Re: [PATCH v7 12/17] mm: replace vm_lock and detached flag with a reference count

[Wei Yang]

On Thu, Dec 26, 2024 at 09:07:04AM -0800, Suren Baghdasaryan wrote:
[...]
> /*
>  * 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 +733,8 @@ 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 +745,20 @@ 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;
> 
>-	if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
>+
>+	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
>+	/* Limit at VMA_REF_LIMIT to leave one count for a writer */
>+	if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
>+						      VMA_REF_LIMIT))) {
>+		rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> 		return false;
>+	}
>+	lock_acquired(&vma->vmlock_dep_map, _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
>@@ -734,10 +766,12 @@ 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 (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
>-		up_read(&vma->vm_lock.lock);
>+	if (unlikely(oldcnt & VMA_LOCK_OFFSET ||
>+		     vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {

I am not sure it worth mention. In case it is too trivial, just ignore.

If (oldcnt & VMA_LOCK_OFFSET), oldcnt + 1 > VMA_REF_LIMIT. This means
__refcount_inc_not_zero_limited() above would return false.

If my understanding is correct, we don't need to check it here.

>+		vma_refcount_put(vma);
> 		return false;
> 	}
>+
> 	return true;
> }
> 
[...]

-- 
Wei Yang
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