Re: [PATCH v4 03/10] mm/vma: rename is_vma_write_only(), separate out shared refcount put

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jan 23, 2026 at 12:12 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> The is_vma_writer_only() function is misnamed - this isn't determining if
> there is only a write lock, as it checks for the presence of the
> VM_REFCNT_EXCLUDE_READERS_FLAG.
>
> Really, it is checking to see whether readers are excluded, with a
> possibility of a false positive in the case of a detachment (there we
> expect the vma->vm_refcnt to eventually be set to
> VM_REFCNT_EXCLUDE_READERS_FLAG, whereas for an attached VMA we expect it to
> eventually be set to VM_REFCNT_EXCLUDE_READERS_FLAG + 1).
>
> Rename the function accordingly.
>
> Relatedly, we use a __refcount_dec_and_test() primitive directly in
> vma_refcount_put(), using the old value to determine what the reference
> count ought to be after the operation is complete (ignoring racing
> reference count adjustments).
>
> Wrap this into a __vma_refcount_put_return() function, which we can then
> utilise in vma_mark_detached() and thus keep the refcount primitive usage
> abstracted.
>
> This function, as the name implies, returns the value after the reference
> count has been updated.
>
> This reduces duplication in the two invocations of this function.
>
> Also adjust comments, removing duplicative comments covered elsewhere and
> adding more to aid understanding.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  include/linux/mmap_lock.h | 66 +++++++++++++++++++++++++++++++--------
>  mm/mmap_lock.c            | 17 +++++-----
>  2 files changed, 63 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index a764439d0276..294fb282052d 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -122,15 +122,22 @@ static inline void vma_lock_init(struct vm_area_struct *vma, bool reset_refcnt)
>         vma->vm_lock_seq = UINT_MAX;
>  }
>
> -static inline bool is_vma_writer_only(int refcnt)
> +/*
> + * This function determines whether the input VMA reference count describes a
> + * VMA which has excluded all VMA read locks.
> + *
> + * In the case of a detached VMA, we may incorrectly indicate that readers are
> + * excluded when one remains, because in that scenario we target a refcount of
> + * VM_REFCNT_EXCLUDE_READERS_FLAG, rather than the attached target of
> + * VM_REFCNT_EXCLUDE_READERS_FLAG + 1.
> + *
> + * However, the race window for that is very small so it is unlikely.
> + *
> + * Returns: true if readers are excluded, false otherwise.
> + */
> +static inline bool __vma_are_readers_excluded(int refcnt)
>  {
>         /*
> -        * With a writer and no readers, refcnt is VM_REFCNT_EXCLUDE_READERS_FLAG
> -        * if the vma is detached and (VM_REFCNT_EXCLUDE_READERS_FLAG + 1) if it is
> -        * attached. Waiting on a detached vma happens only in
> -        * vma_mark_detached() and is a rare case, therefore most of the time
> -        * there will be no unnecessary wakeup.
> -        *
>          * See the comment describing the vm_area_struct->vm_refcnt field for
>          * details of possible refcnt values.
>          */
> @@ -138,18 +145,51 @@ static inline bool is_vma_writer_only(int refcnt)
>                 refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
>  }
>
> +/*
> + * Actually decrement the VMA reference count.
> + *
> + * The function returns the reference count as it was immediately after the
> + * decrement took place. If it returns zero, the VMA is now detached.
> + */
> +static inline __must_check unsigned int
> +__vma_refcount_put_return(struct vm_area_struct *vma)
> +{
> +       int oldcnt;
> +
> +       if (__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt))
> +               return 0;
> +
> +       return oldcnt - 1;
> +}
> +
> +/**
> + * vma_refcount_put() - Drop reference count in VMA vm_refcnt field due to a
> + * read-lock being dropped.
> + * @vma: The VMA whose reference count we wish to decrement.
> + *
> + * If we were the last reader, wake up threads waiting to obtain an exclusive
> + * lock.
> + */
>  static inline void vma_refcount_put(struct vm_area_struct *vma)
>  {
> -       /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
> +       /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt. */
>         struct mm_struct *mm = vma->vm_mm;
> -       int oldcnt;
> +       int newcnt;
>
>         rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> -       if (!__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt)) {
>
> -               if (is_vma_writer_only(oldcnt - 1))
> -                       rcuwait_wake_up(&mm->vma_writer_wait);
> -       }
> +       newcnt = __vma_refcount_put_return(vma);
> +       /*
> +        * __vma_enter_locked() may be sleeping waiting for readers to drop
> +        * their reference count, so wake it up if we were the last reader
> +        * blocking it from being acquired.
> +        *
> +        * We may be raced by other readers temporarily incrementing the
> +        * reference count, though the race window is very small, this might
> +        * cause spurious wakeups.
> +        */
> +       if (newcnt && __vma_are_readers_excluded(newcnt))
> +               rcuwait_wake_up(&mm->vma_writer_wait);
>  }
>
>  /*
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 75dc098aea14..6be1bbcde09e 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -134,21 +134,24 @@ void vma_mark_detached(struct vm_area_struct *vma)
>         vma_assert_attached(vma);
>
>         /*
> -        * We are the only writer, so no need to use vma_refcount_put().
> -        * The condition below is unlikely because the vma has been already
> -        * write-locked and readers can increment vm_refcnt only temporarily
> -        * before they check vm_lock_seq, realize the vma is locked and drop
> -        * back the vm_refcnt. That is a narrow window for observing a raised
> -        * vm_refcnt.
> +        * This condition - that the VMA is still attached (refcnt > 0) - is
> +        * unlikely, because the vma has been already write-locked and readers
> +        * can increment vm_refcnt only temporarily before they check
> +        * vm_lock_seq, realize the vma is locked and drop back the
> +        * vm_refcnt. That is a narrow window for observing a raised vm_refcnt.
>          *
>          * See the comment describing the vm_area_struct->vm_refcnt field for
>          * details of possible refcnt values.
>          */
> -       if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
> +       if (unlikely(__vma_refcount_put_return(vma))) {
>                 /* Wait until vma is detached with no readers. */
>                 if (__vma_enter_locked(vma, true, TASK_UNINTERRUPTIBLE)) {
>                         bool detached;
>
> +                       /*
> +                        * Once this is complete, no readers can increment the
> +                        * reference count, and the VMA is marked detached.
> +                        */
>                         __vma_exit_locked(vma, &detached);
>                         WARN_ON_ONCE(!detached);
>                 }
> --
> 2.52.0
>