Re: [PATCH 3/4] mm: Fix some minor per-VMA lock issues in userfaultfd

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Apr 26, 2024 at 7:45 AM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> Rename lock_vma() to uffd_lock_vma() because it really is uffd specific.
> Remove comment referencing unlock_vma() which doesn't exist.
> Fix the comment about lock_vma_under_rcu() which I just made incorrect.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/userfaultfd.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e6486923263c..defa5109cc62 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -56,17 +56,16 @@ struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
>
>  #ifdef CONFIG_PER_VMA_LOCK
>  /*
> - * lock_vma() - Lookup and lock vma corresponding to @address.
> + * uffd_lock_vma() - Lookup and lock vma corresponding to @address.
>   * @mm: mm to search vma in.
>   * @address: address that the vma should contain.
>   *
> - * Should be called without holding mmap_lock. vma should be unlocked after use
> - * with unlock_vma().
> + * Should be called without holding mmap_lock.
>   *
>   * Return: A locked vma containing @address, -ENOENT if no vma is found, or
>   * -ENOMEM if anon_vma couldn't be allocated.
>   */
> -static struct vm_area_struct *lock_vma(struct mm_struct *mm,
> +static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
>                                        unsigned long address)
>  {
>         struct vm_area_struct *vma;
> @@ -74,9 +73,8 @@ static struct vm_area_struct *lock_vma(struct mm_struct *mm,
>         vma = lock_vma_under_rcu(mm, address);
>         if (vma) {
>                 /*
> -                * lock_vma_under_rcu() only checks anon_vma for private
> -                * anonymous mappings. But we need to ensure it is assigned in
> -                * private file-backed vmas as well.
> +                * We know we're going to need to use anon_vma, so check
> +                * that early.
>                  */
>                 if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
>                         vma_end_read(vma);
> @@ -107,7 +105,7 @@ static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
>  {
>         struct vm_area_struct *dst_vma;
>
> -       dst_vma = lock_vma(dst_mm, dst_start);
> +       dst_vma = uffd_lock_vma(dst_mm, dst_start);
>         if (IS_ERR(dst_vma) || validate_dst_vma(dst_vma, dst_start + len))
>                 return dst_vma;
>
> @@ -1436,7 +1434,7 @@ static int uffd_move_lock(struct mm_struct *mm,
>         struct vm_area_struct *vma;
>         int err;
>
> -       vma = lock_vma(mm, dst_start);
> +       vma = uffd_lock_vma(mm, dst_start);
>         if (IS_ERR(vma))
>                 return PTR_ERR(vma);
>
> @@ -1451,7 +1449,7 @@ static int uffd_move_lock(struct mm_struct *mm,
>         }
>
>         /*
> -        * Using lock_vma() to get src_vma can lead to following deadlock:
> +        * Using uffd_lock_vma() to get src_vma can lead to following deadlock:
>          *
>          * Thread1                              Thread2
>          * -------                              -------
> @@ -1473,7 +1471,7 @@ static int uffd_move_lock(struct mm_struct *mm,
>         err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
>         if (!err) {
>                 /*
> -                * See comment in lock_vma() as to why not using
> +                * See comment in uffd_lock_vma() as to why not using
>                  * vma_start_read() here.
>                  */
>                 down_read(&(*dst_vmap)->vm_lock->lock);
> --
> 2.43.0
>