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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 16, 2023 at 08:34:36PM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox <willy@infradead.org> 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).

Perhaps I misunderstood your description; I thought that a _waiting_
writer would make the count negative, not a successfully acquiring
writer.