> +       /* Check since vm_start/vm_end might change before we lock the VMA */
> +       if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> +               vma_read_unlock(vma);
> +               goto inval;
> +       }
> +
> +       /* Check if the VMA got isolated after we found it */
> +       mas.index = address;
> +       validate = mas_walk(&mas);

Question for Maple Tree experts:

Are you allowed to use mas_walk() like this? If the first mas_walk()
call encountered a single-entry tree, it would store mas->node =
MAS_ROOT, right? And then the second call would go into
mas_state_walk(), mas_start() would return NULL, mas_is_ptr() would be
true, and then mas_state_walk() would return the result of
mas_start(), which is NULL? And we'd end up with mas_walk() returning
NULL on the second run even though the tree hasn't changed?

> +       if (validate != vma) {
> +               vma_read_unlock(vma);
> +               count_vm_vma_lock_event(VMA_LOCK_MISS);
> +               /* The area was replaced with another one. */
> +               vma = validate;
> +               goto retry;
> +       }
> +
> +       rcu_read_unlock();
> +       return vma;
> +inval:
> +       rcu_read_unlock();
> +       count_vm_vma_lock_event(VMA_LOCK_ABORT);
> +       return NULL;
> +}