Re: [PATCH v4 02/10] mm/vma: document possible vma->vm_refcnt values and reference comment

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [260123 15:12]:
> The possible vma->vm_refcnt values are confusing and vague, explain in
> detail what these can be in a comment describing the vma->vm_refcnt field
> and reference this comment in various places that read/write this field.

You may want to indicate what attached/detached means?  Then again, they
get added/removed from a few trees so it gets a bit difficult to spell
out.  Maybe a pointer to your nice doc file?

This is certainly worth doing as is (minus that typo), so

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  include/linux/mm_types.h  | 42 +++++++++++++++++++++++++++++++++++++--
>  include/linux/mmap_lock.h |  7 +++++++
>  mm/mmap_lock.c            |  6 ++++++
>  3 files changed, 53 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index bdbf17c4f26b..12281a1128c9 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -758,7 +758,8 @@ static inline struct anon_vma_name *anon_vma_name_alloc(const char *name)
>   * set the VM_REFCNT_EXCLUDE_READERS_FLAG in vma->vm_refcnt to indiciate to
>   * vma_start_read() that the reference count should be left alone.
>   *
> - * Once the operation is complete, this value is subtracted from vma->vm_refcnt.
> + * See the comment describing vm_refcnt in vm_area_struct for details as to
> + * which values the VMA reference count can be.
>   */
>  #define VM_REFCNT_EXCLUDE_READERS_BIT	(30)
>  #define VM_REFCNT_EXCLUDE_READERS_FLAG	(1U << VM_REFCNT_EXCLUDE_READERS_BIT)
> @@ -989,7 +990,44 @@ struct vm_area_struct {
>  	struct vma_numab_state *numab_state;	/* NUMA Balancing state */
>  #endif
>  #ifdef CONFIG_PER_VMA_LOCK
> -	/* Unstable RCU readers are allowed to read this. */
> +	/*
> +	 * Used to keep track of firstly, whether the VMA is attached, secondly,
> +	 * if attached, how many read locks are taken, and thirdly, if the
> +	 * VM_REFCNT_EXCLUDE_READERS_FLAG is set, whether any read locks held
> +	 * are currently in the process of being excluded.
> +	 *
> +	 * This value can be equal to:
> +	 *
> +	 * 0 - Detached. IMPORTANT: when the refcnt is zero, readers cannot
> +	 * increment it.
> +	 *
> +	 * 1 - Attached and either unlocked or write-locked. Write locks are
> +	 * identified via __is_vma_write_locked() which checks for equality of
> +	 * vma->vm_lock_seq and mm->mm_lock_seq.
> +	 *
> +	 * >1, < VM_REFCNT_EXCLUDE_READERS_FLAG - Read-locked or (unlikely)
> +	 * write-locked with other threads having temporarily incremented the
> +	 * reference count prior to determining it is write-locked and
> +	 * decrementing it again.
> +	 *
> +	 * VM_REFCNT_EXCLUDE_READERS_FLAG - Detached, pending
> +	 * __vma_exit_locked() completion which will decrement the reference
> +	 * count to zero. IMPORTANT - at this stage no further readers can
> +	 * increment the reference count. It can only be reduced.
> +	 *
> +	 * VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either write-locking
> +	 * an attached VMA and has yet to invoke __vma_exit_locked(), OR a
> +	 * thread is detaching a VMA and is waiting on a single spurious reader
> +	 * in order to decrement the reference count. IMPORTANT - as above, no
> +	 * further readers can increment the reference count.
> +	 *
> +	 * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either
> +	 * write-locking or detaching a VMA is waiting on readers to
> +	 * exit. IMPORTANT - as above, no ruther readers can increment the
> +	 * reference count.
> +	 *
> +	 * NOTE: Unstable RCU readers are allowed to read this.
> +	 */
>  	refcount_t vm_refcnt ____cacheline_aligned_in_smp;
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map vmlock_dep_map;
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 5acbd4ba1b52..a764439d0276 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -130,6 +130,9 @@ static inline bool is_vma_writer_only(int refcnt)
>  	 * attached. Waiting on a detached vma happens only in
>  	 * vma_mark_detached() and is a rare case, therefore most of the time
>  	 * there will be no unnecessary wakeup.
> +	 *
> +	 * See the comment describing the vm_area_struct->vm_refcnt field for
> +	 * details of possible refcnt values.
>  	 */
>  	return (refcnt & VM_REFCNT_EXCLUDE_READERS_FLAG) &&
>  		refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
> @@ -249,6 +252,10 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
>  {
>  	unsigned int mm_lock_seq;
>  
> +	/*
> +	 * See the comment describing the vm_area_struct->vm_refcnt field for
> +	 * details of possible refcnt values.
> +	 */
>  	VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
>  		      !__is_vma_write_locked(vma, &mm_lock_seq), vma);
>  }
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 1d23b48552e9..75dc098aea14 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -65,6 +65,9 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
>  	/*
>  	 * If vma is detached then only vma_mark_attached() can raise the
>  	 * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached().
> +	 *
> +	 * See the comment describing the vm_area_struct->vm_refcnt field for
> +	 * details of possible refcnt values.
>  	 */
>  	if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
>  		return 0;
> @@ -137,6 +140,9 @@ void vma_mark_detached(struct vm_area_struct *vma)
>  	 * before they check vm_lock_seq, realize the vma is locked and drop
>  	 * back the vm_refcnt. That is a narrow window for observing a raised
>  	 * vm_refcnt.
> +	 *
> +	 * See the comment describing the vm_area_struct->vm_refcnt field for
> +	 * details of possible refcnt values.
>  	 */
>  	if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
>  		/* Wait until vma is detached with no readers. */
> -- 
> 2.52.0
>