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

[Mateusz Guzik <mjguzik@gmail.com>]

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.

> +	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.

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.

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.