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

[Matthew Wilcox <willy@infradead.org>]

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.

>   * 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;
>  }