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

Suren Baghdasaryan
2025-01-11T20:15:00Z

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!