Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan <surenb@google.com>]

On Sat, Jan 11, 2025 at 3:24 AM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote:
>
> So there were quite a few iterations of the patch and I have not been
> reading majority of the feedback, so it may be I missed something,
> apologies upfront. :)
>
> >  /*
> >   * 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 +742,8 @@ 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 +754,19 @@ 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;
> >
> > -     if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
> > +     /*
> > +      * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited() will fail
> > +      * because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET.
> > +      */
> > +     if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> > +                                                   VMA_REF_LIMIT)))
> >               return false;
> >
>
> Replacing down_read_trylock() with the new routine loses an acquire
> fence. That alone is not a problem, but see below.

Hmm. I think this acquire fence is actually necessary. We don't want
the later vm_lock_seq check to be reordered and happen before we take
the refcount. Otherwise this might happen:

reader             writer
if (vm_lock_seq == mm_lock_seq) // check got reordered
        return false;
                       vm_refcnt += VMA_LOCK_OFFSET
                       vm_lock_seq == mm_lock_seq
                       vm_refcnt -= VMA_LOCK_OFFSET
if (!__refcount_inc_not_zero_limited())
        return false;

Both reader's checks will pass and the reader would read-lock a vma
that was write-locked.

>
> > +     rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
> >       /*
> > -      * Overflow might produce false locked result.
> > +      * 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_lock protection and mm->mm_lock_seq
> > +      * 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
> > @@ -735,9 +775,10 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> >        * This pairs with RELEASE semantics in vma_end_write_all().
> >        */
> >       if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
>
> The previous modification of this spot to raw_read_seqcount loses the
> acquire fence, making the above comment not line up with the code.

Is it? From reading the seqcount code
(https://elixir.bootlin.com/linux/v6.13-rc3/source/include/linux/seqlock.h#L211):

raw_read_seqcount()
    seqprop_sequence()
        __seqprop(s, sequence)
            __seqprop_sequence()
                smp_load_acquire()

smp_load_acquire() still provides the acquire fence. Am I missing something?

>
> I don't know if the stock code (with down_read_trylock()) is correct as
> is -- looks fine for cursory reading fwiw. However, if it indeed works,
> the acquire fence stemming from the lock routine is a mandatory part of
> it afaics.
>
> I think the best way forward is to add a new refcount routine which
> ships with an acquire fence.

I plan on replacing refcount_t usage here with an atomic since, as
Hillf noted, refcount is not designed to be used for locking. And will
make sure the down_read_trylock() replacement will provide an acquire
fence.

>
> Otherwise I would suggest:
> 1. a comment above __refcount_inc_not_zero_limited saying there is an
>    acq fence issued later
> 2. smp_rmb() slapped between that and seq accesses
>
> If the now removed fence is somehow not needed, I think a comment
> explaining it is necessary.
>
> > @@ -813,36 +856,33 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> >
> >  static inline void vma_assert_locked(struct vm_area_struct *vma)
> >  {
> > -     if (!rwsem_is_locked(&vma->vm_lock.lock))
> > +     if (refcount_read(&vma->vm_refcnt) <= 1)
> >               vma_assert_write_locked(vma);
> >  }
> >
>
> This now forces the compiler to emit a load from vm_refcnt even if
> vma_assert_write_locked expands to nothing. iow this wants to hide
> behind the same stuff as vma_assert_write_locked.

True. I guess I'll have to avoid using vma_assert_write_locked() like this:

static inline void vma_assert_locked(struct vm_area_struct *vma)
{
        unsigned int mm_lock_seq;

        VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
                                          !__is_vma_write_locked(vma,
&mm_lock_seq), vma);
}

Will make the change.

Thanks for the feedback!