Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 16, 2024 at 01:44:45PM -0800, Suren Baghdasaryan wrote:
> On Mon, Dec 16, 2024 at 1:38 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Dec 16, 2024 at 11:24:13AM -0800, Suren Baghdasaryan wrote:
> > > +static inline void vma_refcount_put(struct vm_area_struct *vma)
> > > +{
> > > +     int refcnt;
> > > +
> > > +     if (!__refcount_dec_and_test(&vma->vm_refcnt, &refcnt)) {
> > > +             rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> > > +
> > > +             if (refcnt & VMA_STATE_LOCKED)
> > > +                     rcuwait_wake_up(&vma->vm_mm->vma_writer_wait);
> > > +     }
> > > +}
> > > +
> > >  /*
> > >   * Try to read-lock a vma. The function is allowed to occasionally yield false
> > >   * locked result to avoid performance overhead, in which case we fall back to
> > > @@ -710,6 +728,8 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
> > >   */
> > >  static inline bool vma_start_read(struct vm_area_struct *vma)
> > >  {
> > > +     int oldcnt;
> > > +
> > >       /*
> > >        * Check before locking. A race might cause false locked result.
> > >        * We can use READ_ONCE() for the mm_lock_seq here, and don't need
> > > @@ -720,13 +740,20 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> > >       if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
> > >               return false;
> > >
> > > +
> > > +     rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
> > > +     /* Limit at VMA_STATE_LOCKED - 2 to leave one count for a writer */
> > > +     if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> > > +                                                   VMA_STATE_LOCKED - 2))) {
> > > +             rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> > >               return false;
> > > +     }
> > > +     lock_acquired(&vma->vmlock_dep_map, _RET_IP_);
> > >
> > >       /*
> > > +      * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
> > >        * False unlocked result is impossible because we modify and check
> > > +      * vma->vm_lock_seq under vma->vm_refcnt protection and mm->mm_lock_seq
> > >        * modification invalidates all existing locks.
> > >        *
> > >        * We must use ACQUIRE semantics for the mm_lock_seq so that if we are
> > > @@ -734,10 +761,12 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> > >        * after it has been unlocked.
> > >        * This pairs with RELEASE semantics in vma_end_write_all().
> > >        */
> > > +     if (oldcnt & VMA_STATE_LOCKED ||
> > > +         unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
> > > +             vma_refcount_put(vma);
> >
> > Suppose we have detach race with a concurrent RCU lookup like:
> >
> >                                         vma = mas_lookup();
> >
> >         vma_start_write();
> >         mas_detach();
> >                                         vma_start_read()
> >                                         rwsem_acquire_read()
> >                                         inc // success
> >         vma_mark_detach();
> >         dec_and_test // assumes 1->0
> >                      // is actually 2->1
> >
> >                                         if (vm_lock_seq == vma->vm_mm_mm_lock_seq) // true
> >                                           vma_refcount_put
> >                                             dec_and_test() // 1->0
> >                                               *NO* rwsem_release()
> >
> 
> Yes, this is possible. I think that's not a problem until we start
> reusing the vmas and I deal with this race later in this patchset.
> I think what you described here is the same race I mention in the
> description of this patch:
> https://lore.kernel.org/all/20241216192419.2970941-14-surenb@google.com/
> I introduce vma_ensure_detached() in that patch to handle this case
> and ensure that vmas are detached before they are returned into the
> slab cache for reuse. Does that make sense?

So I just replied there, and no, I don't think it makes sense. Just put
the kmem_cache_free() in vma_refcount_put(), to be done on 0.

Anyway, my point was more about the weird entanglement of lockdep and
the refcount. Just pull the lockdep annotation out of _put() and put it
explicitly in the vma_start_read() error paths and vma_end_read().

Additionally, having vma_end_write() would allow you to put a lockdep
annotation in vma_{start,end}_write() -- which was I think the original
reason I proposed it a while back, that and having improved clarity when
reading the code, since explicitly marking the end of a section is
helpful.