Re: [PATCH v4 1/5] mm: Make per-VMA locks available universally

On Fri, Aug 7, 2026 at 6:14 PM Matthew Wilcox <[email protected]> wrote:
>
> On Thu, Aug 06, 2026 at 01:05:44PM -0700, Suren Baghdasaryan wrote:
> > +++ b/kernel/bpf/stackmap.c
> > @@ -272,13 +272,8 @@ struct stack_map_vma_lock {
> >  /*
> >   * Acquire a stable read-side reference on the VMA covering @ip.
> >   *
> > - * With CONFIG_PER_VMA_LOCK=y this returns a VMA with its per-VMA read
> > - * lock held and mmap_lock dropped, so the caller may sleep.
> > - *
> > - * With CONFIG_PER_VMA_LOCK=n it returns a VMA with mmap_lock still
> > - * held; the caller must snapshot any fields it needs and pin vm_file
> > - * with get_file() before stack_map_unlock_vma() drops mmap_lock, as
> > - * the VMA may be split, merged, or freed after that.
> > + * This returns a VMA with its per-VMA read lock held and mmap_lock
> > + * dropped, so the caller may sleep.
>
> I don't know if BPF is compatible with !MMU or not, but the comment
> is inconsistent with the code.  How about:
>
>  * On NOMMU configurations, returns with the mmap_lock held.  If the MMU
>  * is enabled, the per-VMA lock will be held instead.  The lock
>  * should be released with stack_map_unlock_vma() which will release the
>  * appropriate lock.  Once the lock is released, the VMA may be freed.

Good point. I'll update the comment here. Thanks Matthew!

>
> >   * Returns NULL on failure, in which case no lock is held.
> >   */
> > @@ -288,7 +283,6 @@ stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip)
> >       struct mm_struct *mm = lock->mm;
> >       struct vm_area_struct *vma;
> >
> > -     /* noop under !CONFIG_PER_VMA_LOCK */
> >       vma = lock_vma_under_rcu(mm, ip);
> >       if (vma) {
> >               lock->vma = vma;
> > @@ -308,21 +302,20 @@ stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip)
> >               return NULL;
> >       }
> >
> > -#ifdef CONFIG_PER_VMA_LOCK
> > +#ifdef CONFIG_MMU
> >       if (!vma_start_read_locked(vma)) {
> >               mmap_read_unlock(mm);
> >               return NULL;
> >       }
> >       mmap_read_unlock(mm);
> >  #endif
> > -
> >       lock->vma = vma;
> >       return vma;
> >  }