Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Apr 12, 2024 at 8:31 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Apr 12, 2024 at 04:19:27PM +0100, Matthew Wilcox wrote:
> > On Fri, Apr 12, 2024 at 09:53:29AM -0500, Suren Baghdasaryan wrote:
> > > Unless vmf_anon_prepare() already explains why vma->anon_vma poses a
> > > problem for per-vma locks, we should have an explanation there. This
> > > comment would serve that purpose IMO.
> >
> > I'll do you one better; here's some nice kernel-doc for
> > vmd_anon_prepare():
>
> And here's a followup patch to fix some minor issues in uffd.
>
>  - Rename lock_vma() to uffd_lock_vma() because it really is uffd
>    specific.

I'm planning to expand the scope of lock_vma() and reuse it for
/proc/pid/maps reading under per-VMA locks. No objection to renaming
it for now but I'll likely rename it back later once it's used in more
places.

>  - Remove comment referencing unlock_vma() which doesn't exist.
>  - Fix the comment about lock_vma_under_rcu() which I just made
>    incorrect.
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index f6267afe65d1..a32171158c38 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;
>
> @@ -1437,7 +1435,7 @@ 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);
>
> @@ -1452,7 +1450,7 @@ 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
>          * -------                              -------
> @@ -1474,7 +1472,7 @@ 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);