Re: [patch V2 m@/6] perf/core: Split out AUX buffer allocation

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 11, 2025 at 09:06:41AM +0200, Thomas Gleixner wrote:
> The code logic in perf_mmap() is incomprehensible and has been source of
> subtle bugs in the past. It makes it impossible to convert the atomic_t
> reference counts to refcount_t.
> 
> There is not really much, which is shared between the ringbuffer and AUX
> buffer allocation code since the mlock limit calculation and the
> accounting has been split out into helper functions.
> 
> Move the AUX buffer allocation code out and integrate the call with a
> momentary workaround to allow skipping the remaining ringbuffer related
> code completely. That workaround will be removed once the ringbuffer
> allocation is moved to its own function as well.
> 
> No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V2: Fixup invers condition and add the dropped flags setup back - Lorenzo
>     Fixup subject line to match the content
> ---
>  kernel/events/core.c |  137 +++++++++++++++++++++++++++++----------------------
>  1 file changed, 78 insertions(+), 59 deletions(-)
> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6970,12 +6970,79 @@ static void perf_mmap_account(struct vm_
>  	atomic64_add(extra, &vma->vm_mm->pinned_vm);
>  }
>  
> +static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
> +			 unsigned long nr_pages)
> +{
> +	long user_extra = nr_pages, extra = 0;
> +	struct perf_buffer *rb = event->rb;
> +	u64 aux_offset, aux_size;
> +	int ret, rb_flags = 0;
> +
> +	/*
> +	 * AUX area mapping: if rb->aux_nr_pages != 0, it's already
> +	 * mapped, all subsequent mappings should have the same size
> +	 * and offset. Must be above the normal perf buffer.
> +	 */
> +	aux_offset = READ_ONCE(rb->user_page->aux_offset);
> +	aux_size = READ_ONCE(rb->user_page->aux_size);
> +
> +	if (aux_offset < perf_data_size(rb) + PAGE_SIZE)
> +		return -EINVAL;
> +
> +	if (aux_offset != vma->vm_pgoff << PAGE_SHIFT)
> +		return -EINVAL;
> +
> +	/* Already mapped with a different offset */
> +	if (rb_has_aux(rb) && rb->aux_pgoff != vma->vm_pgoff)
> +		return -EINVAL;
> +
> +	if (aux_size != nr_pages * PAGE_SIZE)
> +		return -EINVAL;
> +
> +	/* Already mapped with a different size */
> +	if (rb_has_aux(rb) && rb->aux_nr_pages != nr_pages)
> +		return -EINVAL;
> +
> +	if (!is_power_of_2(nr_pages))
> +		return -EINVAL;
> +
> +	/* If this succeeds, subsequent failures have to undo it */
> +	if (!atomic_inc_not_zero(&rb->mmap_count))
> +		return -EINVAL;
> +
> +	/* If mapped, attach to it */
> +	if (rb_has_aux(rb)) {
> +		atomic_inc(&rb->aux_mmap_count);
> +		return 0;

so this was: ret = 0; goto unlock;, which then would've also taken the
!ret branch and done perf_mmap_account(), no?

> +	}
> +
> +	if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> +		atomic_dec(&rb->mmap_count);
> +		return -EPERM;
> +	}
> +
> +	if (vma->vm_flags & VM_WRITE)
> +		rb_flags |= RING_BUFFER_WRITABLE;
> +
> +	ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
> +			   event->attr.aux_watermark, rb_flags);
> +	if (ret) {
> +		atomic_dec(&rb->mmap_count);
> +		return ret;
> +	}
> +
> +	atomic_set(&rb->aux_mmap_count, 1);
> +	rb->aux_mmap_locked = extra;
> +	perf_mmap_account(vma, user_extra, extra);
> +	atomic_inc(&event->mmap_count);
> +	return 0;
> +}

These two aux and rb split out patches seem like they're trying to take
too big a step. Let me try and do the same with smaller steps.

If only to try and find bugs.