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

From: John Johansen <john.johansen@canonical.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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: Thu, 2 May 2019 12:33:04 -0700	[thread overview]
Message-ID: <88ef3979-5821-886f-3b53-c16fa325048e@canonical.com> (raw)
In-Reply-To: <20190502105158.2hluemukrdz5hbus@linutronix.de>

On 5/2/19 3:51 AM, Sebastian Andrzej Siewior wrote:
> On 2019-05-01 14:29:17 [-0700], John Johansen wrote:
>> On 4/30/19 7:47 AM, Sebastian Andrzej Siewior wrote:
>>> 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.
>>>
>> I wouldn't exactly call taking a lock speedy. Getting an available buffer
>> or returning it is indeed quick. The allocation fall back not so much.
> 
> Based on testing it happens only in the beginning. We could also start
> with 2,3,4 pre allocated buffers or so.
> My testing was most likely limited and I did not exceed two.
> 

yeah lets have a few preallocated

>>>> * 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.
>>>
>>
>> yeah, I am not really a fan of a potential infinite loop trying to allocate
>> memory. It may be worth retrying once or twice but potentially infinitely
>> spinning on failed allocation really isn't acceptable.
> 
> It shouldn't spin infinitely because even if kmalloc() does not return
> any memory, one of the other CPUs should return their buffer at some
> point. However, if you don't like it I could add two retries and return
> NULL + fixup callers. On the other hand if the other CPUs BUG() with the
> buffers then yes, we may spin.
> So limited retries it is?
> 
yes please