Re: [PATCH v4 09/10] mm/vma: update vma_assert_locked() to use lockdep

[Lorenzo Stoakes]

On Mon, Jan 26, 2026 at 04:44:07PM +0000, Lorenzo Stoakes wrote:
> On Mon, Jan 26, 2026 at 02:42:00PM +0100, Vlastimil Babka wrote:
> > On 1/23/26 21:12, Lorenzo Stoakes wrote:
> > > We can use lockdep to avoid unnecessary work here, otherwise update the
> > > code to logically evaluate all pertinent cases and share code with
> > > vma_assert_write_locked().
> > >
> > > Make it clear here that we treat the VMA being detached at this point as a
> > > bug, this was only implicit before.
> > >
> > > Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> >
> > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> Thanks!
>
> >
> > Nit:
> >
> > > ---
> > >  include/linux/mmap_lock.h | 41 +++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 39 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > > index 23bde4bd5a85..4a0aafc66c5d 100644
> > > --- a/include/linux/mmap_lock.h
> > > +++ b/include/linux/mmap_lock.h
> > > @@ -322,19 +322,56 @@ int vma_start_write_killable(struct vm_area_struct *vma)
> > >  	return __vma_start_write(vma, __vma_raw_mm_seqnum(vma), TASK_KILLABLE);
> > >  }
> > >
> > > +/**
> > > + * vma_assert_write_locked() - assert that @vma holds a VMA write lock.
> > > + * @vma: The VMA to assert.
> > > + */
> > >  static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> > >  {
> > >  	VM_WARN_ON_ONCE_VMA(!__is_vma_write_locked(vma), vma);
> > >  }
> > >
> > > +/**
> > > + * vma_assert_locked() - assert that @vma holds either a VMA read or a VMA write
> > > + * lock and is not detached.
> > > + * @vma: The VMA to assert.
> > > + */
> > >  static inline void vma_assert_locked(struct vm_area_struct *vma)
> > >  {
> > > +	unsigned int refcnt;
> > > +
> > > +	/*
> > > +	 * If read-locked or currently excluding readers, then the VMA is
> > > +	 * locked.
> > > +	 */
> > > +#ifdef CONFIG_LOCKDEP
> > > +	if (lock_is_held(&vma->vmlock_dep_map))
> > > +		return;
> >
> > Wouldn't this work a tiny bit better?
> >
> > 	if (!lock_is_held(&vma->vmlock_dep_map))
> > 		vma_assert_write_locked(vma);
> > 	return;
>
> Hm yeah could do, I guess we don't need to mix the 'uncertain' stuff below with
> the lockdep-certainty, at this point we _know_ there is no read lock/readers
> being excluded exclusive lock so it can only be a write lock.
>
> Will test locally to make sure sane then send a fix-patch. :)

Actually I think I can get rid of this #ifdef here, will fold it into patch...