Re: [PATCH RESEND v3 02/10] mm/vma: document possible vma->vm_refcnt values and reference comment

Vlastimil Babka
2026-01-22T16:48:10Z

On 1/22/26 14:01, Lorenzo Stoakes wrote:
> The possible vma->vm_refcnt values are confusing and vague, explain in
> detail what these can be in a comment describing the vma->vm_refcnt field
> and reference this comment in various places that read/write this field.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks, very useful. Forgive my nitpicks :) It's because it's tricky so best
try to be as precise as possible, I believe.

> ---
> include/linux/mm_types.h | 39 +++++++++++++++++++++++++++++++++++++--
> include/linux/mmap_lock.h | 7 +++++++
> mm/mmap_lock.c | 6 ++++++
> 3 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 94de392ed3c5..e5ee66f84d9a 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -758,7 +758,8 @@ static inline struct anon_vma_name *anon_vma_name_alloc(const char *name)
> * set the VM_REFCNT_EXCLUDE_READERS_FLAG in vma->vm_refcnt to indiciate to
> * vma_start_read() that the reference count should be left alone.
> *
> - * Once the operation is complete, this value is subtracted from vma->vm_refcnt.
> + * See the comment describing vm_refcnt in vm_area_struct for details as to
> + * which values the VMA reference count can be.
> */
> #define VM_REFCNT_EXCLUDE_READERS_BIT (30)
> #define VM_REFCNT_EXCLUDE_READERS_FLAG (1U << VM_REFCNT_EXCLUDE_READERS_BIT)
> @@ -989,7 +990,41 @@ struct vm_area_struct {
> struct vma_numab_state *numab_state; /* NUMA Balancing state */
> #endif
> #ifdef CONFIG_PER_VMA_LOCK
> - /* Unstable RCU readers are allowed to read this. */
> + /*
> + * Used to keep track of the number of references taken by VMA read or
> + * write locks. May have the VM_REFCNT_EXCLUDE_READERS_FLAG set

I wonder about the "or write locks" part. The process of acquiring it uses
VM_REFCNT_EXCLUDE_READERS_FLAG but then the writer doesn't hold a 1
refcount? (the sentence could be read it way IMHO) It's vma being attached
that does, AFAIK?

> + * indicating that a thread has entered __vma_enter_locked() and is
> + * waiting on any outstanding read locks to exit.
> + *
> + * This value can be equal to:
> + *
> + * 0 - Detached.

Is it worth saying that readers can't increment the refcount?

> + * 1 - Unlocked or write-locked.

"Attached and either unlocked or write-locked." ?

(see how "write-locked" isn't reflected, I argued above)

> + *
> + * >1, < VM_REFCNT_EXCLUDE_READERS_FLAG - Read-locked or (unlikely)
> + * write-locked with other threads having temporarily incremented the
> + * reference count prior to determining it is write-locked and
> + * decrementing it again.

Ack.

> + * VM_REFCNT_EXCLUDE_READERS_FLAG - Detached, pending
> + * __vma_exit_locked() completion which will decrement the reference
> + * count to zero. IMPORTANT - at this stage no further readers can
> + * increment the reference count. It can only be reduced.
> + *
> + * VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - Either an attached VMA pending
> + * __vma_exit_locked() completion which will decrement the reference
> + * count to one, OR a detached VMA waiting on a single spurious reader
> + * to decrement reference count. IMPORTANT - as above, no further
> + * readers can increment the reference count.
> + *
> + * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - VMA is waiting on readers,

"VMA is waiting" sounds weird? a thread might be, but VMA itself?
(similarly in the previous paragraph)

> + * whether it is attempting to acquire a write lock or attempting to
> + * detach. IMPORTANT - as above, no ruther readers can increment the
> + * reference count.
> + *
> + * NOTE: Unstable RCU readers are allowed to read this.
> + */
> refcount_t vm_refcnt ____cacheline_aligned_in_smp;
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> struct lockdep_map vmlock_dep_map;
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 5acbd4ba1b52..a764439d0276 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -130,6 +130,9 @@ static inline bool is_vma_writer_only(int refcnt)
> * 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.
> */
> return (refcnt & VM_REFCNT_EXCLUDE_READERS_FLAG) &&
> refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
> @@ -249,6 +252,10 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
> {
> unsigned int mm_lock_seq;
>
> + /*
> + * See the comment describing the vm_area_struct->vm_refcnt field for
> + * details of possible refcnt values.
> + */
> VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
> !__is_vma_write_locked(vma, &mm_lock_seq), vma);
> }
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 1d23b48552e9..75dc098aea14 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -65,6 +65,9 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
> /*
> * If vma is detached then only vma_mark_attached() can raise the
> * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached().
> + *
> + * See the comment describing the vm_area_struct->vm_refcnt field for
> + * details of possible refcnt values.
> */
> if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
> return 0;
> @@ -137,6 +140,9 @@ void vma_mark_detached(struct vm_area_struct *vma)
> * 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))) {
> /* Wait until vma is detached with no readers. */
> --
> 2.52.0