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

Suren Baghdasaryan
2024-12-16T21:45:01Z

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?

>
>
> > return false;
> > }
> > +
> > return true;
> > }