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Date: Fri, 16 Jan 2026 19:15:48 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>, Chris Mason
 <clm@meta.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sebastian
 Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v2] percpu: add basic double free check
Message-Id: <20260116191548.7df814c2a9eea1a9fa3c4cb5@linux-foundation.org>
In-Reply-To: <20260116023216.14515-1-dennis@kernel.org>
References: <20260116023216.14515-1-dennis@kernel.org>
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On Thu, 15 Jan 2026 18:32:16 -0800 Dennis Zhou <dennis@kernel.org> wrote:

> This adds a basic double free check by validating the first bit of the
> allocation in alloc_map and bound_map are set. If the alloc_map bit is
> not set, then this means the area is currently unallocated. If the
> bound_map bit is not set, then we are not freeing from the beginning of
> the allocation.
> 
> This is a respin of [1] adding the requested changes from me and
> Christoph.
> 
> ...
>
> @@ -1276,18 +1277,24 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int alloc_bits,
>  static int pcpu_free_area(struct pcpu_chunk *chunk, int off)
>  {
>  	struct pcpu_block_md *chunk_md = &chunk->chunk_md;
> +	int region_bits = pcpu_chunk_map_bits(chunk);
>  	int bit_off, bits, end, oslot, freed;
>  
>  	lockdep_assert_held(&pcpu_lock);
> -	pcpu_stats_area_dealloc(chunk);
>  
>  	oslot = pcpu_chunk_slot(chunk);
>  
>  	bit_off = off / PCPU_MIN_ALLOC_SIZE;
> +	if (unlikely(bit_off < 0 || bit_off >= region_bits))
> +		return 0;

This (which looks sensible) wasn't changelogged?

> @@ -2242,6 +2252,13 @@ void free_percpu(void __percpu *ptr)
>  
>  	spin_lock_irqsave(&pcpu_lock, flags);
>  	size = pcpu_free_area(chunk, off);
> +	if (size == 0) {
> +		spin_unlock_irqrestore(&pcpu_lock, flags);
> +
> +		if (__ratelimit(&_rs))
> +			WARN(1, "percpu double free or bad ptr\n");

Is ratelimiting really needed?  A WARN_ON_ONCE is enough to tell people
that this kernel is wrecked?

> +		return;
> +	}

The patch does appear to do that which it set out to do.  But do we
want to do it?  Is there a history of callers double-freeing percpu
memory?  Was there some bug which would have been more rapidly and
easily solved had this change been in place?