Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

[Suren Baghdasaryan]

On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote:
> > > @@ -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).