Re: [PATCH v3 12/15] perf: Split out the RB allocation

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Aug 12, 2025 at 12:39:10PM +0200, Peter Zijlstra wrote:
> Move the RB buffer allocation branch into its own function.
>
> Originally-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Ah yes of course, the very next patch :)

One nit below about ret assignment, though I wonder if you address later?

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  kernel/events/core.c |  145 +++++++++++++++++++++++++--------------------------
>  1 file changed, 73 insertions(+), 72 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6970,6 +6970,75 @@ static void perf_mmap_account(struct vm_
>  	atomic64_add(extra, &vma->vm_mm->pinned_vm);
>  }
>
> +static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
> +			unsigned long nr_pages)
> +{
> +	long extra = 0, user_extra = nr_pages;
> +	struct perf_buffer *rb;
> +	int rb_flags = 0;
> +
> +	nr_pages -= 1;
> +
> +	/*
> +	 * If we have rb pages ensure they're a power-of-two number, so we
> +	 * can do bitmasks instead of modulo.
> +	 */
> +	if (nr_pages != 0 && !is_power_of_2(nr_pages))
> +		return -EINVAL;
> +
> +	WARN_ON_ONCE(event->ctx->parent_ctx);
> +
> +	if (event->rb) {
> +		if (data_page_nr(event->rb) != nr_pages)
> +			return -EINVAL;
> +
> +		if (atomic_inc_not_zero(&event->rb->mmap_count)) {
> +			/*
> +			 * Success -- managed to mmap() the same buffer
> +			 * multiple times.
> +			 */
> +			perf_mmap_account(vma, user_extra, extra);
> +			atomic_inc(&event->mmap_count);
> +			return 0;
> +		}
> +
> +		/*
> +		 * Raced against perf_mmap_close()'s
> +		 * atomic_dec_and_mutex_lock() remove the
> +		 * event and continue as if !event->rb
> +		 */
> +		ring_buffer_attach(event, NULL);
> +	}
> +
> +	if (!perf_mmap_calc_limits(vma, &user_extra, &extra))
> +		return -EPERM;
> +
> +	if (vma->vm_flags & VM_WRITE)
> +		rb_flags |= RING_BUFFER_WRITABLE;
> +

I hadn't noticed before actually, but it's really nice that we only assign
rb from below for the newly allocated rb, and refer to the prior one via
event->rb above.

Which neatly addressed my prior review feedback on this, very nice, cheers!

> +	rb = rb_alloc(nr_pages,
> +		      event->attr.watermark ? event->attr.wakeup_watermark : 0,
> +		      event->cpu, rb_flags);
> +
> +	if (!rb)
> +		return -ENOMEM;
> +
> +	atomic_set(&rb->mmap_count, 1);
> +	rb->mmap_user = get_current_user();
> +	rb->mmap_locked = extra;
> +
> +	ring_buffer_attach(event, rb);
> +
> +	perf_event_update_time(event);
> +	perf_event_init_userpage(event);
> +	perf_event_update_userpage(event);
> +
> +	perf_mmap_account(vma, user_extra, extra);
> +	atomic_inc(&event->mmap_count);
> +
> +	return 0;
> +}
> +
>  static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
>  			 unsigned long nr_pages)
>  {
> @@ -7050,10 +7119,8 @@ static int perf_mmap(struct file *file,
>  {
>  	struct perf_event *event = file->private_data;
>  	unsigned long vma_size, nr_pages;
> -	long user_extra = 0, extra = 0;
> -	struct perf_buffer *rb = NULL;
> -	int ret, flags = 0;
>  	mapped_f mapped;
> +	int ret;
>
>  	/*
>  	 * Don't allow mmap() of inherited per-task counters. This would
> @@ -7079,8 +7146,6 @@ static int perf_mmap(struct file *file,
>  	if (vma_size != PAGE_SIZE * nr_pages)
>  		return -EINVAL;
>
> -	user_extra = nr_pages;
> -
>  	mutex_lock(&event->mmap_mutex);
>  	ret = -EINVAL;

This ret assignment is redundant now, since in all cases you assign ret
below.

I wonder if you get rid of this in a later patch though?

>
> @@ -7094,74 +7159,10 @@ static int perf_mmap(struct file *file,
>  		goto unlock;
>  	}
>
> -	if (vma->vm_pgoff == 0) {
> -		nr_pages -= 1;
> -
> -		/*
> -		 * If we have rb pages ensure they're a power-of-two number, so we
> -		 * can do bitmasks instead of modulo.
> -		 */
> -		if (nr_pages != 0 && !is_power_of_2(nr_pages))
> -			goto unlock;
> -
> -		WARN_ON_ONCE(event->ctx->parent_ctx);
> -
> -		if (event->rb) {
> -			if (data_page_nr(event->rb) != nr_pages)
> -				goto unlock;
> -
> -			if (atomic_inc_not_zero(&event->rb->mmap_count)) {
> -				/*
> -				 * Success -- managed to mmap() the same buffer
> -				 * multiple times.
> -				 */
> -				ret = 0;
> -				perf_mmap_account(vma, user_extra, extra);
> -				atomic_inc(&event->mmap_count);
> -				goto unlock;
> -			}
> -
> -			/*
> -			 * Raced against perf_mmap_close()'s
> -			 * atomic_dec_and_mutex_lock() remove the
> -			 * event and continue as if !event->rb
> -			 */
> -			ring_buffer_attach(event, NULL);
> -		}
> -
> -		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> -			ret = -EPERM;
> -			goto unlock;
> -		}
> -
> -		if (vma->vm_flags & VM_WRITE)
> -			flags |= RING_BUFFER_WRITABLE;
> -
> -		rb = rb_alloc(nr_pages,
> -			      event->attr.watermark ? event->attr.wakeup_watermark : 0,
> -			      event->cpu, flags);
> -
> -		if (!rb) {
> -			ret = -ENOMEM;
> -			goto unlock;
> -		}
> -
> -		atomic_set(&rb->mmap_count, 1);
> -		rb->mmap_user = get_current_user();
> -		rb->mmap_locked = extra;
> -
> -		ring_buffer_attach(event, rb);
> -
> -		perf_event_update_time(event);
> -		perf_event_init_userpage(event);
> -		perf_event_update_userpage(event);
> -		ret = 0;
> -
> -		perf_mmap_account(vma, user_extra, extra);
> -		atomic_inc(&event->mmap_count);
> -	} else {
> +	if (vma->vm_pgoff == 0)
> +		ret = perf_mmap_rb(vma, event, nr_pages);
> +	else
>  		ret = perf_mmap_aux(vma, event, nr_pages);
> -	}
>
>  unlock:
>  	mutex_unlock(&event->mmap_mutex);
>
>