On Fri, Apr 12, 2024 at 8:31 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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);
