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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: Wed, 1 May 2019 14:29:17 -0700
Message-ID: <02d7772b-5d06-1c32-b089-454547fbe08b@canonical.com> (raw)
In-Reply-To: <20190430144725.gd6r3aketxuqdyir@linutronix.de>

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.


>> * 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.

>> * 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.
> 

Indeed most if not all (I'd have to dig to be sure) the changes made in #2
were original done because of the move to the per cpu buffers and blocking
pre-emption.

The problem was with the allocation of the buffer needing to be GFP_ATOMIC
some times.

  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-05 13:34 Sebastian Andrzej Siewior
2019-04-05 13:34 ` [PATCH 2/2] apparmor: Switch to GFP_KERNEL where possible Sebastian Andrzej Siewior
2019-05-07 19:57   ` John Johansen
2019-04-15 10:50 ` [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches Sebastian Andrzej Siewior
2019-04-28 23:56 ` John Johansen
2019-04-30 14:47   ` Sebastian Andrzej Siewior
2019-05-01 21:29     ` John Johansen [this message]
2019-05-02 10:51       ` Sebastian Andrzej Siewior
2019-05-02 13:17         ` Tetsuo Handa
2019-05-02 13:47           ` Sebastian Andrzej Siewior
2019-05-02 14:10             ` Tetsuo Handa
2019-05-03 11:48               ` [PATCH v2] " Sebastian Andrzej Siewior
2019-05-03 11:51                 ` [PATCH v3] " Sebastian Andrzej Siewior
2019-05-03 12:41                   ` Tetsuo Handa
2019-05-03 14:12                     ` [PATCH v4] " Sebastian Andrzej Siewior
2019-05-07 19:57                       ` John Johansen
2019-10-02  8:59                         ` Sebastian Andrzej Siewior
2019-10-02 15:47                           ` John Johansen
2019-10-02 15:52                             ` Sebastian Andrzej Siewior
2019-05-02 19:33         ` [PATCH 1/2] " John Johansen

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