[PATCH RESEND v3 03/10] mm/vma: rename is_vma_write_only(), separate out shared refcount put

Lorenzo Stoakes
2026-01-22T13:03:36Z

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 finnicky __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() function, which we can then utilise
in vma_mark_detached() and thus keep the refcount primitive usage
abstracted.

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>
---
include/linux/mmap_lock.h | 62 +++++++++++++++++++++++++++++++--------
mm/mmap_lock.c | 18 +++++-------
2 files changed, 57 insertions(+), 23 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index a764439d0276..0b3614aadbb4 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -122,15 +122,27 @@ 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)
+/**
+ * are_readers_excluded() - Determine whether @refcnt describes a VMA which has
+ * excluded all VMA read locks.
+ * @refcnt: The VMA reference count obtained from vm_area_struct->vm_refcnt.
+ *
+ * We may be raced by other readers temporarily incrementing the reference
+ * count, though the race window is very small, this might cause spurious
+ * wakeups.
+ *
+ * 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 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 +150,42 @@ static inline bool is_vma_writer_only(int refcnt)
refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
}

+static inline bool __vma_refcount_put(struct vm_area_struct *vma, int *refcnt)
+{
+ int oldcnt;
+ bool detached;
+
+ detached = __refcount_dec_and_test(&vma->vm_refcnt, &oldcnt);
+ if (refcnt)
+ *refcnt = oldcnt - 1;
+ return detached;
+}
+
+/**
+ * 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 refcnt;
+ bool detached;

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);
- }
+ detached = __vma_refcount_put(vma, &refcnt);
+ /*
+ * __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.
+ */
+ if (!detached && are_readers_excluded(refcnt))
+ rcuwait_wake_up(&mm->vma_writer_wait);
}

/*
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 75dc098aea14..ebacb57e5f16 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -130,25 +130,23 @@ EXPORT_SYMBOL_GPL(__vma_start_write);

void vma_mark_detached(struct vm_area_struct *vma)
{
+ bool detached;
+
vma_assert_write_locked(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.
- *
* 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))) {
+ detached = __vma_refcount_put(vma, NULL);
+ if (unlikely(!detached)) {
/* 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