Re: [PATCH] mm/vma: predicate vma_assert_stabilised() on CONFIG_DEBUG_VM

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Tue, Nov 10, 2026 at 04:18:35PM +0000, Lorenzo Stoakes (ARM) wrote:
> On Tue, Nov 10, 2026 at 04:07:48PM +0000, Lorenzo Stoakes (ARM) wrote:
> > Andrew - could you update as below?
> >
> > Thanks!
> >
> > On Tue, Nov 10, 2026 at 03:10:40PM +0000, Lorenzo Stoakes (ARM) wrote:
> > [ ... ]
> > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > > index 9ccbbcbd7aca..9c3a9c6525c1 100644
> > > --- a/include/linux/mmap_lock.h
> > > +++ b/include/linux/mmap_lock.h
> > [ ... ]
> > >  /**
> > >   * vma_assert_stabilised() - assert that this VMA cannot be changed from
> > >   * underneath us either by having a VMA or mmap lock held.
> > > @@ -418,9 +423,18 @@ static inline void vma_assert_locked(const struct vm_area_struct *vma)
> > >   *
> > >   * With lockdep disabled we may sometimes race with other threads acquiring the
> > >   * mmap read lock simultaneous with our VMA read lock.
> > > + *
> > > + * Assert only if CONFIG_DEBUG_VM is set.
>
> Also this should say:
>
> 'Assert only if CONFIG_DEBUG_VM or CONFIG_LOCKDEP is set.'
>
> > >   */
> > >  static inline void vma_assert_stabilised(const struct vm_area_struct *vma)
> > >  {
> > > +	if (!IS_ENABLED(CONFIG_DEBUG_VM))
> >
> > Actually I think this should be:
> >
> > 	if (!IS_ENABLED(CONFIG_DEBUG_VM) || !IS_ENABLED(CONFIG_LOCKDEP))
>
> Sorry this should be...
>
> 	if (!IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_LOCKDEP))
>
> (I have a bit of a fever today, can you tell? :P)
>
> Thanks!

OK OK actually. Sorry. The change isn't required, it's fine :P

Please leave the patch as it is.

If !CONFIG_DEBUG_VM but CONFIG_LOCKDEP we don't actually assert anything.

Since this:

	if (IS_ENABLED(CONFIG_LOCKDEP)) {
		if (lockdep_is_held(&vma->vm_mm->mmap_lock))
			return;
	}

Is the only thing that would run in the functino and early-exit if the condition
is hit.

Then it defers to vma_assert_locked() which then reaches:

	if (IS_ENABLED(CONFIG_LOCKDEP)) {
		if (!lock_is_held(__vma_lockdep_map(vma)))
			vma_assert_write_locked(vma);
		return;
	}

And vma_assert_write_locked() uses VM_WARN_ON_ONCE_VMA().

(flu vaccine on sunday, seems to have impacted my brain :P)

--
Cheers, Lorenzo