Re: [PATCH RESEND v3 04/10] mm/vma: add+use vma lockdep acquire/release defines

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 22, 2026 at 11:32 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > The code is littered with inscrutable and duplicative lockdep incantations,
> > replace these with defines which explain what is going on and add
> > commentary to explain what we're doing.
> >
> > If lockdep is disabled these become no-ops. We must use defines so _RET_IP_
> > remains meaningful.
> >
> > These are self-documenting and aid readability of the code.
> >
> > Additionally, instead of using the confusing rwsem_*() form for something
> > that is emphatically not an rwsem, we instead explicitly use
> > lock_[acquired, release]_shared/exclusive() lockdep invocations since we
> > are doing something rather custom here and these make more sense to use.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Very nice! Thank you.
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>
> > ---
> >  include/linux/mmap_lock.h | 35 ++++++++++++++++++++++++++++++++---
> >  mm/mmap_lock.c            | 10 +++++-----
> >  2 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > index 0b3614aadbb4..da63b1be6ec0 100644
> > --- a/include/linux/mmap_lock.h
> > +++ b/include/linux/mmap_lock.h
> > @@ -78,6 +78,36 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> >
> >  #ifdef CONFIG_PER_VMA_LOCK
> >
> > +/*
> > + * VMA locks do not behave like most ordinary locks found in the kernel, so we
> > + * cannot quite have full lockdep tracking in the way we would ideally prefer.
> > + *
> > + * Read locks act as shared locks which exclude an exclusive lock being
> > + * taken. We therefore mark these accordingly on read lock acquire/release.
> > + *
> > + * Write locks are acquired exclusively per-VMA, but released in a shared
> > + * fashion, that is upon vma_end_write_all(), we update the mmap's seqcount such
> > + * that write lock is de-acquired.
> > + *
> > + * We therefore cannot track write locks per-VMA, nor do we try. Mitigating this
> > + * is the fact that, of course, we do lockdep-track the mmap lock rwsem.
> > + *
> > + * We do, however, want to indicate that during either acquisition of a VMA
> > + * write lock or detachment of a VMA that we require the lock held be exclusive,
> > + * so we utilise lockdep to do so.
> > + */
> > +#define __vma_lockdep_acquire_read(vma) \

One question I forgot to ask. Are you adding "__" prefix to indicate
no other users should be using them or for some other reason?

> > +       lock_acquire_shared(&vma->vmlock_dep_map, 0, 1, NULL, _RET_IP_)
> > +#define __vma_lockdep_release_read(vma) \
> > +       lock_release(&vma->vmlock_dep_map, _RET_IP_)
> > +#define __vma_lockdep_acquire_exclusive(vma) \
> > +       lock_acquire_exclusive(&vma->vmlock_dep_map, 0, 0, NULL, _RET_IP_)
> > +#define __vma_lockdep_release_exclusive(vma) \
> > +       lock_release(&vma->vmlock_dep_map, _RET_IP_)
> > +/* Only meaningful if CONFIG_LOCK_STAT is defined. */
> > +#define __vma_lockdep_stat_mark_acquired(vma) \
> > +       lock_acquired(&vma->vmlock_dep_map, _RET_IP_)
> > +
> >  static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> >  {
> >         seqcount_init(&mm->mm_lock_seq);
> > @@ -176,8 +206,7 @@ static inline void vma_refcount_put(struct vm_area_struct *vma)
> >         int refcnt;
> >         bool detached;
> >
> > -       rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> > -
> > +       __vma_lockdep_release_read(vma);
> >         detached = __vma_refcount_put(vma, &refcnt);
> >         /*
> >          * __vma_enter_locked() may be sleeping waiting for readers to drop
> > @@ -203,7 +232,7 @@ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int
> >                                                               VM_REFCNT_LIMIT)))
> >                 return false;
> >
> > -       rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
> > +       __vma_lockdep_acquire_read(vma);
> >         return true;
> >  }
> >
> > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> > index ebacb57e5f16..9563bfb051f4 100644
> > --- a/mm/mmap_lock.c
> > +++ b/mm/mmap_lock.c
> > @@ -72,7 +72,7 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
> >         if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
> >                 return 0;
> >
> > -       rwsem_acquire(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
> > +       __vma_lockdep_acquire_exclusive(vma);
> >         err = rcuwait_wait_event(&vma->vm_mm->vma_writer_wait,
> >                    refcount_read(&vma->vm_refcnt) == tgt_refcnt,
> >                    state);
> > @@ -85,10 +85,10 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
> >                         WARN_ON_ONCE(!detaching);
> >                         err = 0;
> >                 }
> > -               rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> > +               __vma_lockdep_release_exclusive(vma);
> >                 return err;
> >         }
> > -       lock_acquired(&vma->vmlock_dep_map, _RET_IP_);
> > +       __vma_lockdep_stat_mark_acquired(vma);
> >
> >         return 1;
> >  }
> > @@ -97,7 +97,7 @@ static inline void __vma_exit_locked(struct vm_area_struct *vma, bool *detached)
> >  {
> >         *detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG,
> >                                           &vma->vm_refcnt);
> > -       rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> > +       __vma_lockdep_release_exclusive(vma);
> >  }
> >
> >  int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
> > @@ -199,7 +199,7 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
> >                 goto err;
> >         }
> >
> > -       rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
> > +       __vma_lockdep_acquire_read(vma);
> >
> >         if (unlikely(vma->vm_mm != mm))
> >                 goto err_unstable;
> > --
> > 2.52.0