On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox [off-list ref] wrote:

On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote:

quoted

quoted

@@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma)
 static inline bool vma_read_trylock(struct vm_area_struct *vma)
 {
    /* Check before locking. A race might cause false locked result. */
-   if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
+   if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
            return false;

-   if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
+   if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count)))
            return false;

+   /* If atomic_t overflows, restore and fail to lock. */
+   if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) {
+           if (atomic_dec_and_test(&vma->vm_lock->count))
+                   wake_up(&vma->vm_mm->vma_writer_wait);
+           return false;
+   }
+
    /*
     * Overflow might produce false locked result.
     * False unlocked result is impossible because we modify and check
     * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq
     * modification invalidates all existing locks.
     */
-   if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
-           up_read(&vma->vm_lock->lock);
+   if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
+           if (atomic_dec_and_test(&vma->vm_lock->count))
+                   wake_up(&vma->vm_mm->vma_writer_wait);
            return false;
    }

With this change readers can cause writers to starve.
What about checking waitqueue_active() before or after increasing
vma->vm_lock->count?

I don't understand how readers can starve a writer.  Readers do
atomic_inc_unless_negative() so a writer can always force readers
to fail.

I think the point here was that if page faults keep occuring and they
prevent vm_lock->count from reaching 0 then a writer will be blocked
and there is no reader throttling mechanism (no max time that writer
will be waiting).