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

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Message-ID: <20250811090416.GC1613200@noisy.programming.kicks-ass.net>
Date: Mon, 11 Aug 2025 11:04:16 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Kees Cook <kees@...nel.org>
Subject: Re: [patch V2 m@/6] perf/core: Split out AUX buffer allocation

On Mon, Aug 11, 2025 at 09:06:41AM +0200, Thomas Gleixner wrote:
> @@ -7055,51 +7122,15 @@ static int perf_mmap(struct file *file,
>  		}
>  
>  	} else {
> -		/*
> -		 * 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.
> -		 */
> -		u64 aux_offset, aux_size;
> -
> -		rb = event->rb;
> -		if (!rb)
> -			goto aux_unlock;
> -
> -		aux_mutex = &rb->aux_mutex;
> -		mutex_lock(aux_mutex);
> -
> -		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)
> -			goto aux_unlock;
> -
> -		if (aux_offset != vma->vm_pgoff << PAGE_SHIFT)
> -			goto aux_unlock;
> -
> -		/* already mapped with a different offset */
> -		if (rb_has_aux(rb) && rb->aux_pgoff != vma->vm_pgoff)
> -			goto aux_unlock;
> -
> -		if (aux_size != nr_pages * PAGE_SIZE)
> -			goto aux_unlock;
> -
> -		/* already mapped with a different size */
> -		if (rb_has_aux(rb) && rb->aux_nr_pages != nr_pages)
> -			goto aux_unlock;
> -
> -		if (!is_power_of_2(nr_pages))
> -			goto aux_unlock;
> -
> -		if (!atomic_inc_not_zero(&rb->mmap_count))
> -			goto aux_unlock;
> -
> -		if (rb_has_aux(rb)) {
> -			atomic_inc(&rb->aux_mmap_count);
> -			ret = 0;
> -			goto unlock;
> +		if (!event->rb) {
> +			ret = -EINVAL;
> +		} else {
> +			scoped_guard(mutex, &event->rb->aux_mutex)
> +				ret = perf_mmap_aux(vma, event, nr_pages);
>  		}
> +		// Temporary workaround to split out AUX handling first
> +		mutex_unlock(&event->mmap_mutex);
> +		goto out;
>  	}
>  
>  	if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> @@ -7132,28 +7163,16 @@ static int perf_mmap(struct file *file,
>  		perf_event_init_userpage(event);
>  		perf_event_update_userpage(event);
>  		ret = 0;
> -	} else {
> -		ret = rb_alloc_aux(rb, event, vma->vm_pgoff, nr_pages,
> -				   event->attr.aux_watermark, flags);
> -		if (!ret) {
> -			atomic_set(&rb->aux_mmap_count, 1);
> -			rb->aux_mmap_locked = extra;
> -		}
>  	}
> -
>  unlock:
>  	if (!ret) {
>  		perf_mmap_account(vma, user_extra, extra);
>  		atomic_inc(&event->mmap_count);
> -	} else if (rb) {
> -		/* AUX allocation failed */
> -		atomic_dec(&rb->mmap_count);
>  	}
> -aux_unlock:
> -	if (aux_mutex)
> -		mutex_unlock(aux_mutex);
>  	mutex_unlock(&event->mmap_mutex);
>  
> +// Temporary until RB allocation is split out.
> +out:
>  	if (ret)
>  		return ret;

Its a bit of a struggle getting back to work after spending 3 weeks on a
beach with the kids, but doesn't this remove the perf_mmap_account()
call out from under aux_mutex?

Notably, perf_mmap_close() seems to rely on this being the case.