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

On Mon, Aug 10, 2026 at 09:52:11AM +0100, Lorenzo Stoakes (ARM) wrote:

On Sat, Aug 08, 2026 at 02:12:50AM +0100, Matthew Wilcox wrote:

quoted

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:

<requisite nommu rant>

I do think there are components that simply don't think to depend on CONFIG_MMU
even though they do.

In fact more than think - have run into exactly that before.

It's another thing that speaks to nommu being a legacy barnacle that bashes us
on the head fairly regularly for little to no gain (and nobody is testing it for
tip kernel AFAICT).

</requisite nommu rant>

quoted

 * 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.

I mean I suppose it's accurate but I don't love the idea of essentially implying
nommu+bpf is a thing and also treating it as so important that it must be called
out here.

I'd rather it be inaccurate for nommu as are most comments in mm and mm-adjacent
components, it's kinda implied in general. Those who care can look at the code.

Actually scratch that I'm wrong, firstly Suren's series explicitly does some
nommu-specific logic in the bpf code and secondly BPF already has nommu-specific
stuff in it.

It seems nommu bpf is (kinda) a supported path.

quoted

quoted

  * Returns NULL on failure, in which case no lock is held.
  */

--
Cheers, Lorenzo