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

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Message-ID: <20250811093658.GD1613200@noisy.programming.kicks-ass.net>
Date: Mon, 11 Aug 2025 11:36:58 +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:
> +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;
> +	}
> +
> +	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);

Ooh, you squirreled a perf_mmap_account call in here.