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 F2C3F266580; Tue, 2 Sep 2025 07:35:17 +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=1756798518; cv=none; b=NIoM/zYtcBByDHWhG98N2hp9pXEBur/MV5Vy2lcNLbahUsMHwG+vdGJiDp0mFW5oiptY1V80ByL8CtrWT5sXw+XLN8qDflzF/RJjUcGGOLuShaNxYznH3RrnnfQAG0DNL5GOAhAqkqDvVqEM7iEwCQ85y3FYYzl0ww4d5n40V94= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756798518; c=relaxed/simple; bh=E/WY5o6a97fVHU8e5uy2CoWMRAn7yHxp4l1Tt05M7pQ=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=ot7dRY1wfxlddNIYHJQPAwm4H6glBU27FzHgRkByax6o/OliYyMolYMBz6w3pXRW3y30HHNDbdzVGlKosFt7TqQFvOsm+M17C/KrOrNsKYPXl5Qgz/5MCi+hhgDuA1ojbWCMSC9HfTTgoB+brBwB+obqFe1CzAnzYlUmVD9Cqsc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nymPyFHa; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="nymPyFHa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 32F30C4CEED; Tue, 2 Sep 2025 07:35:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756798517; bh=E/WY5o6a97fVHU8e5uy2CoWMRAn7yHxp4l1Tt05M7pQ=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=nymPyFHaAcgHY3kjoLFAWTfg/VeqM5171gZw/UhNyg9PSulFo70S9+Uf89ywYHI2F TP0kj0tj7CVr8uk31H7r6i8kgwp/Vz/lIMQJfsFXZ+qe5QdumAEUtB7oaB6fWS30oA ba0B1Z4l66xfrJs0oNuIbiVCvOJzTxdFntl4jR8aunxFyTXYVkITEOMVs9zZvzFT9I BksleDrnd05NzHesNhvNdLu7bpN3EkUHHVKhhYu27OCgSambrPhwWIieAEK8n3eQys 9QgBDGPKGDm6KMWAW2efFr5R/C+a3M3oSggjnqSZSBuK2CHxWS5GxKmt453CYagsKq 0tCjlMKVdJOnA== Date: Tue, 2 Sep 2025 16:35:14 +0900 From: Masami Hiramatsu (Google) To: Luo Gengkun Cc: Steven Rostedt , mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, Mark Rutland Subject: Re: [PATCH] tracing: Fix tracing_marker may trigger page fault during preempt_disable Message-Id: <20250902163514.f877d9c96e913f08c0c6b0b1@kernel.org> In-Reply-To: References: <20250819105152.2766363-1-luogengkun@huaweicloud.com> <20250819135008.5f1ba00e@gandalf.local.home> <436e4fa7-f8c7-4c23-a28a-4e5eebe2f854@huaweicloud.com> <20250829082604.1e3fd06e@gandalf.local.home> <20250902005645.8c6436b535731a4917745c5d@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-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 Tue, 2 Sep 2025 11:47:32 +0800 Luo Gengkun wrote: > > On 2025/9/1 23:56, Masami Hiramatsu (Google) wrote: > > On Fri, 29 Aug 2025 08:26:04 -0400 > > Steven Rostedt wrote: > > > >> [ Adding arm64 maintainers ] > >> > >> On Fri, 29 Aug 2025 16:29:07 +0800 > >> Luo Gengkun wrote: > >> > >>> On 2025/8/20 1:50, Steven Rostedt wrote: > >>>> On Tue, 19 Aug 2025 10:51:52 +0000 > >>>> Luo Gengkun wrote: > >>>> > >>>>> Both tracing_mark_write and tracing_mark_raw_write call > >>>>> __copy_from_user_inatomic during preempt_disable. But in some case, > >>>>> __copy_from_user_inatomic may trigger page fault, and will call schedule() > >>>>> subtly. And if a task is migrated to other cpu, the following warning will > >>>> Wait! What? > >>>> > >>>> __copy_from_user_inatomic() is allowed to be called from in atomic context. > >>>> Hence the name it has. How the hell can it sleep? If it does, it's totally > >>>> broken! > >>>> > >>>> Now, I'm not against using nofault() as it is better named, but I want to > >>>> know why you are suggesting this change. Did you actually trigger a bug here? > >>> yes, I trigger this bug in arm64. > >> And I still think this is an arm64 bug. > > I think it could be. > > > >>>> > >>>>> be trigger: > >>>>> if (RB_WARN_ON(cpu_buffer, > >>>>> !local_read(&cpu_buffer->committing))) > >>>>> > >>>>> An example can illustrate this issue: > > You've missed an important part. > > > >>>>> process flow CPU > >>>>> --------------------------------------------------------------------- > >>>>> > >>>>> tracing_mark_raw_write(): cpu:0 > >>>>> ... > >>>>> ring_buffer_lock_reserve(): cpu:0 > >>>>> ... > > preempt_disable_notrace(); --> this is unlocked by ring_buffer_unlock_commit() > > > >>>>> cpu = raw_smp_processor_id() cpu:0 > >>>>> cpu_buffer = buffer->buffers[cpu] cpu:0 > >>>>> ... > >>>>> ... > >>>>> __copy_from_user_inatomic(): cpu:0 > > So this is called under preempt-disabled. > > > >>>>> ... > >>>>> # page fault > >>>>> do_mem_abort(): cpu:0 > >>>> Sounds to me that arm64 __copy_from_user_inatomic() may be broken. > >>>> > >>>>> ... > >>>>> # Call schedule > >>>>> schedule() cpu:0 > > If this does not check the preempt flag, it is a problem. > > Maybe arm64 needs to do fixup and abort instead of do_mem_abort()? > > My kernel was built without CONFIG_PREEMPT_COUNT, so the preempt_disable() > does nothing more than act as a barrier. In this case, it can pass the > check by schedule(). Perhaps this is another issue? OK, I got it. Indeed, in that case, we have no way to check this happens in the preempt critical section. Anyway, as in discussed here, __copy_from_user_inatomic() is for the internal function, so I'm also OK to this patch. Reviewed-by: Masami Hiramatsu (Google) BTW, currently we just write a fault message if the __copy_from_user_*() hits a fault, but I think we can retry with normal __copy_from_user() to a kernel buffer and copy it in the ring buffer as slow path. Thank you, -- Masami Hiramatsu (Google)