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

On Fri, Aug 7, 2026 at 6:14 PM Matthew Wilcox [off-list ref] wrote:

On Thu, Aug 06, 2026 at 01:05:44PM -0700, Suren Baghdasaryan wrote:

quoted

+++ 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!

quoted

  * 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;
 }