linux-kernel - Re: [PATCH v3 04/15] perf: Move perf_mmap_calc

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Message-ID: <8fe920fc-83e5-4892-9457-45da6ed23a09@lucifer.local>
Date: Wed, 13 Aug 2025 06:28:08 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org,
        torvalds@...uxfoundation.org, mingo@...nel.org, namhyung@...nel.org,
        acme@...hat.com, kees@...nel.org
Subject: Re: [PATCH v3 04/15] perf: Move perf_mmap_calc_limits() into both rb
 and aux branches

On Tue, Aug 12, 2025 at 12:39:02PM +0200, Peter Zijlstra wrote:
>   if (cond) {
>     A;
>   } else {
>     B;
>   }
>   C;
>
> into
>
>   if (cond) {
>     A;
>     C;
>   } else {
>     B;
>     C;
>   }
>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Seems sensible enough, guessing trade off on duplication vs. complexity of
having to keep track of 'C' being in place for both and I assume makes
later refactorings easier :)

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

> ---
>  kernel/events/core.c |   26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7054,6 +7054,16 @@ static int perf_mmap(struct file *file,
>  			ring_buffer_attach(event, NULL);
>  		}
>
> +		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> +			ret = -EPERM;
> +			goto unlock;
> +		}
> +
> +		WARN_ON(!rb && event->rb);
> +
> +		if (vma->vm_flags & VM_WRITE)
> +			flags |= RING_BUFFER_WRITABLE;
> +
>  	} else {
>  		/*
>  		 * AUX area mapping: if rb->aux_nr_pages != 0, it's already
> @@ -7100,17 +7110,17 @@ static int perf_mmap(struct file *file,
>  			ret = 0;
>  			goto unlock;
>  		}
> -	}
>
> -	if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> -		ret = -EPERM;
> -		goto unlock;
> -	}
> +		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
> +			ret = -EPERM;
> +			goto unlock;
> +		}
>
> -	WARN_ON(!rb && event->rb);
> +		WARN_ON(!rb && event->rb);
>
> -	if (vma->vm_flags & VM_WRITE)
> -		flags |= RING_BUFFER_WRITABLE;
> +		if (vma->vm_flags & VM_WRITE)
> +			flags |= RING_BUFFER_WRITABLE;
> +	}
>
>  	if (!rb) {
>  		rb = rb_alloc(nr_pages,
>
>