Re: [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: John Johansen <john.johansen@canonical.com>
Cc: linux-security-module@vger.kernel.org,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	tglx@linutronix.de
Subject: Re: [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches
Date: Tue, 30 Apr 2019 16:47:25 +0200	[thread overview]
Message-ID: <20190430144725.gd6r3aketxuqdyir@linutronix.de> (raw)
In-Reply-To: <ae17e2a3-7d08-5863-4fba-66ddeac11541@canonical.com>

On 2019-04-28 16:56:59 [-0700], John Johansen wrote:
> So digging into why the history of the per cpu buffers in apparmor.
> We used to do buffer allocations via kmalloc and there were a few reasons
> for the switch 
> 
> * speed/lockless: speaks for it self, mediation is already slow enough

it is shared among all CPUs but it is a small/quick operation to
add/return a buffer.

> * some buffer allocations had to be done with GFP_ATOMIC, making them
>   more likely to fail. Since we fail closed that means failure would
>   block access. This actually became a serious problem in a couple
>   places. Switching to per cpu buffers and blocking pre-empt was
>   the solution.

GFP_KERNEL is allowed to use IO/SWAP and ATOMIC has emergency pools. The
new approach won't return a NULL pointer, simply spin to either allocate
new memory or get one which was just returned.

> * in heavy use cases we would see a lot of buffers being allocated
>   and freed. Which resulted in locking slow downs and also buffer
>   allocation failures. So having the buffers preallocated allowed us
>   to bound this potential problem.
> 
> This was all 6 years ago. Going to a mem pool certainly could help,
> reduce the memory foot print, and would definitely help with
> preempt/real time kernels.
> 
> A big concern with this patchset is reverting back to GFP_KERNEL
> for everything. We definitely were getting failures due to allocations
> in atomic context. There have been lots of changes in the kernel over
> the last six years so it possible these cases don't exist anymore. I
> went through and built some kernels with this patchset and have run
> through some testing without tripping that problem but I don't think
> it has seen enough testing yet.

Do you want apply #1 now and #2 later? I audited the ATOMIC->KERNEL
changes manually and I didn't see any atomic context. It looked like the
only reason for ATOMIC was the preempt_disable() due to the memory pool.

Sebastian