From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 24B0F18CC13 for ; Sun, 18 Jan 2026 01:09:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768698565; cv=none; b=FTCIyMMywinAqcTVAnkZCd0RVevfOlO7y5fQmTUnopc9H0ZpIWjYuzShESjU8mp6BCfT/50NB18LvsUQseBwGU0mwQr9fltoULJaztUJVzaogD5PEYb6pJBxovqlT2kHPtZFjGsnHA910TsFojF+GIgdPYjBVophDiikSsqutZc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768698565; c=relaxed/simple; bh=TTkhei+cPeMgKIMcltIoko3GtbvF9Mp6tYUNOAa15ms=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=Q7/s/xDYqdIyvkn3QyIOpNd2Ikg51EXVcBD/2qaMEfMlByZCVCT0B5eClgx08z+I8d+hs/J2v6wuKTycKFJ2K4t9Y6fUHbWIOx1pbFG4Bhl6axdA5UHMqrXy6uVpjsoOeEQ/9u3Ul8OTIN3dbDwhcYrfluNHEgZ8k3wkeVd2ADo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=ZfJMiAml; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="ZfJMiAml" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7588AC4CEF7; Sun, 18 Jan 2026 01:09:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1768698564; bh=TTkhei+cPeMgKIMcltIoko3GtbvF9Mp6tYUNOAa15ms=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=ZfJMiAmlDGllLVBJIBviMsDm7OBsxRoPAs2LV1hY2IfIOc4eb1DiIVufmycGG6+0E Q4BZfbhB4e2gzuk7gjB2DxFs+zVWTIabOXqUtRYEHD/XxewYtEo03XLRGFyOiwJ1OG HaDQdLlRTGlhYO+HOasOau8rHN45FcgZmvljfWSg= Date: Sat, 17 Jan 2026 17:09:23 -0800 From: Andrew Morton To: Dennis Zhou Cc: Tejun Heo , Christoph Lameter , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior Subject: Re: [PATCH v2] percpu: add basic double free check Message-Id: <20260117170923.71d856cdc65505e1ea841ef2@linux-foundation.org> In-Reply-To: References: <20260116023216.14515-1-dennis@kernel.org> <20260116191548.7df814c2a9eea1a9fa3c4cb5@linux-foundation.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 16 Jan 2026 21:15:33 -0800 Dennis Zhou wrote: > On Fri, Jan 16, 2026 at 07:15:48PM -0800, Andrew Morton wrote: > > On Thu, 15 Jan 2026 18:32:16 -0800 Dennis Zhou 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? > > > > Sorry that's my fault. I can respin and add it if you'd like. Yes please, I'm thinking a respin is needed anyway.... > > > @@ -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? > > > > I can see running multiple tests that might give me additional debug / > signal to how badly I screwed up. In production a WARN_ON_ONCE is > definitely more than enough, but might as well offer the chance to try > and trigger it more than once. If this is happening at development-time then developer can turn that into WARN_ON() or whatever. I dunno, I do feel that WARN_ON_ONCE is sufficient but no strong feelings. > > > + 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? > > > > Originally, Sebastian posted he ran into the issue where he double freed > in [1] (linked in patch). Maybe he can elaborate how that bug was > introduced. > > Wrt do we want to do it - I think it doesn't hurt and makes it more > explicit that something very wrong occurred. Percpu memory really > expects users to be good samaritans. If you do happen to accidentally > double free without the warning, in a contrived case you could > experience unexplained behavior for some time before crashing in a spot > that would leave your head scratching. If anything I think there could > be an argument to fail louder. > > [1] https://lore.kernel.org/linux-mm/20250904143514.Yk6Ap-jy@linutronix.de/ Could you please get this justification into the changelog? It's pretty important - explain to the world why we feel that Linux needs alteration and what benefit we're providing.